Improvements in and relating to photonics

Abstract

An all-optical switch 1 comprises an optical router which receives an optical signal and routes the signal conditional on single-photon detection. The router comprises an optical resonator 14, a first

Description

Field of the Invention The present invention relates to optical switches and / or optical signal routers and particularly, although not exclusively, to single-photon triggered optical switches and / or optical signal routers. Background Many advantages are gained by using integrated photonics as a platform for quantum technologies. These include ease of integration with more traditional photonic technologies, the potential to integrate thousands of components on a single chip, and the relatively cheap, fast and robust nature of information carried in the form of single photons or other quantum states of light. However, because light signals do not mutually interact in the same way as, for example, two electrons do via the coulomb force, a drawback of photonics is the difficulty in engineering a coupling between optical signals. As a result of this, constructing all-optical logic gates between optical signals is challenging. Effective interactions between optical signals can be engineered by electrically detecting all or part of one of two signals so as to convert it into an electronic signal. The electronic signal may then be amplified and used to drive or modulate the other one of the two optical signals (e.g., a target optical signal). Unfortunately, this approach introduces some latency associated with the detection, amplification and optical-to-electronic conversion processes. This latency means that the target optical signal typically must be passed through an optical delay line, which is often several metres in length thereby taking up much valuable physical space on an integrated chip. In addition, propagation losses will also be incurred in an optical delay line. This is especially undesirable when dealing with single photons in quantum information applications, because information encoded onto the quantum states of single photons cannot be amplified or copied. These factors mean that a need exists for an effective way to switch and / or controllably couple light involving single photons or other quantum states of light. This is needed for optical quantum technologies, where fast switching is needed for, e.g., multiplexing probabilistic sources, providing the necessary feedforward for quantum teleportation, or conditional operations for quantum gates and ultimately optical quantum information processing. The present invention has been devised in light of the above considerations. Summary of the Invention Mindful of the issues above, the inventors have recognised that optical quantum technologies will benefit from the fundamental capability to do fast switching conditioned on single photon detection. Removal of the need for bulky and / or off-chip amplification of the electrical signal generated from a single photon detector reduces a latency penalty. In order to reduce this latency, the inventors have realised that modulation of a target signal may be done directly from the voltage pulse produced by a single photon detector, and that an advantage may be gained by the use of a sensitive structure such as an optical resonator that will respond to a relatively weak signal. Preferably, the use of a non-standard material platform such as thin-film lithium niobate which has a high electro-optic coefficient (or to use carrier injection in CMOS materials) may be used in this regard. The inventors have realised that a photon may be detected and an optical switch or re-router may be operated in response to that so as to re-route or switch a target optical signal without the need for off-chip amplification and the associated long delay lines and incurred losses. In a general sense the invention provides a method to re-route or switch a target optical signal conditional on the detection of one or more photons in a manner that does not require a bulky and / or off-chip electronic amplifier. This innovation may be implemented preferably by using the small (tens to hundreds of mV) voltage output from (e.g., known and available) single photon detectors to modulate the resonance of one or more optical resonators. For example, the optical resonator may in general couple multiple optical modes (e.g., waveguide modes). Optical signals propagating in one of the waveguide modes may be distributed via the resonator(s) across the modes (e.g., the waveguide modes), in a manner that depends on the resonance of the resonator(s) and the coupling between the resonator and optical modes (e.g., the waveguides). Thus, an initial optical signal may be distributed differently depending on the detection of a single photon. This may provide a fast single-photon transducer which may enable performance of optical transformations conditional on the detection of a single photon, and thus removes the need for long delay lines and the requirement for real time feedforward logic with long electronic connections. At its most general, the invention is to implement optical switching using an optical resonator into and out of which an optical signal (e.g., travelling optical modes), of a given optical signal frequency, can selectively couple by electro-optically modulating the resonance characteristics (e.g., resonance optical frequency) of one or more optical resonators in response to a signal produced by the single-photon detector (SPD). The detection of a photon can thereby control how (and when) the optical signal couples into and / or out of the optical resonator(s). In this way, the route of the optical signal (e.g., travelling optical modes of light) may be switched in response to (i.e., condition upon) the detection of a single photon. That single photon can be prepared by any desired optical or photonic process occurring within a part of an optical or photonic circuit of which the optical switch forms a part or is optically coupled to. Examples of optical or photonic circuits include photonic information processor circuits, photonic quantum computer circuits, photonic communications circuits and components and the like. The quantum states of travelling optical modes conditionally routed by the optical switching may then be subject to further optical processing, such as quantum-optical processing, as desired. Here, the term ‘quantum-optical processing’ is intended to include a reference to the processing of an optical signal by a process defined by treating the optical signal as possessing a quantum state which may be manipulated according to defined quantum processes to achieve a desired quantum state. The ‘quantum-optical processing’ may comprise applying a process corresponding to the application of a quantum operator to the optical signal (e.g., a ‘displacement operator’, a ‘phase shift operator’ etc.). The ‘quantum-optical processing’ may require the optical signal to be defined in terms of a quantum state or states e.g., representing a photon or a stream of photons rather than a classical electromagnetic wave. For example, ‘quantum-optical processing’ may comprise entanglement. Noting that quantum states of travelling optical modes can be entangled, e.g., by mixing the modes at an appropriately chosen multiport. The simplest example is the superposition of two travelling modes by an optical beam splitter. A combination of beam splitters with measuring instruments in certain output channels may therefore be a method for engineering quantum states of travelling optical fields. For example, ‘quantum-optical processing’ may comprise generation of ‘squeezed’ states from the optical signal. For example, ‘quantum-optical processing’ may comprise generation of phase-shifted quantum states from the optical signal. For example, ‘quantum-optical processing’ may comprise generation of ‘displaced’ quantum states from the optical signal (e.g., to displace a state in phase space by a desired magnitude). An optical resonator may comprise a ring resonator, a racetrack resonator, a disk resonator, a bow-tie resonator, a photonic crystal ring resonator, a photonic crystal cavity resonator (2D or 1D cavities), a semiconductor micro-pillar cavity resonator. A ring resonator structure, racetrack resonator structure has been found to be particularly beneficial as it can blend high quantity-factor (Q), relatively easier integration into an optical circuit, and greater efficiency of modulation. In a first aspect, the invention may provide an optical switch for receiving an optical signal from an optical signal source and for outputting the optical signal, the optical switch comprising an optical router configured for routing the optical signal conditional on the detection of a single photon, the optical router comprising: an optical resonator; a first optical pathway part comprising the optical resonator, and a second optical pathway part; an optical input coupling part for receiving the optical signal and for coupling the received optical signal into the first optical pathway part and into the second optical pathway part; and, an optical modulator coupled to a single-photon detector unit configured to output a detection signal in response to said detection of a single photon, the optical modulator being configured, in response to the detection signal, to modulate the refractive index of the optical resonator to change an optical phase shift incurred by the optical signal upon passage through the optical resonator such that an optical interference results in a routing of the output optical signal to one of the first and second optical pathway parts selected conditional on the detection of a single photon. The optical switch may comprise an optical output coupling part for receiving the optical signal via the first optical pathway part and for receiving the optical signal via the second optical pathway part such that the optical signal as received via the first optical pathway part optically interferes with the optical signal as received via the second optical pathway part, and for outputting the result of the optical interference to the first optical pathway part and to the second optical pathway part. Accordingly, an optical resonator rapidly, controllably and in response to a single photon detection signal, changes an optical phase shift incurred by the optical signal to control how the optical signal from / on the first optical pathway part optically interferes with the optical signal from / on the second optical pathway part. This controlled and conditional phase shift change has the effect of controlling the net effect of the optical interference in terms of the distribution of optical intensity (or power), or of probability density in quantum mechanical terms, of the result of the optical interference output to the first optical pathway part and to the second optical pathway part. A pulse of light may be defined, for example, as having a finite duration (r) in time, and a finite extent (ct / h) in space in its direction of motion, where c is the speed of light in a vacuum and n is the refractive index of the material through which the pulse travels (e.g., a waveguide material). If the dimensions of the pulse are large compared to the wavelength of light but small compared to the dimensions of the optical switch then the light pulse acts as a particle, or a group of particles, localised at the pulse position and moving at the speed of light in that material. An ordinary plane wave of light of definite wavelength A is spread over all space and therefore cannot represent a light pulse which is localised in a narrow region of space, described by a “wave packet”. A wave packet comprises a group of waves having slightly different wavelengths relative to each other, spanning a wavelength range 2AA from Ao - AA to Ao + AA, with amplitudes and phases being such that the group of waves interfere constructively only over a small region of space corresponding to the location of the particle(s). The group of waves destructively interfere elsewhere in space such that the amplitude of the wave packet falls to zero rapidly at distances spaced from the location of the wave packet. The amplitude, E, of a wave packet (in one dimension, for simplicity) will, in general, resemble the curve 3 shown in Fig. 1. A wave packet may be represented by taking a plane wave and integrating it over a small range of wavelengths. As a simple example, example: k0+Ak r sin[A / c(x — x0)] EM = I dkexp[ik(x — x0)] = 2--------------exp[ifc0(x — x0)] J (x - XO) k0-&k Here, i = V^l, k0 = 2tt / A0 is a wave number, Ak = -2nAA / A2, and x0 is the centre position of the wave packet which extends either side of this position in space, x. The intensity of this wave packet, corresponding to a probability density in quantum mechanical terms, is given by: |E(x)|2 = 4 sin2[A / c(x — x0)] (x - xo)2 The intensity is maximum when x = x0 and falls to zero when x - x0 = n / kk. Similarly, a so-called “Gaussian wave packet” is generated by applying the same principles to a group of waves whereby each wave of a given wave number k is weighted by a Gaussian weight f(k - k0) that is dependent upon that wave number as follows: f(k- kg) = exp (fc ~ kg)2' 2(A / c)2 , E(x) = f(k — k0) x exp[i / c(x - x0)] dk Thus: (k - koy 2(Afc)2 x exp[ifc(x — x0)]dk = 427tkk X exp lk0(x — x0) (x — x0)2(Afc)2’ 2 Noting that Ax is a measure of the spatial width of the wave packet, this is related to the range of wave numbers Lk forming the wave packet according to the constraint: AxA / c = 1. To represent the motion of this wave packet through space, at the speed of light, we note that a wave with wave number k oscillates with a frequency to = ck / n so that: E(x,t) = J exp (k-koy 2^kY x exp[i{ / c(x — x0) — w^k^tJldk = V27rAfc x exp[i{k0[(x - x0) - ct / n]} — (x — x0 - ct / n)2(Afc)2 / 2] The wave packet 3 illustrated in Fig. 1 schematically shows the modulus of the real part, |Re{E(x,t)}|, of a Gaussian wave packet of this form. The intensity of this wave packet, corresponding to a probability density in quantum mechanical terms, is given by: |E(x,t)|2 = 2tt(A / c)2 x exp[-(x - x0 - ct / n)2(A / c)2] The group velocity of the wave packet, VCroup = dte / dk, corresponds to the speed of motion of the centre of the wave packet at which the phase, <p = {k(x - x0) - to(fc)t}, of the moving wave packet satisfied the condition: d<p / dk = 0. This gives: d<p / dk = {(x — x0) — doj / dk x t} = 0 Thus, Vcroup = da) / dk = (x- x^ / t = c / n In this way, a wave packet (e.g., see item 3 of Fig. 1), representing either a pulse of light (e.g., laser light) or a sole photon, travels through the optical switch apparatus, and may be routed as disclosed herein. The interference of this wave packet as between versions of itself (e.g., self-interference, quantum mechanically-speaking) on the first and second optical pathway, or as between separated parts of the wave packet (classically-speaking) may be carefully controlled by the optical switch to achieve routing of the wave packet. The inventors have appreciated the importance of ensuring that most of, or substantially / approximately all of, the wave packet is successfully routed to a desired route output without significant deformation of, or loss from, the wave packet in the process. This comes from a realisation that the spatial and temporal locality of the wave packet arises from the fact that the wave packet comprises a spectral spread, or bandwidth, Afc, of optical wavelengths / frequencies, and that optical processes and components of the optical switch should most preferably be substantially or approximately “frequency agnostic” across the whole of (or at least most of) the spectral spread, or bandwidth, A / c, so that all of (or at least most of) the frequency components of the wave packet “feel” the same optical interactions within the optical switch as the wave packet is routed through / by the optical switch. The result is most preferably that the whole of (or at least most of) the wave packet is routed as one through / by the optical switch, so that practically no component part of the wave packet (or a negligible amount of the wave packet) is mis-directed / mis-routed through / by the optical switch by virtue of the routing effects of the optical switch being different for different component parts (frequencies) of the wave packet. The invention aims to address this. The inventors have appreciated the importance of providing a sufficiently low degree of distinguishability (or a sufficiently high degree of indistinguishability) between the wave packet as it appears when output from the optical switch upon the first optical pathway and the wave packet as it appears when output from the optical switch upon the second optical pathway. This may be aided by providing a sufficiently low difference between: (a) a first time interval between the inputting of the wave packet to the optical switch and the subsequent outputting of the wave packet from the optical switch upon the first optical pathway; and, (b) a second time interval between the inputting of the wave packet to the optical switch and the subsequent outputting of the wave packet from the optical switch upon the second optical pathway. By minimising the difference (relative delay, t) between the first and second time intervals, distinguishability of the wave packet as it appears when routed out to the first optical pathway, and as it appears when routed out to the second optical pathway may be reduced. A high degree of indistinguishability in optical signals routed by an optical switch is a highly-desirable property or requirement when used in quantum information processing whereby a timing of the delivery / output of an optical signal to downstream optical processes is preferably independent of the state (routing) of the optical switch. A significant relative delay, t, between the first and second time intervals may damage the desired distribution of entanglement between photons in a quantum information processing circuit. Indistinguishability may be quantified in terms of the photon coincidence rate at the two outputs of a beam-splitting interferometer according to the methods described in reference [1]: “Measurement of subpicosecond time intervals between two photons by interference”; C. K. Hong, Z. Y. Ou, and L. Mandel; Phys. Rev. Lett. 59, 2044 - Published 2 November 1987. A graph of the photon coincidence rate at the two outputs of a beam-splitting interferometer may contain a so-called Hong-Ou-Mandel (HOM) ’dip’ (e.g., see Fig. 5(d)). When the photon as output from one of two interferometer output optical pathways is identical to the photon as output from the other one of the two interferometer output optical pathways, complete indistinguishability is attained and the HOM dip falls to zero, whereas when there is complete distinguishability the HOM dip disappears. A graph of the photon coincidence rate is such that the x-axis may correspond to a separation (time delay, r) of photon wave packets as output from one of two interferometer output optical pathways, while the y-axis corresponds to normalized coincidence / overlap of optical output intensity (or probability density) of the wave packet arising from the two interferometer output optical pathways. This may be thought of as a measure of a redistribution of the wave packet (or parts of it) as between the first and second optical pathway parts at the output of the optical switch. The form of the photon coincidence rate including a HOM dip for “Gaussian wave packet” is generally as follows (see equation (11) of ref. [1]): yM =A(1- Bexp{—C[r - t0]2}) Here, the terms A, B and C are constants whose value depends upon the properties of the optical switch and the optical interference it imposes on the wave packet. For example, in an idealised case, if 4=0.5, and B = 1.0, then y(r) varies from a value of 0.5 for large values of |t - t0|, and ‘dips’ to a value of 0 (zero) when: t - t0 = 0. The term t0 represents a particular value of the relative time delay, t, noted above, at which the coincidence / overlap reaches a minimum. Preferably, the photon coincidence rate, y(r), of the optical switch is such that: y(0) = 0, or as close to this result as is practicable, such as: y(0) <0.2, or y(0) <0.1, or y(0) <0.05, or y(0) <0.01. Preferably, t0 = 0, or as close to this result as is practicable. Preferably, y(r0) = 0, or as close to this result as is practicable. The time-dependent voltage output of a single photon detector is used to modulate (i.e., shift) a resonant frequency of one or more optical resonators of the optical switch which changes how the input light accumulates optical phase from interaction with the optical resonator(s). The inventors have found that a large enough voltage output from the single photon detector can cause a large enough shift in the resonant frequency of the optical resonator(s) such that input light may be caused to accumulate from an optical resonator an optical phase controllable to control optical interferences and, thereby, control the routing of the input light by the optical switch. The time scale at which the cessation of the modulated change in accumulated optical phase takes place has been found to be comparable to the time scale of the time-dependent voltage output of a single photon detector. For example, optical interference may result in a distribution of output optical intensity (or power), or of probability density, concentrated upon output to the first optical pathway part and not upon output to the second optical pathway part, or vice versa. The distribution of output optical intensity (or power), or of probability density, may be such that the proportion (as a %) of the total output optical intensity (or power), or of probability density, as concentrated upon one of the first and second optical pathway parts, is at least about 80%, or at least about 85%, or at least about 90%, or at least about 95% or at least about 99%. For example, expressed in terms of switching fidelity, the optical switch may provide a switching fidelity, as between optical outputs upon the first and second optical pathway parts, of at least about 0.7, or at least about 0.75, or at least about 0.8, or at least about 0.85, or at least about 0.9, or at least about 0.95, or at least about 0.99. Switching fidelity measures the degree of overlap between two states, i.e., how similar two states are with respect to one another, giving a value of 1.0 for perfect fidelity and a value of 0 (zero) for no fidelity at all. For example, switching fidelity may measure the degree of overlap between the following two states: (a) the state corresponding to the distribution of output optical intensity (or power), or of probability density, concentrated upon output to the first optical pathway part; (b) the state corresponding to the distribution of output optical intensity (or power), or of probability density, concentrated upon output to the second optical pathway part. For example, switching fidelity may be considered to be the fidelity of two identical photons being input separately into a respective one of two separate identical copies of the optical switch, where one of the optical switches is switched to re-route one of the photons and the other optical switch is not. The overlap of the re-routed photon's optical intensity, or probability density, with the not-re-routed photon's optical intensity, or probability density, provides a measure of switching fidelity. Perfect performance gives complete overlap (fidelity = 1.0) of the two photons and less-than-perfect performance gives less than complete overlap (fidelity <1.0). This quantifies how well the actual behaviour of the optical switch conforms to an ideal behaviour of this system. The optical switch may comprise the optical signal source. The optical signal may comprise a given optical signal frequency bandwidth, and the optical resonator may be configured to resonate at a resonant optical frequency within a resonance bandwidth determined by a refractive index of the optical resonator. The optical modulator may be configured to modulate the refractive index of the optical resonator so as to modulate the resonance bandwidth to change from being a resonance bandwidth centred at an optical frequency higher (or lower) than a centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency lower (or higher) than the centre of the optical signal frequency bandwidth. Alternatively, the optical modulator may be configured to modulate the refractive index of the optical resonator so as to modulate the resonance bandwidth to change from being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than or lower than the centre of the optical signal frequency bandwidth. It is to be understood that a resonance bandwidth of an optical resonator may be set by factors determined during fabrication, such as physical dimensions and other characteristics, it is also determined - after it has been fabricated - by the refractive index of the optical resonator. The refractive index of the optical resonator may be controllably adjusted after fabrication of the resonator, as is discussed in more detail below. References herein to the term “...bandwidth...” may be considered to include a reference to a continuous range of frequencies spanning a given frequency band. In the context of an optical wave packet or an optical pulse, for example, references herein to the term “...bandwidth...” may include a reference to a width (e.g., full width at half maximum (FWHM), or a standard deviation) of a distribution of optical frequencies present in the optical wave packet or optical pulse. References herein to the term “...resonance bandwidth...” may be considered to include a reference to a continuous range of frequencies including not only the exact resonance frequency, wres, but also including closely neighbouring frequencies that differ from the exact resonance frequency by not more than one half of the value of the resonance line-width (e.g., FWHM), f, of the spectral resonance profile of the optical resonator. For example, the terms “...resonance bandwidth...” may be considered to include a reference to a continuous range of frequencies, to, which satisfy the following condition: - r / 2) <to <(wres + r / 2) Accordingly, the optical resonator may be considered to resonate at any optical frequency within the resonance bandwidth, and all such optical frequencies may be considered to be “resonant optical frequencies” for practical purposes. For example, light of frequency to may be considered to reside within the “...resonance bandwidth...” of a resonator possessing a quality factor, Q, within the meaning of the term used herein, if the following condition is met: tores(l - 1 / 2$) <to <tores(l + 1 / 2$) Here, Q = a>res / T. It will be understood that the terms “...modulate the resonance bandwidth...” may be considered to include a reference to a change, shift or translation of the spectral location or position of the resonance bandwidth (e.g., the position of its centre). Noting that the resonance bandwidth surrounds the exact resonance frequency, a)res, which may be at or close to the bandwidth centre, this means that a change, shift or translation of the spectral location or position of the resonance bandwidth corresponds to a change, shift or translation of the spectral location or position of the resonance frequency within it. Thus, a change (modulation) of the frequency position of the resonance bandwidth may comprise a change / shift (modulation) in the resonance frequency. It is also noted that a modulation of the refractive index of the optical resonator may result in a change in the size of the resonance line-width (e.g., FWHM), r, of the spectral resonance profile of the optical resonator. Thus, a change in the size of the resonance bandwidth (due to a change in line-width of the spectral resonance profile) alone or together with a change in the spectral position of the resonance bandwidth (and of the resonance frequency) may occur in response to a modulation of the refractive index of the optical modulator. Either or both effects may contribute to the end result of the resonance bandwidth either including or excluding a given optical signal frequency, as desired. The optical resonator may be arranged to change an optical phase shift incurred by the first optical signal portion by passage through the optical resonator by at least about 3tt / 4 radians, or by about n radians. The optical signal source and the optical resonator may be arranged such that optical signal is detuned from resonance with the optical resonator by a detuning such that an optical phase shift incurred by the first optical signal portion by passage through the optical resonator is about -rr / 2 radians (or, in an alternative, about tt / 2 radians) in the absence of (or, in an alternative, in response to) of the detection signal, and is about +n / 2 radians (or, in the alternative, about -tt / 2 radians) in response to (or, in the alternative, in the absence of) of the detection signal. Alternatively, the optical signal source and the optical resonator may be arranged such that the optical signal is substantially resonant with the optical resonator such that an optical phase shift incurred by the first optical signal portion by passage through the optical resonator is about +tt radians in the absence of (or, in an alternative, in response to) of the detection signal, and changes by at least about +0.75tt radians and less than about +1.25tt radians (preferably about +n radians) in response to (or, in the alternative, in the absence of) of the detection signal. The inventors have found that this controlled modulation of the phase shift imposed upon a signal by the optical resonator is particularly effective in achieving optical switching with a high switching fidelity. The second optical pathway part may comprise an optical phase-shifter part configured to impose a preset optical phase shift incurred by the optical signal upon passage through the optical phase-shifter part to the optical output coupling part. The inventors have found that, in some embodiments, an improvement of the switching fidelity of the optical switch may be achieved. The pre-set optical phase shift may be about tt / 2 radians. In some examples, the pre-set optical phase shift may be about (2n + 1)tt / 2 radians, where n is any integer, for example about n / 2 radians, and the optical signal source and the optical resonator may be arranged such that optical signal is detuned from resonance with the optical resonator by a detuning such that an optical phase shift incurred by the first optical signal portion by passage through the optical resonator is about -tt / 2 radians (or, in an alternative, about n / 2 radians) in the absence of (or in response to) of the detection signal, and is about +tt / 2 radians (or, in the alternative, about —tt / 2 radians) in response to (or in the absence of) of the detection signal. Alternatively, in the absence of the pre-set optical phase shift (or if the pre-set optical phase shift is set to zero or is negligible), the optical signal source and the optical resonator may be arranged such that the optical signal is substantially resonant with the optical resonator such that an optical phase shift incurred by the first optical signal portion by passage through the optical resonator is about +tt radians in the absence of (or, in an alternative, in response to) of the detection signal, and changes by at least about +0.75tt radians and less than about +1.25n radians (preferably about +n radians) in response to (or, in the alternative, in the absence of) of the detection signal. The optical phase-shifter part may comprise an optical resonator. Alternatively, the optical phase-shifter part may be operable to induce a pre-set refractive index change in the material of the second optical pathway part induced by effects including, but not limited to, thermo-optic, electro-optic, carrier-injection, piezo-electric, birefringent, micro-electro-mechanical, strain-inducing, or acousto-optic. Examples of means to achieve this effect are readily available to the person skilled in the art. The optical input coupling part may be configured to couple the received optical signal into the first optical pathway part and into the second optical pathway part by redistributing (e.g., splitting) the optical signal (e.g., its intensity; or its probability density distribution, in quantum terms) amongst the first optical pathway part and the second optical pathway. The optical input coupling part may be configured to redistribute (e.g., split) the optical signal in equal measure (e.g., 50 / 50 % split) amongst the first optical pathway part and into the second optical pathway. This redistribution may be considered to be a division, of splitting of the received optical signal into first and second optical signal portions (e.g., signal intensity portions or probability density portions). The optical output coupling part may be configured to couple the redistributed parts of the optical signal on the first optical pathway part and on the second optical pathway part by recombining (e.g., merging) the redistributed parts of the optical signal optical signal (e.g., its intensity; or its probability density distribution, in quantum terms) so that optical interference can occur. The resulting optical signal may be redistributed (e.g., split) to one of the first optical pathway part and the second optical pathway according to result of the optical self-interference. The optical output coupling part may be configured to redistribute (e.g., split) the optical signal substantially wholly (or preferentially) into the first optical pathway part or substantially wholly (or preferentially) into the second optical pathway. The optical resonator of the first optical pathway part may comprise an asymmetric pair of optically coupled optical resonators comprising a primary optical resonator and a secondary optical resonator whereby a quality factor, QI, of the primary optical resonator differs from a quality factor, Q2, of the secondary optical resonator. A resonator may be described by two different Q-factors. The intrinsic Q-factor depends only on the coupling to unwanted, loss modes representing losses from the resonator. A high intrinsic Q-factor corresponds to low unwanted losses and is generally desirable. When including losses arising from the coupling of light into the resonator, the total Q-factor of the resonator becomes lower than its intrinsic Q -factor value and is referred to in the art as the “loaded” Q-factor. It is to be noted that a reference to a quality factor may refer to an “intrinsic” quality factor of an optical resonator, or may refer to a “loaded” quality factor of an optical resonator as the context requires. An “intrinsic” quality factor may refer to a quality factor as defined herein being the ratio of the resonance frequency, (ores, and the full width at halfmaximum (FWHM) bandwidth, r, of the resonance: Qo = (ores / r in a context where optical losses from the resonator incurred by the coupling of the resonator to an optical waveguide, other or optical mode, are negligible or omitted. A “loaded” quality factor may refer to a quality factor as defined herein being the ratio of the resonance frequency, a)res, and the full width at half-maximum (FWHM) bandwidth, r, of the resonance: QL = a)res / r in a context where optical losses from the resonator incurred by the coupling of the resonator to an optical waveguide, or other optical mode, are not negligible and are included. Accordingly, an intrinsic quality factor Qo, is related to the losses caused by scattering, absorption and radiative process. A coupling quality factor Qc is related to optical losses from the resonator incurred by the coupling of the resonator to an optical waveguide, or other optical mode. These two quality factors together define the loaded quantity factor, as follows: r _ i _ i 1 ^res Ql Qo Qc Depending on the dominating loss processes, the coupling regime can be identified as “under coupled” (Qo <Qc), “critically coupled” (Qo = Qc = 2QL) or “over coupled” (Qo >Qc). As an example, consider a ring resonator of circular radius R formed from a waveguide of refractive index n, and consider the coupling of that resonator to another waveguide. The coupling rate per each round trip of an intracavity field of angular frequency co, is related to the coupling quality factor Qc as follows: 2irRna) Qc= c|k|2 Here, the factor k is the coupling coefficient between a ring resonator and the waveguide, and c is the speed of light. As the coupling is increased, the light tends to leave the resonator sooner via the waveguide / mode thereby increasing the losses due to the coupling and decreasing Qc. The losses inside the cavity may be described by an attenuation factor, a, via the expression exp(-<z27r / ?). When the losses are small exp(-a27rf?) « 1 - a2nR, whereby a is related to the intrinsic quality factor by: rrn Q _ __ Vo Xa In simulations disclosed and discussed herein with reference to the accompanying drawings, the attenuation factor, a, is assumed to be negligibly small. This is equivalent to assuming an infinite intrinsic Q-factor, Qo- However, practically-speaking, it can correctly be said that, in simulations disclosed and discussed herein with reference to the accompanying drawings, the results of the simulations are practically applicable to a very high degree of accuracy to implementations using ring resonators of high intrinsic Q-factor, Qo >1,000,000, and remain relevant to smaller values of Qo as appropriate. The coupling to waveguides described by the k parameter values discussed herein with reference to the accompanying drawings thereby determine the coupling Q-factor, Qc, which determines the loaded Q-factor, Ql. In simulations discussed herein with reference to the accompanying drawings, the loaded Q-factor, Ql, ranges from 1000s to 100,000s depending on the k parameter values specified. Thus, in specifying that a high intrinsic Q-factor is required, and subsequently specifying any k parameter coupling strengths, the loaded Q-factor is uniquely determined. A reference herein to parts being “optically coupled” may include a reference to a direct optical coupling between parts to transfer optical energy, intensity or probability density (in quantum mechanical terms) directly between the parts in question such as, or example (but without limitation), a direct evanescent optical coupling. If the context requires, a reference herein to parts being “optically coupled” may include a reference to an indirect optical coupling between two parts to transfer optical energy, intensity or probability density (in quantum mechanical terms) indirectly between the two parts in question such as, or example via an intermediate part(s) (e.g., an intervening optical waveguide etc.) to which the two parts are each directly optically coupled. The optical modulator may be configured to modulate the refractive index of one (e.g., only one, and not both) of the primary optical resonator and the secondary optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the one optical resonator. The one optical resonator may be the secondary optical resonator, in some examples. The optical modulator may comprise a plurality of separate optical modulator parts wherein, in response to the detection signal, each optical modulator part is configured simultaneously to modulate the refractive index of a respective one of the primary optical resonator and the secondary optical resonator to change (e.g., simultaneously) a respective optical phase shift incurred by the optical signal upon passage through the respective optical resonator. The optical resonator of the first optical pathway part may define a first optical resonator and the second optical pathway part may comprise a second optical resonator, and the optical modulator may comprise a first optical modulator part and a second optical modulator part wherein, in response to the detection signal, the first optical modulator part is configured to modulate the refractive index of said first optical resonator and simultaneously the second optical modulator part is configured to modulate the refractive index of said second optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the first and second optical resonators. The first optical resonator and the second optical resonator may each comprise a said asymmetric pair of optically coupled optical resonators. The optical modulator may comprise a plurality of separate optical modulator parts wherein, in response to the detection signal, each optical modulator part may be configured simultaneously to modulate the refractive index of the secondary optical resonator of a respective one of the first optical resonator and the second optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the secondary optical resonator thereof. The optical input coupling part may be configured for coupling the received optical signal into the first optical pathway part and into the second optical pathway part so as to impart a pre-set optical phase shift upon the optical signal coupled into the first optical pathway part relative to the optical signal coupled into the second optical pathway part. The pre-set optical phase difference may be about (2n + 1)tt / 2 radians, where n is any integer, for example about n / 2 radians. The optical input coupling part may be configured to impart a pre-set optical phase shift upon the received optical signal and for coupling the result into the first optical pathway part and into the second optical pathway part so as to impart substantially no optical phase difference upon the optical signal coupled into the first optical pathway part relative to the optical signal coupled into the second optical pathway part. The second optical pathway part may comprise an optical resonator configured to impose a pre-set optical phase shift incurred by the optical signal upon passage through the optical phase-shifter part to the optical output coupling part. The pre-set optical phase difference may be about (2n + 1)tt / 2 radians, where n is any integer, for example about n / 2 radians. The optical input coupling part may comprise a directional optical coupler and the optical output coupling part comprises a directional optical coupler. The optical input coupling part, the first optical pathway part, the second optical pathway part and the optical output coupling part may collectively form a Mach-Zehnder interferometer. The first optical pathway part may define a first interferometer arm of the Mach-Zehnder interferometer and the second optical pathway part defines a second interferometer arm of the Mach-Zehnder interferometer. The optical input coupling part may comprise a first optical resonator. The optical output coupling part may comprise a second optical resonator. In this way, an optical resonator, such as a looped (e.g., ring) optical resonator, my serve a dual purpose of not only imposing optical interferences upon the optical signal, but also performing the input or output coupling functions of the optical input coupling part or the optical output coupling part. This has been found to be an efficient and cost-effective way of utilising an optical resonator within the optical switch. The second optical resonator may comprise the optical resonator comprising a part of the first optical pathway part. Accordingly, the dual roles of the optical modulation and optical output coupling may be performed using the optical resonator of the first optical pathway part. The optical resonator comprising a part of the first optical pathway part may be optically coupled to the second optical pathway part. Accordingly, a routing of the optical signal to the second optical pathway part may be achieved. The optical modulator may comprise a first optical modulator part and a second optical modulator part wherein, in response to the detection signal, the first optical modulator part may be configured to modulate the refractive index of said first optical resonator and simultaneously the second optical modulator part may be configured to modulate the refractive index of the second optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the first and second optical resonators. The first optical modulator part and the second optical modulator part may be configured respectively to apply a modulation to change the refractive index of the first optical resonator and the second optical resonator by substantially the same change. The changes to the refractive index of the first and second optical resonators may be substantially the same magnitude. The changes to the refractive index of the first and second optical resonators may be the same polarity (e.g., both an increase (+ve) or both a decrease (-ve)). The first optical modulator part and the second optical modulator part may be configured respectively to apply a modulation to change the refractive index of the first optical resonator and the second optical resonator by opposite respective changes of substantially the same size. The changes to the refractive index of the first and second optical resonators may be substantially the same magnitude and the opposite polarity (e.g., one an increase (+ve) and the other a decrease (-ve)). The optical resonator may comprise a plurality of separate optical resonators including the optical resonator comprising a part of the first optical pathway part, and the optical modulator may comprise a plurality of separate optical modulator parts wherein, in response to the detection signal, each optical modulator part is configured simultaneously to modulate the refractive index of a respective one of the plurality of optical resonators to change a respective optical phase shift incurred by the optical signal upon passage through the respective optical resonator. The respective modulations of refractive index of two or more of (e.g., each of) the plurality of separate optical resonators may apply a respective refractive index change of substantially the same magnitude (e.g., a shared / common change). The respective modulations of refractive index of two or more of (e.g., each of) the plurality of separate optical resonators may apply a respective refractive index change of substantially the same polarity (e.g., each an increase (+ve) or each a decrease (-ve)). The respective modulations of refractive index of each of the plurality of separate optical resonators may apply a respective refractive index change of substantially the same magnitude (e.g., a shared / common change) wherein the polarity of the respective modulations of refractive index of one or more of, but not each of, the optical resonators amongst the plurality of separate optical resonators, differ from the polarity of the respective modulations of refractive index of one or more other optical resonators amongst the plurality of separate optical resonators (e.g., some an increase (+ve) and some a decrease (-ve)). For example, the plurality of separate optical resonators may comprise one or more pairs of optically coupled optical resonators. The respective modulations of refractive index of two optical resonators of a given pair of optical resonators may apply a respective refractive index change of substantially the same magnitude and opposite respective polarity (e.g., one an increase (+ve) and the other a decrease (-ve)). The respective modulations of refractive index of each of the plurality of separate optical resonators may apply a respective refractive index change substantially simultaneously. The plurality of separate optical resonators may comprise one or more optically coupled pairs of optically coupled optical resonators. In some examples, each of the one or more optically coupled pairs of optically coupled optical resonators comprises at least one of said first optical resonator (e.g., resonator comprising a part of the first optical pathway part) and said second optical resonator (e.g., resonator comprising a part of the second optical pathway part). In some examples, an optical resonator of one or more of the pairs of optically coupled optical resonators may be optically coupled to an optical resonator of another one or more of the pairs of optically coupled optical resonators. In this way, for example, two pairs of optically coupled optical resonators may each contribute one outer optical resonator and one inner optical resonator whereby the inner optical resonator of one pair is optically coupled to the inner optical resonator of the other pair, thereby forming an inner pair of optically coupled optical resonators. The outer optical resonators of the two pairs may be in optical communication with each other via the inner pair of optically coupled optical resonators they form. The respective modulations of refractive index respectively of the two optical resonators (e.g., inner and outer resonators) of any one of the two pairs of optically coupled optical resonators, may apply a respective refractive index change of substantially the same magnitude and opposite respective polarity (e.g., one an increases (+ve) and the other a decrease (-ve)). A modulation may be applied to increase the refractive index of the outer optical resonator of a given pair, and a modulation may be applied to the refractive index of the inner optical resonator of the given pair. These modulations may be applied simultaneously. The plurality of separate optical resonators may comprise a pair of optically coupled optical resonators in which one optical resonator of the pair of optical resonators comprises said first optical resonator (e.g., resonator comprising a part of the first optical pathway part) and said second optical resonator (e.g., resonator comprising a part of the second optical pathway part), the plurality of separate optical resonators further comprising a third optical resonator and a fourth optical resonator wherein the first optical pathway part comprises the third optical resonator and the second optical pathway part comprises the fourth optical resonator. The third optical resonator may be arranged to receive the optical signal from the optical output coupling part and to output the received optical signal as an optical output from the optical switch. The fourth optical resonator may be arranged to receive the optical signal from the optical output coupling part and to output the received optical signal as an optical output from the optical switch. The optical modulator may comprise a first optical modulator part, a second optical modulator part, a third optical modulator part and fourth optical modulator part wherein, in response to the detection signal, respectively the first optical modulator part, the second optical modulator part, the third optical modulator part and the fourth optical modulator part may be configured to modulate simultaneously the refractive index of the first optical resonator, the second optical resonator, the third optical resonator and the fourth optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the respective optical resonators. The second optical modulator part may be configured to apply to the second optical resonator (e.g., resonator comprising a part of the second optical pathway part) a refractive index change of substantially the same magnitude and opposite respective polarity (e.g., an increase (+ve) and to the others a decrease (-ve), or vice versa) to apply to a refractive index change applied to the first, third and fourth optical resonators. These modulations may be applied simultaneously. The optical switch may comprise the optical signal source. The optical signal source may be arranged to provide a source of single photons (e.g., each with a Gaussian wavepacket) emitted individually and in isolation (i.e., not as an ensemble of photons). The optical signal source may be configured to output an optical signal in the form of an optical pulse comprising either a plurality of photons, or not more than a single photon. The optical switching assembly may comprise a laser unit configured to generate a pulsed laser light output for input into the optical input coupling part. The optical input signal could be a coherent pulse, or a single photon of source light. A pulsed optical input may be switched (or not) by the detection (or lack of detection) of a single photon. A conditioned single photon gate or switch may be provided, whereby the detection of a single photon causes a switching operation to happen on a second photon or light pulse (i.e., the source light). For example, the laser unit may comprise an optical output port that is optically coupled to the optical input coupling part (e.g., to an input port thereof, or via any suitable optical coupler at a position along a waveguide leading to the optical input coupling part). In this way, the laser light output from the laser unit may provide the optical input signal to be into the optical input coupling part. The optical signal may comprise a given optical signal frequency bandwidth, and the (or each, if more than one) optical resonator may be configured to resonate at a respective resonant optical frequency within a resonance bandwidth determined by a refractive index of the optical resonator. The optical modulator may be configured to modulate the refractive index of the optical resonator so as to modulate the (or any one or more of the) resonance bandwidth(s) to either: change from being a resonance bandwidth centred at an optical frequency lower than a centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than or lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that excludes the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that excludes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth. In another aspect, the invention may provide an optical switch for receiving an optical signal from an optical signal source and for outputting the optical signal, the optical switch having; an optical router configured for routing the optical signal conditional on the detection of a single photon, the optical router comprising a Mach-Zehnder optical interferometer comprising: an input optical coupling part for receiving the optical signal and splitting the received optical signal into a first optical signal portion and a second optical signal portion; a first optical interferometer arm and a second optical interferometer arm for receiving, respectively, the first and second optical signal portions; an optical resonator optically coupled to the first interferometer arm and configured to resonate at optical frequencies within a resonance bandwidth determined by a refractive index of the optical resonator; an output optical coupler for receiving and recombining the first and second optical signal portions from the first and second optical interferometer arms; an optical modulator coupled to a single-photon detector unit configured to modulate the refractive index of the optical resonator to change an optical phase shift incurred by the first optical signal portion by passage through the optical resonator by at least about 3tt / 4 radians such that said recombining of the two signal portions by the output optical coupler results in a routing of said output optical signal to one of two optical output ports thereof selected conditional on said detection of a single photon. The modulation signal may be configured to modulate the refractive index of the optical resonator to change an optical phase shift incurred by the first optical signal portion by passage through the optical resonator by about it radians. In yet another aspect, the invention may provide an optical switch for receiving an optical signal from an optical signal source and for outputting the optical signal, the optical switch comprising an optical router configured for routing the optical signal conditional on the detection of a single photon, the optical router comprising: a first optical pathway part comprising an optical resonator, and a second optical pathway part comprising a further (e.g., second) optical resonator wherein the optical resonator and the further optical resonator are optically coupled to form a pair of optically coupled optical resonators; an optical input coupling part for receiving the optical signal and for coupling the received optical signal into the first optical pathway part and into the second optical pathway part; and, an optical modulator coupled to a single-photon detector unit configured to output a detection signal in response to said detection of a single photon, the optical modulator being configured, in response to the detection signal, to modulate the refractive index of the optical resonator and the further (e.g., second) optical resonator to change an optical phase shift incurred by the optical signal upon passage through the optical resonator and the further (e.g., second) optical resonator such that an optical interference results in a routing of the output optical signal to one of the first and second optical pathway parts selected conditional on the detection of a single photon. An optical coupling between the optical resonator and the further optical resonator may provide the optical input coupling part. The optical switch may comprise an optical output coupling part for receiving the optical signal via the first optical pathway part and for receiving the optical signal via the second optical pathway part such that the optical signal as received via the first optical pathway part optically interferes with the optical signal as received via the second optical pathway part, and for outputting the result of the optical interference to the first optical pathway part and to the second optical pathway part. An optical coupling between the optical resonator and the further optical resonator may provide the optical output coupling part. The optical resonator and the further optical resonator may each be formed from a respective pair of optically coupled optical resonators. The optical signal may comprise a given optical signal frequency bandwidth, and the (or each, if more than one) optical resonator may be configured to resonate at a respective resonant optical frequency within a resonance bandwidth determined by a refractive index of the optical resonator. The optical modulator may be configured to modulate the refractive index of the optical resonator and the further optical resonator so as to modulate the respective resonance bandwidths thereof selectively and individually to either: change from being a resonance bandwidth centred at an optical frequency lower than a centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than or lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that excludes the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that excludes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth. In the optical switch, each optical resonator may comprise a ring resonator. A ring resonator may be evanescently optically coupled to the first and / or second optical pathway part, and / or to another ring resonator, as appropriate. The refractive index of any modulated optical resonator(s) of the optical switch may be changed along only a limited section of the length / circumference of the optical resonator(s), or preferably along the entire length / circumference of the optical resonator(s). Changing the refractive index along only a limited section of the length / circumference of the optical resonator(s) may induce more optical backscattering and may be less efficient (i.e., have a lower resonant shift for the same electric signal) than if the refractive index were changed along the entire length / circumference of the optical resonator(s). For example, the optical modulator may be operable to implement local refractive index changes to the material of the optical resonator(s) at local parts (i.e., not the whole of) the optical pathway defined by the optical resonator(s) it modulates. Alternatively, the optical modulator may be operable to implement global refractive index changes to the material of the optical resonator(s) at substantially all parts (i.e., the whole of) the optical pathway defined by the optical resonator(s) it modulates. The optical modulator may be operable to implement refractive index changes induced by effects including, but not limited to, thermooptic, electro-optic, carrier-injection, piezo-electric, birefringent, micro-electro-mechanical, strain-inducing, or acousto-optic. The first optical pathway part may be evanescently optically coupled to optical resonator(s) of the optical switch (e.g., physically and materially separated from the optical resonator but in sufficient proximity to permit respective evanescent electromagnetic fields, or quantum modes / states, to couple across the separation). Alternatively, the first optical pathway part may be physically optically coupled to optical(s) of the optical switch resonator (e.g., in physical contact with, or integrally formed with, or optically bonded to the optical resonator). The first optical pathway part and the second optical pathway part may each comprise a respective optical waveguide that is optically coupled to one or more of: the optical input coupling part; one or more of the optical resonator(s) of the optical switch; the optical output coupling part, at an evanescent optical coupling region of the optical switch. For example, one or more of the optical resonator(s) of the optical switch may comprise a looped waveguide such as a ring optical resonator (e.g., a micro-ring optical resonator), a racetrack resonator, or a disk resonator, that is physically spaced from the optical waveguide of the first and / or second optical pathway part by a spacing (which may be occupied with a material or substance as appropriate) so as to be evanescently optically coupled to the optical waveguide. Alternatively, the optical waveguide of the first and / or second optical pathway part may comprise one or more of the optical resonators of the optical switch, which may be formed integrally with (e.g., within) the optical waveguide as an optical waveguide structure containing an optical resonator cavity (e.g., a photonic crystal cavity resonator (2D or 1D cavities), or a semiconductor micro-pillar cavity resonator). The optical resonator may comprise a looped optical resonator (e.g., a ring optical resonator, a optical racetrack resonator, a photonic crystal ring optical resonator, a disk optical resonator), a bow-tie optical resonator, a photonic crystal cavity optical resonator (2D or 1D cavities), or a semiconductor micro-pillar cavity optical resonator. A looped optical resonator structure (e.g., especially a ring, racetrack, or photonic crystal ring optical resonator, and also a disk optical resonator) has been found to be particularly beneficial for use as an optical resonator in any example of the present invention as it can blend high quantity-factor (Q), relatively easier integration into an optical circuit, and greater efficiency of modulation. Desirably, an optical resonator in any example of the present invention may comprise an intrinsic Q-factor of not less than 1,000,000. An optical resonator in any example of the present invention may comprise a loaded Q-factor of not less than 100,000, or not less than 200,000. This has the benefit of permitting a relatively small modulation in the resonance frequency of the optical resonator to produce the effect of a very large relative change (e.g., % change) on the transmission of the optical signal between the optical resonator and the optical signal propagating within the optical waveguide of the first or second optical pathway part. This relative change is reflected in a corresponding relative change in the optical output signal intensity from the switch. These Q-factors (e.g., loaded Q-factors) may be less than 1,000,000 in the case of using a SPAD that can output a larger voltage, providing a bigger frequency shift. Thus, scenarios exist where it is beneficial to employ a Q-factor of less than 1,000,000, such as when rerouting a spectrally broader optical signal. An optical resonator in any example of the present invention is preferably a single-mode waveguide, but may be a multi-mode waveguide. An optical resonator(s) of the optical switch may be critically coupled to the optical waveguide of the first and / or second optical pathway part, or to another optical resonator(s) of the optical switch as the case may be, or may be over coupled (or under coupled) to the optical waveguide of the first and / or second optical pathway part, or other optical component as the case may be. In a further aspect, the invention may provide an optical or photonic circuit comprising the optical switch In another aspect, the invention may provide photonic information processor circuit or chip, or a photonic quantum computer circuit or chip, or a photonic communications circuit or chip, comprising the optical switch. In yet another aspect, the invention may provide a method for optical switching for receiving an optical signal from an optical signal source and for outputting the optical signal for routing the optical signal conditional on the detection of a single photon, the method comprising: providing an optical resonator; providing a first optical pathway part comprising the optical resonator, and a second optical pathway part; providing an optical modulator coupled to a single-photon detector unit configured to output a detection signal in response to said detection of a single photon; and, by an optical input coupling part, receiving the optical signal and coupling the received optical signal into the first optical pathway part and into the second optical pathway part; by an optical output coupling part, receiving the optical signal via the first optical pathway part and receiving the optical signal via the second optical pathway part such that the optical signal as received via the first optical pathway part optically interferes with the optical signal as received via the second optical pathway part, and outputting the result of the optical interference to the first optical pathway part and to the second optical pathway part; and, by the optical modulator, in response to the detection signal, modulating the refractive index of the optical resonator to change an optical phase shift incurred by the optical signal upon passage through the optical resonator such that the optical interference results in a routing of the output optical signal to one of said first and second optical pathway parts selected conditional on said detection of a single photon. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows a schematic diagram of an optical switch comprising a Mach-Zehnder interferometer in the first one of two switching states. Figure 2 shows a schematic diagram of the optical switch of Fig. 1 in the second one of two switching states. Figure 3(a) and 3(b) each show a transmission spectrum from an optical input port to (a) a first optical output port and (b) a second optical output port of the optical switch of Fig. 1 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch. Figure 4 shows an optical phase imposed upon an optical signal as transmitted by the optical switch of Fig. 1 as a function of the detuning (refractive index modulation) of a ring optical resonator of the optical switch, and a relative position of an optical frequency distribution of a Gaussian wave packet optical pulse in each one of two switching states of the optical switch. Figure 5(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 1; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 1; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 1; (d) a “HOM dip” graph associated with transmission of the a Gaussian wave packet optical pulse by the switch of Fig. 1. Figure 6 shows a schematic diagram of an optical switch comprising a Mach-Zehnder interferometer in first one of two switching states. Figure 7 shows a schematic diagram of the optical switch of Fig. 6 in second one of two switching states. Figure 8 shows an optical phase imposed upon an optical signal as transmitted by a ring optical resonator of the optical switch of Fig. 6 as a function of the detuning (refractive index modulation) of a ring optical resonator of the optical switch, and a relative position of an optical frequency distribution of a Gaussian wave packet optical pulse in each one of two switching states of the optical switch. Figure 9(a), (b) and (c) show a transmission spectrum from an optical input port to (a) a first optical output port and (b) a second optical output port of the optical switch of Fig. 6 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch, and (c) an optical frequency distribution of an input Gaussian wave packet optical pulse together with the relative spectral position of the transmission spectrum of a second optical output port of the optical switch of Fig. 6, and the transmission optical phase of a ring optical resonator thereof, as a function of increasing detuning (refractive index modulation) of the ring optical resonator of the optical switch, as the switch transitions from one switching state to another switching state. Figure 10(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 6; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 6; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 6; (d) a “HOM dip” graph associated with transmission of a Gaussian wave packet optical pulse by the switch of Fig. 6. Figure 11 shows a schematic diagram of an optical switch comprising a Mach-Zehnder interferometer in first one of two switching states. Figure 12 shows a schematic diagram of the optical switch of Fig. 11 in second one of two switching states. Figure 13 shows an optical phase imposed upon an optical signal as transmitted by each ring optical resonator of the optical switch of Fig. 11 as a function of the detuning (refractive index modulation) of a ring optical resonator of the optical switch, and a relative difference in imposed optical phase between the two ring resonators in one of two switching states of the optical switch. Figure 14 shows an optical frequency distribution of an input Gaussian wave packet optical pulse together with the relative spectral position of the transmission spectrum of a second optical output port of the optical switch of Fig. 11, and the transmission optical phase of each ring optical resonator thereof, as a function of increasing detuning (refractive index modulation) of the ring optical resonator of the optical switch, as the switch transitions from one switching state to another switching state. Figure 15(a) and 15(b) each show a transmission spectrum from an optical input port to (a) a first optical output port and (b) a second optical output port of the optical switch of Fig. 11 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch. Figure 16(a), (b), (c) and (d) show, respectively: (a) an optical transmission spectrum of the one of the two optical output ports of the optical switch in one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 11; (b) a “HOM dip” graph associated with transmission of a Gaussian wave packet optical pulse by the switch of Fig. 11; (c) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 11; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 11. Figure 17(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to a variant of the optical switch of Fig. 11; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of a variant of the optical switch of Fig. 11; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of a variant of the optical switch of Fig. 11; (d) a “HOM dip” graph associated with transmission of a Gaussian wave packet optical pulse by a variant of the switch of Fig. 11. Figure 18 shows a schematic diagram of optical phases applied to an optical signal as it passes through an optical switch comprising a Mach-Zehnder interferometer in each one of two switching states. Figure 19 shows a schematic diagram of an interferometer providing an optical switch in first one of two switching states. Figure 20 shows a schematic diagram of the optical switch of Fig. 19 In second one of two switching states. Figure 21(a) and 21(b) each show a transmission spectrum of (a) a first optical output port and (b) a second optical output port of the optical switch of Fig. 19 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch. Figure 22(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 19; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 19; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 19; (d) a “HOM dip” graph associated with transmission of a Gaussian wave packet optical pulse by the switch of Fig. 19. Figure 23 shows an optical frequency distribution of an input Gaussian wave packet optical pulse together with the relative spectral position of the transmission spectrum of a first optical output port of the optical switch of Fig. 19, and the optical transmission spectrum of each ring optical resonator thereof, as a function of increasing detuning (refractive index modulation) of the ring optical resonator of the optical switch, as the switch transitions from one switching state to another switching state. Figure 24 shows a schematic diagram of an interferometer providing an optical switch in first one of two switching states. Figure 25 shows a schematic diagram of the optical switch of Fig. 24 in second one of two switching states. Figure 26(a) and 26(b) each show a transmission spectrum of (a) a first optical output port and (b) a second optical output port of the optical switch of Fig. 24 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch. Figure 27(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 24; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 24; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 24; (d) a “HOM dip” graph associated with transmission of a Gaussian wave packet optical pulse by the switch of Fig. 24. Figure 28(a) and 28(b) each show a transmission spectrum of (a) a first optical output port and (b) a second optical output port of a variant of the optical switch of Fig. 24 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch. Figure 29(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to a variant of the optical switch of Fig. 24; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of a variant of the optical switch of Fig. 24; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of a variant of the optical switch of Fig. 24; (d) a “HOM dip” graph associated with transmission of a Gaussian wave packet optical pulse by a variant of the switch of Fig. 24. Figure 30 shows an optical frequency distribution of an input Gaussian wave packet optical pulse together with the relative spectral position of the transmission spectrum of a first optical output port of a variant of the optical switch of Fig. 24, and the optical transmission spectrum of each ring optical resonator thereof, as a function of increasing detuning (refractive index modulation) of the ring optical resonator of the optical switch, as the switch transitions from one switching state to another switching state. Figure 31 shows a schematic diagram of an interferometer providing an optical switch in first one of two switching states. Figure 32 shows a schematic diagram of the optical switch of Fig. 31 in second one of two switching states. Figure 33(a) and 33(b) each show a transmission spectrum from an input port to (a) a first optical output port and (b) a second optical output port of the optical switch of Fig. 31 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch. Figure 34(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 31; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 31; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 31; (d) a “HOM dip” graph associated with transmission of the a Gaussian wave packet optical pulse by the switch of Fig. 31. Figure 35 shows an optical frequency distribution of an input Gaussian wave packet optical pulse together with the relative spectral position of the transmission spectrum of a second optical output port of the optical switch of Fig, 31, and the transmission optical phase of the coupled ring optical resonator thereof, as a function of increasing detuning (refractive index modulation) of the ring optical resonator of the optical switch, as the switch transitions from one switching state to another switching state. Figure 36 shows a schematic diagram of an interferometer providing an optical switch in first one of two switching states. Figure 37 shows a schematic diagram of the optical switch of Fig. 36 in second one of two switching states. Figure 38(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 36; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 36; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 36; (d) a “HOM dip” graph associated with transmission of the a Gaussian wave packet optical pulse by the switch of Fig. 36. Figure 39 shows a schematic diagram of an interferometer providing an optical switch in first one of two switching states. Figure 40 shows a schematic diagram of the optical switch of Fig. 39 in second one of two switching states. Figure 41(a) and 41(b) each show a transmission spectrum of (a) a first optical output port and (b) a second optical output port of a variant of the optical switch of Fig. 39 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch. Figure 42(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 39; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 39; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 39; (d) a “HOM dip” graph associated with transmission of the a Gaussian wave packet optical pulse by the switch of Fig. 39. Figure 43 shows a schematic diagram of an interferometer providing an optical switch in first one of two switching states. Figure 44 shows a schematic diagram of the optical switch of Fig. 39 in second one of two switching states. Figure 45(a) and 45(b) each show a transmission spectrum of (a) a first optical output port and (b) a second optical output port of a variant of the optical switch of Fig. 39 as a function of increasing detuning (refractive index modulation) of a ring optical resonator of the optical switch. Figure 46(a), (b), (c) and (d) show, respectively: (a) a spectral power distribution of a Gaussian wave packet optical pulse input to the optical switch of Fig. 43; (b) an optical transmission spectrum of one of the two optical output ports of the optical switch in one of two switching states, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from two output ports of the optical switch of Fig. 43; (c) an optical transmission spectrum of the other one of the two optical output ports of the optical switch in the other one of the two switching states of the optical switch, together with a spectral power output distribution of an input Gaussian wave packet optical pulse as output from both of two output ports of the optical switch of Fig. 43; (d) a “HOM dip” graph associated with transmission of the a Gaussian wave packet optical pulse by the switch of Fig. 43. Figures 47(a), (b) show schematic diagrams of an optical transmission spectrum of a modulated optical ring resonator in a switching / routing device. Figures 48(a), (b) show schematic diagrams of an optical transmission spectrum of a modulated optical ring resonator in a switching / routing device. Figure 49 shows the time profile of the electrical signal generated by a single-photon detector. Figures 50(a), (b), (c) shows optical resonators. Figures 51(A), (B), (C), (D), and (E) show optical waveguides. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. In the following disclosures, with reference to the drawings, like items shown in the drawings are assigned like reference symbols. A Ring Resonator: The Classical Description An optical ring resonator, as employed in any example of the invention disclosed herein, typically consists of the following elements described with reference to Fig. 1 but applicable to any of the figures herein showing a ring optical resonator. Typically, a first optical waveguide section 8 is coupled to a circular second optical waveguide section 14 defining the ring resonator. The first optical waveguide section may be a linear waveguide or may itself be a ring optical resonator (e.g., see Fig. 24, Fig. 31, Fig. 36, Fig. 39, Fig. 43). Light 7 (e.g., an optical pulse or photon) input to the ring resonator via the first optical waveguide section is coupled (i.e., transferred) into the circular optical waveguide section according to the strength of an optical coupling present at a coupling point, or short coupling region, 12 of the first optical waveguide section where it is in close proximity to (or in contact with) the circular optical waveguide section 14. Due to the presence of this optical coupling, a proportion of light 9 may be transferred into the circular optical waveguide section from the input light 7 within the first optical waveguide section. The quantity so transferred depends upon the strength of optical coupling provided by the coupling point, or short coupling region, as well as the wavelength of the light undergoing the coupling in question. By contrast, a proportion of light 11 may be fully transmitted along the first optical waveguide section, past the coupling region 12. That proportion depends sensitively on the proportion of light that is coupled into the circular optical waveguide section and, therefore, is not transmitted directly beyond the coupling region 12. The proportion of directly transmitted light 11 relative to the amount of light 9 entering the resonator, is defined as the transmittance, T, of the optical ring resonator, and is given by the formula: a2 + r2 - 2arcos(^) 1 + a2r2 — 2 ar cos (ip) Here, the parameter r is the self-coupling coefficient of light between the first waveguide 8 and the circular waveguide part 14, and the parameter a is a loss parameter related to the optical power attenuation coefficient a through the equation a2 = exp(-aL), with L being the circumferential length of the circular waveguide part. Here, <p is the single-pass phase shift, defined as: 2nn V=—L Here, n is the effective refractive index of the propagating mode in the circular waveguide part, and A is the wavelength of the light in free space. The transmittance is minimized when cos(<p) = 1.0, which implies that the condition for minimum transmittance, as far as the phase shift is concerned, is: (p = 2mn Here, m is an integer. It follows that the values of the optical wavelengths minimizing the transmittance (i.e., the resonant wavelengths) are given by the formula: L 2nR 2™, = —n =---n m m Consequently, the values of the optical frequencies minimizing the transmittance (i.e., the resonant frequencies) are given by the formula: c c ~~ ”””— —— Ares 2nRn Here, R is the radius of the circular waveguide. Consequently, the transmittance takes the form: When a = r , the transmittance is zero. This condition is known as “critical coupling”. When a <r, the resonator is said to be “under coupled”, and when a >r, the resonator is said to be “over coupled”. Optical resonators of examples of the invention may be individually “critical coupled”, or “under coupled”, or “over coupled” as appropriate. For implementations of the invention employing a ring resonator coupled to a single optical waveguide, the inventors have found it to be preferable to configure a ring resonator to be “over coupled” to a waveguide. An important result of the above analysis is that the resonant wavelength is given by A = Ares, which practically implies that it is proportional to the effective refractive index, n, of the propagating mode in the circular waveguide. In more detail, supposing that the refractive index, n, changes by An, then: 2nR ^res * O + An) ^resO T ^^res m and c ^3 2nR(n + ^n)m = ^3° “ ^3 Here, Ares0 (o»res0) is the resonant wavelength (frequency) corresponding to the refractive index, n. It follows that: An &Ares ft ^resO and An ^res ft ^resO ~ ^Mres Consequently, the relative change in the resonant wavelength (frequency) is equal to the relative change in the effective refractive index. Accordingly, an important parameter regarding the ability to vary the resonance wavelength (frequency) of an optical ring resonator is the dependence of the refractive index experienced by a propagating mode of light with the resonator. This is a measure of the sensitivity of the system when the refractive index, n, is modulated (An) deliberately as a means to modulate (Alres) the resonant wavelength of the system, or modulate (An>res) the resonant frequency of the system. Spectral power output distributions disclosed herein with reference to the drawings, as output from output ports of the optical switch and arising from an input Gaussian wave packet optical pulse, are calculated semi-classically according to operators used to describe a quantum system. This is more appropriate when the input optical signal comprises either only a single photon, or a few photons, or a rapidly changing optical pulse, when the optical processes taking place are less adequately described by purely classical physics and are accurately described by quantum mechanical operators. Examples EXAMPLE 1 In this example, the optical switch is configured for receiving an optical signal 3 from an optical signal source 30 as a pulse of laser light (or a photon). The optical switch is arranged for routing the input signal by passing it through an optical router of the switch configured for routing the optical signal conditional on the detection of a single photon. Figure 1 shows a schematic diagram of an optical switch comprising a Mach-Zehnder interferometer in first one of two switching states. Figure 2 shows a schematic diagram of the optical switch of Fig. 1 in second one of two switching states. Fig. 1 and Fig. 2 each illustrate the optical switch according to an embodiment of the invention employing an optical interferometer including a first waveguide 8 forming a first optical pathway part comprising an optical ring resonator 14, and a second waveguide 10 forming a second optical pathway part. The ring resonator is controllable for routing an input optical signal 3 (e.g., an optical pulse) received into an input optical port 2 of the second waveguide part 10 from the optical signal source (e.g., a laser unit) in optical communication with the optical switch, for onward propagation along the second waveguide part 10 for input to the optical interferometer. The switching state (routing) is put into effect by the action of the ring resonator, as described herein, conditional on the detection of a single photon (19a / 19b). The optical switch provides a single-photon triggered optical signal switch / router. In particular, the optical router comprises an optical input coupling part in the form of an input directional coupler 6 formed by an evanescent optical coupling of respective adjacent sections of the first waveguide 8 and the second waveguide 10 being in suitably close physical proximity. The coupling constants of the input directional coupler 6 are such that the directional coupler redistributes substantially 50% of the input signal 7 from the second waveguide into the first waveguide with substantially 50% of the input signal 5 remaining in the second waveguide. As a result, a substantially equal redistribution of the initial optical signal 3 is shared amongst the first optical pathway part and the second optical pathway part. The optical input coupling part 6 is thereby configured for receiving the optical signal 3 and for coupling the received optical signal into the first optical pathway part 8 and into the second optical pathway part 10. It may accurately be described by, and have the effect of, a unitary scattering matrix. Accordingly, a redistribution (5, 7) of the intensity, or probability density, of the input optical signal is formed as between the first and second optical pathways parts. The ring optical resonator 14 is directly optically coupled to the first waveguide part 8, such that the first optical pathway part comprises the optical resonator. The optical switch comprises an optical modulator 28 coupled to a single-photon detector unit 24 configured to output a detection signal in response to said detection of a single photon (19a / 19b). The optical modulator 28 is configured, in response to the detection signal, to modulate the refractive index of the optical resonator 14 to change an optical phase shift incurred by the optical signal 9 upon passage through the optical resonator such that the optical interference occurring at the optical output coupling part 16 results in a routing of the output optical signal 17 to one of said first and second optical pathway parts selected conditional on said detection of a single photon. More generally, for arbitrary inputs, the switched and unswitched outputs may preferably be orthogonal (or as close to orthogonal as possible). The optical ring resonator 14 is configured to resonate at resonant optical frequencies within a resonance bandwidth determined by a refractive index, n, of the optical resonator. The first waveguide part 8 is optically coupled to the optical ring resonator 14 by the physical proximity of the first waveguide part to the ring resonator at a coupling region 12. An optical input port of the single-photon detector is coupled to the optical output end of a feed waveguide 22 for conveying photons to the single photon detector. An optical input end of the feed waveguide 22 is optically coupled to a source (not shown) of single photons, or other quantum optical state such as a squeezed state, for receiving single photons therefrom and for guiding the single photons to the optical input port of the single-photon detector for detection. This source of single photons may be any suitable source readily apparent and available to the person skilled in the art. The single-photon detector unit 24 is electrically coupled to an electro-optical modulator 28 which is configured to receive the electrical detection signal and to output an electrical modulation signal in response to the electrical detection signal. An optional electrical contact part 26 electrically links the single-photon detector 24 and the electro-optical modulator 28. This electrical contact part serves to provide signal processing or amplification of the electrical detection signal produced by single-photon detector unit 24 and may comprise signal filtering elements or capacitive elements. It may be used to generate a larger output voltage pulse, such as by passing the signal through a cascading array of nanowire detectors to amplify the output pulse from a single detector. Alternatively, or in addition, the electrical contact part 24 may comprise an impedancematching taper, or / and may comprise a compact low power signal amplifier. It is to be noted that the presence and arrangement of the feed waveguide 22, the single-photon detector unit 24, and optional electrical contact part 26, as described above, is also present in the other examples of the invention disclosed herein with reference to the accompanying drawings. For the avoidance of doubt, the above description of these parts with reference to Fig. 1 is equally applicable to the same items shown as present in the figures showing the other examples, and is not repeated hereafter. The electro-optical modulator 28 is configured to modulate the refractive index, n, of the material forming the waveguide defining the optical ring resonator 14 by the electrical modulation signal, in response to detection of a single photon by the single-photon detector unit. The effect of the electrical modulation signal is to modulate (&a)res) the spectral position, a>res0, of the centre of the resonance bandwidth of the optical ring resonator. The effect of the modulation in spectral position is to permit an optical input signal 9 to modulate the amount of an optical phase accumulated by an optical signal 9 within the ring resonator thereby to modulate the optical interference occurring at the optical output coupling part 16 when the redistributed optical signal 11 as received via the first optical pathway part optically interferes with the redistributed optical signal 13 as received via the second optical pathway part. As shown in Fig. 1, in the absence 19a of a photon in the feed waveguide 22 to the single-photon detector 24, no detection of a photon takes place and, consequently no electrical detection signal is generated. This means that the electro-optical modulator is not driven to modulate, An, the refractive index of the material of the waveguide forming the ring resonator (i.e., the modulator is “OFF”). Consequently, no modulation, Aeores, of the resonance frequency (or resonance wavelength modulation, AAres) takes place. A first pre-set optical phase shift will be accumulated by an optical signal 9 within the ring resonator. By contrast, as shown in Fig. 2, upon detection of one or more photons 19b via the feed waveguide 22, the single-photon detector 24 produces a photon detection electrical signal which drives the electro-optical modulator 28 (i.e., switches the modulator “ON”) to change the resonant frequency (wavelength) of the ring resonator 14, and the spectral position of the bandwidth of the resonance profile. The result is that a second pre-set optical phase shift will be accumulated by an optical signal 9 within the ring resonator. The optical router also comprises an optical output coupling part 16 configured for receiving the redistributed optical signal 11 via the first optical pathway part and for receiving the redistributed optical signal 13 via the second optical pathway part such that the redistributed optical signal as received via the first optical pathway part optically interferes with the redistributed optical signal as received via the second optical pathway part. The output coupling part 16 is arranged to output the result 17 of the optical interference to an optical output port 20 the first optical pathway part and to an optical output port 18 of the second optical pathway part. Furthermore, the optical router comprises an optical output coupling part in the form of an output directional coupler 16 formed by an evanescent optical coupling of respective adjacent sections of the first waveguide 8 and the second waveguide 10 being in suitably close physical proximity. The coupling constants of the output directional coupler 16 are configured such that the directional coupler may redistribute substantially 50% of the optical signal 11 received from the first waveguide 8 into the second waveguide 10 and substantially 50% of the optical signal 11 remaining in the first waveguide. The coupling constants of the output directional coupler 16 are configured such that, simultaneously, the directional coupler may redistribute substantially 50% of the optical signal 13 received from the second waveguide 10 into the first waveguide 8 and substantially 50% of the optical signal 13 remaining in the second waveguide. As a result, a substantially equal redistribution of the optical signal (11, 13) from the first optical pathway part and the second optical pathway part is shared amongst the first optical pathway part and the second optical pathway part. The first optical pathway part 8 thereby defines a first interferometer arm of the Mach-Zehnder interferometer and the second optical pathway part 10 defines a second interferometer arm of the Mach-Zehnder interferometer. Accordingly, a final redistribution 17 of the intensity, or probability density, of the input optical signal is formed as between the first and second optical pathways parts, according to the nature of the optical interference that occurs within the output directional coupler. That optical interference is modulated according to the modulation of the refractive index of the ring optical resonator 14 and the optical phase shift it applies to the optical signal 9 passing through it. In the first switching state of the optical switch, as shown in Fig.1, an optical phase shift of about n radians is applied to the optical signal 9 passing through the ring resonator 14 and is such as to control the optical interference within the output directional coupler to fully, or at least mostly, redistribute the intensity, or probability density, of the input optical signal (11, 13) to the second optical pathway part 10 for output via the optical output port 18 thereof. Little of, or a negligible amount of, the intensity, or probability density, of the input optical signal is redistributed to the optical output port 20 of the first optical pathway 8. In the second switching state of the optical switch, as shown in Fig.2, an optical phase shift of about 0.2?r radians is applied to the optical signal 9 passing through the ring resonator 14, which differs from the optical phase shift applied to the optical signal 9 when in the first switching state, by a phase difference of -0.8 x n radians and is such as to control the optical interference within the output directional coupler to fully, or at least mostly, redistribute the intensity, or probability density, of the input optical signal (11, 13) to the first optical pathways part 8 for output via the optical output port 20 thereof. Little of, or a negligible amount of, the intensity, or probability density, of the input optical signal is redistributed to the optical output port 18 of the second optical pathway 10. The optical input coupling part 6 forms a 50 / 50% beam-splitting directional optical coupler and the optical output coupling part 16 also forms a 50 / 50% beam-splitting directional optical coupler, each configured to split an optical signal input to any one input port thereof, into two equal parts output on a respective one of two output ports. The input and output directional couplers may each accurately be described by, and have the effect of, a respective unitary scattering matrix. Optimal parameters of the ring optical resonator 14 and modulator 28 in conjunction specifically with a Gaussian photon spectrum with a frequency FHWM 0.33 [ieV (= 2 ns temporally) and a single-photon detector (SNSPD) voltage output corresponding to a spectral shift A = 2 peV, are as follows: (1) Ring-waveguide coupling constant: k = 2.7 fieV (2) Ring quality factor: Q=300,000 (3) Detuning (21) of the ring resonance frequency of the ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum: No photon detection: A = 0 p.eV With photon detection: A = 2 [ieV Given that this is the amount of detuning provided, the parameters given by (1) and (2) above are found to be optimal. Figure 3(a) shows a transmission spectrum (32 to 48) from the input optical port 2 of the second waveguide part 10 to the optical output port 20 of the first optical pathway part 8 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the ring optical resonator 14 from coincidence with the centre frequency (34) of the optical wave packet frequency spectrum. Here, the refractive index modulation applied to the ring changes the detuning from A = 0 / zeF (transmission spectrum 32) to A = 2 (transmission spectrum 48). Figure 3(b) shows a transmission spectrum (36 to 54) from the input optical port 2 of the second waveguide part 10 to the optical output port 18 of the second optical pathway part as a function of increasing detuning (refractive index modulation) of the resonance frequency of the ring optical resonator 14 from coincidence with the centre frequency (34) of the optical wave packet frequency spectrum. Here, the refractive index modulation applied to the ring changes the detuning from A = 0 / zeF (transmission spectrum 36) to A = 2 / zeF (transmission spectrum 54). This arrangement works by accessing the maximum zr ring transmission phase change when in the “no photon detection” state (Fig. 1), and an as low a phase as possible (~ 0.2zr for the above parameters) when in the “photon detection” state (Fig. 2). The ring-waveguide coupling constant, k, which determines the transmission spectral width of the ring resonator, may be optimised to balance, on the one hand, encompassing as much as possible of the wave packet frequency spectrum when in the “no photon detection" state (Fig. 1), favouring larger k values to broaden the transmission spectral width, and, on the other hand, excluding as much as possible of the wave packet frequency spectrum in when in the “photon detection” state (Fig. 2), favouring smaller k values to make the transmission spectral width more narrow. A cumulative spectral shift of hAa> = 2 / zeF = 6.1 GHz (angular) is provided which corresponds to a modulation in wavelength of the spectral position of the resonance wavelength by: AX = 4pm at 1=1550 nm. In the present calculations, a Gaussian wave packet spectrum is assumed with a spectral width (FWHM) of 0.33peV or 2ns temporal width. Fig. 4 shows an optical phase 38 imposed upon an optical signal wave packet (40, 42) as relevant to the optical switch of Fig. 1 and Fig. 2 as a function of the detuning A (refractive index modulation) of the ring optical resonator of the optical switch, and a relative position of an optical frequency distribution of the Gaussian wave packet optical pulse 3 in each one of two switching states of the optical switch. In the “no photon detected” state (4 = 0 fieV; Fig. 1) the centre frequency of the optical frequency distribution of the Gaussian wave packet optical pulse 3 coincides with the resonance frequency of the ring optical resonator 14. Note that the y-axis on Fig. 4 is the transmission phase relative to the on-resonance transmission phase of n radians which is the phase imparted on the optical wave packet when the optical switch is in the first switch state. This means that the phase imparted in the first switch state is 0 + n = n, and the phase imparted in the second switch state is -0.8tt + n = 0.2tt, as indicated in Fig. 4. In the “photon detected” state (2! = 2 peV; Fig. 2) the centre frequency of the optical frequency distribution of the Gaussian wave packet optical pulse 3 is detuned from the resonance frequency of the ring optical resonator 14 to such an extent that there is almost no overlap between the optical frequency distribution of the Gaussian wave packet optical pulse when the ring resonator is in the “no photon detected” state (4 = 0 iieV; Fig. 1). Note that, when the optical switch is in the "photon detected state", the phase imparted onto the photon is substantially and significantly different to the phase imparted onto the photon when the optical switch is in the "no photon detected state". Fig. 5(a) shows a spectral power distribution 46 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 1. Fig. 5(b) shows the optical transmission spectrum 48 of the optical output port 18 of the second optical pathway 10 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 1), together with spectral power output distributions of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10 (see curve 50) and as output from the optical output port 20 of the first optical pathway 8 (see curve 52). The relative output power as between the optical output port 20 of the first optical pathway 8 and the optical output port 18 of the second optical pathway 10 is, about 12.04% and about 87.96% respectively. Fig. 5(c) shows an optical transmission spectrum 54 of the optical output port 20 of the first optical pathway 8 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 2), together with a spectral power output distributions of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8 (see curve 46) and as output from the optical output port 18 of the second optical pathway 10 (see curve 56). The relative output power as between the optical output port 20 of the first optical pathway 8 and the optical output port 18 of the second optical pathway 10 is, about 89.28%and about 10.72% respectively, with a switching fidelity of 0.7853, this number being the product of 87.96% (for the first state) and 89.28% (for the second state). Fig. 5(d) shows a graph of the photon coincidence rate 58 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10 and as output from the optical output port 20 of the first optical pathway 8 by the switch as between the switching states shown in Fig. 1 and of Fig. 2. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a non-zero relative time delay of about 0.75ns as between arrival of the wave packet at the output port 20 of the first optical pathway 8 relative to the arrival time at the output port 18 of the second optical pathway 10. This arises from the increased time delay incurred by the wave packet when traversing the optical resonator 14 in the first switch state (Fig. 1) as compared to the corresponding time delay when in the second switch state (Fig.2). This increased delay is such that the when the centre of the wave packet 17 crosses a notional position 21 at the output port 18 of the second optical pathway 8 (see Fig. 1) when routed that way, it will not yet have crossed an equivalent such notional position 21 at the output port 20 of the first optical pathway 8 (see Fig. 2) when routed the other way. An indistinguishability of 0.9046 is achieved between the wave packet with this relative delay between the two routing states, and an indistinguishability of 0.993 is achieved at the minimum of the HOM dip, calculated by notionally applying the same 0.75ns to the wave packet arrival time at the output port 18 of the second optical pathway 10 to achieve time-coincidence at the two output ports. Thus, the optical switch shown in Fig. 1 and Fig.2 is configured for receiving an optical signal from an optical signal source 30 and for outputting the optical signal, the optical switch having; an optical router configured for routing the optical signal conditional on the detection of a single photon 19b, the optical router comprising a Mach-Zehnder optical interferometer comprising: an input optical coupling part 6 for receiving the optical signal and splitting the received optical signal into a first optical signal portion 7 and a second optical signal portion 5; a first optical interferometer arm 8 and a second optical interferometer arm 10 for receiving, respectively, the first and second optical signal portions; an optical resonator 14 optically coupled to the first interferometer arm and configured to resonate at optical frequencies within a resonance bandwidth determined by a refractive index of the optical resonator; an output optical coupler 16 for receiving and recombining the first and second optical signal portions (11, 13) from the first and second optical interferometer arms; an optical modulator 28 coupled to a single-photon detector unit 24 configured to output a detection signal in response to the detection of a single photon 19b, the optical modulator being configured, in response to the detection signal, to modulate the refractive index of the optical resonator to change an optical phase shift incurred by the first optical signal portion by passage through the optical resonator by at least about 3tt / 4 radians (e.g., -0.8 x n, see Fig. 4) such that the recombining of the two signal portions by the output optical coupler 16 results in a routing of the output optical signal 17 to one of two optical output ports (18, 20) thereof selected conditional on the detection of a single photon. EXAMPLE 2 Fig. 6 and Fig. 7 each illustrate a variant of the optical switch according to the embodiment of the invention described herein with reference to Fig. 1 and Fig. 2. The variant comprises the provision of a controllable phase-shifter unit 60 upon the second waveguide 10 forming a second optical pathway part. The controllable phase-shifter unit 60 is configured to apply a preset phase shift of about n / 2 radians to the optical signal 5 travelling along the waveguide 10 of the second optical pathway part. The optical phase-shifter part may comprise a ring optical resonator, such as is shown in Fig. 11 and Fig. 12. Alternatively, the optical phase-shifter unit 60 may be operable to induce a pre-set refractive index change in the material of the second optical pathway part induced by effects including, but not limited to, thermo-optic, electro-optic, carrier-injection, piezo-electric, birefringent, micro-electro-mechanical, straininducing, or acousto-optic. Examples of means to achieve this effect are readily available to the person skilled in the art. The effects of incorporating the phase-shifter unit 60 are shown in Fig. 8, Fig. 9 and Fig. 10. Otherwise, the structure and operation of the optical switch is substantially the same is the switch according to the embodiment of the invention described herein with reference to Fig. 1 and Fig. 2. Optimal parameters of the ring optical resonator 14 and modulator 28 in conjunction specifically with a Gaussian photon spectrum with a frequency FHWM 0.33 [ieV (= 2 ns temporally) and a single-photon detector (SNSPD) voltage output corresponding to a spectral shift 4 = 2 fieV, are as follows: (1) Ring-waveguide coupling constant: k = 3.9 (2) Ring quality factor: ¢=200,000 (3) Detuning (4) of the ring resonance frequency of the ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum: No photon detection: 4 = -1 fieV With photon detection: 4 = +1 / j.eV Given that this is the amount of detuning provided, the parameters given by (1) and (2) above are found to be optimal. Adding a constant n / 2 phase on the second optical pathway part 10 causes the switching fidelity to be improved. This arrangement gives a different role of the ring optical resonator 14, as compared to ‘Example 1 ’ above, and that new role is to impart opposite optical phase shifts of +n / 2 radians or -n / 2 radians depending on photon detection, or non-detection. This is schematically illustrated in Fig. 8 which shows an optical phase 66 imposed upon an optical signal wave packet (62, 64) as transmitted by the optical switch of Fig. 6 and Fig. 7 as a function of the detuning 4 (refractive index modulation) of the ring optical resonator of the optical switch, and a relative position of an optical frequency distribution of the Gaussian wave packet optical pulse 3 in each one of two switching states of the optical switch. In the “no photon detected” state (4 = -1 / ieV; Fig. 6) the centre frequency of the optical frequency distribution of the Gaussian wave packet optical pulse 3 is detuned below the resonance frequency of the ring optical resonator 14. In the “photon detected” state (4 = +1 ^ieV; Fig. 7) the centre frequency of the optical frequency distribution of the Gaussian wave packet optical pulse 3 is detuned above the resonance frequency of the ring optical resonator 14. The detuning in the “no photon detected” state (4 = -1 Fig. 6) is such that there is almost no overlap between the optical frequency distribution of the Gaussian wave packet optical pulse when the ring resonator is in the “photon detected” state (4 = 1 neV; Fig. 7). Note that, when the optical switch is in the "photon detected state", the phase imparted onto the photon is substantially and significantly different to the phase imparted onto the photon when the optical switch is in the "no photon detected state". Achieving an optical phase shift modulation that changes from +tt / 2 radians or -77- / 2 radians is easier than achieving an optical phase modulation that changes from between tt radians and 0 radians, such as discussed herein with reference to Fig. 4, since a 0 (zero) radian phase shift is only achieved at a detuning relatively far from the resonance frequency of the ring optical resonator 14. In this way, the ring optical resonator is modulated to act as another (variant) photon-detection dependent optical signal phase modulator. Figure 9(a), upper sequence, shows a transmission spectrum (67a to 67d) from the input optical port 2 of the second waveguide part 10 to the optical output port 20 of the first optical pathway part 8 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the ring optical resonator 14 from coincidence with the centre frequency (34) of the optical wave packet frequency spectrum. Here, the refractive index modulation applied to the ring changes the detuning from 4 = 0 fieV (transmission spectrum 67a) to A = 2 [ieV (transmission spectrum 67d). Figure 9(b), middle sequence, shows a transmission spectrum (68a to 68d) from the input optical port 2 of the second waveguide part 10 to the optical output port 18 of the second optical pathway part as a function of increasing detuning (refractive index modulation) of the resonance frequency of the ring optical resonator 14 from coincidence with the centre frequency (34) of the optical wave packet frequency spectrum. Here, the refractive index modulation applied to the ring changes the detuning from A = 0 (transmission spectrum 68a) to A = 2 iieV (transmission spectrum 68d). Fig. 9(c) shows an optical frequency distribution 62 of the input Gaussian wave packet optical pulse 3 together with the relative spectral position of the transmission spectrum (68a, 68b, 68c, 68d) of a second optical output port of the optical switch of Fig. 6, and the transmission optical phase spectrum (66a, 66b, 66c, 66d) of the ring optical resonator thereof. Four graphs are shown with each one corresponding to one of four different (increasing) values of spectral detuning A (refractive index modulation) of the ring optical resonator of the optical switch, whereby A = -1 / zeF -> A = +l^eKin three steps of equal size, as the switch transitions from one switching state to another switching state. In the first switching state (leftmost graph; no photon detected) the centre of the optical frequency distribution 62 of the input Gaussian wave packet optical pulse 3 is coincident with the point on the optical phase spectrum curve 66a of the ring resonator corresponding to an optical phase shift of +zr / 2 radians, and is also coincident with a peak in the transmission optical spectrum 68a for output 17 from the second optical pathway 18. In the second switching state (rightmost graph; photon detected) the centre of the optical frequency distribution 62 (see 64 of Fig. 8) of the input Gaussian wave packet optical pulse 3 is coincident with the point on the optical phase curve 66d of the ring resonator corresponding to an optical phase shift of -zr / 2 radians, and is also coincident with a minimum (zero transmission) in the transmission optical phase spectrum 68d for output from the second optical pathway 18. As a result, the optical wave packet 17 is instead routed for output from the first optical pathway 20. The ring resonator 14 imparts a +tt / 2 or -zr / 2 phase depending on photon detection. The configuration gives an optical transmission spectrum (68a to 68d) with a closely-spaced maximum and minimum, separated by a spectral distance corresponding to the detuning change (here 2 / zeF). The optical cavity formed by the ring resonator has a spectral width now optimised to approximately match twice the available spectral shift achievable by modulating the refractive index of the ring resonator, such that the photon spectrum centres at the spectral positions corresponding to +7r / 2 or -zr / 2 points of the cavity transmission phase spectrum 66a and 66d. Note that: k = 3.9 [ieV = 2 x hAa) = 4 / zeF. Fig. 10(a) shows a spectral power distribution 62 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 6. Fig. 10(b) shows the optical transmission spectrum 68a of the optical output port 18 of the second optical pathway 10 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 6), together with a spectral power output distributions of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10 (see curve 68) and as output from the optical output port 20 of the first optical pathway 8 (see curve 69). The relative output power as between the optical output port 18 of the second optical pathway 10 and the optical output port 20 of the first optical pathway 8 is, about 97.88% and about 2.12% respectively. Fig. 10(c) shows an optical transmission spectrum 67d of the optical output port 20 of the first optical pathway 8 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 7), together with a spectral power output distributions of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10 (see curve 61) and as output from the optical output port 20 of the first optical pathway 8 (see curve 62). The relative output power as between the optical output port 18 of the second optical pathway 10 and the optical output port 20 of the first optical pathway 8 is, about 2.12% and about 97.88% respectively, with a switching fidelity of 0.9581. Fig. 10(d) shows a graph of the photon coincidence rate 74 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10 and as output from the optical output port 20 of the first optical pathway 8 by the switch as between the switching states shown in Fig. 6 and of Fig. 7. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a relative time delay of about 0 ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway 8 relative to the arrival time at the output port 18 of the second optical pathway 10. A distinguishability of 0.997 is achieved between the wave packet with this relative delay between the two routing states. EXAMPLE 3 Fig. 11 and Fig. 12 each illustrate a variant of the optical switch according to the embodiment of the invention described herein with reference to Fig. 6 and Fig. 7. The variant comprises the provision of a phase-shifter unit in the form of a second ring resonator 34 optically coupled at a coupling region 32 to the second waveguide 10 forming a second optical pathway part. The second ring resonator 34 is identical to the ring resonator 14 upon the first waveguide 8 forming a first optical pathway part. This second ring resonator 34 is configured to apply a pre-set phase shift of about tt / 2 radians to the optical signal 5 travelling along the waveguide 10 of the second optical pathway part. Otherwise, the structure and operation of the optical switch is substantially the same is the switch according to the embodiment of the invention described herein with reference to Fig. 1 and Fig. 2, or Fig. 6 and Fig. 7. Optimal parameters of the ring optical resonator 14 and modulator 28, for an operating optical wavelength of 1550 nm, are as follows: (1) Ring-waveguide coupling constant: k = 3.9 / ieV (2) Ring quality factor: <2=200,000 (3) Detuning (2!) of the ring resonance frequency of the ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum: No photon detection: A = -1 / zeV With photon detection: A = +1 / zeV (4) Second ring resonator coupling constant: k = 3.9 / zeF (5) Detuning (4) of the ring resonance frequency of the second ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum fixed at: A = 1 / ieV. Adding an identical (unmodulated) ring to the second optical pathway 10 acts to provide the zr / 2 phase shift in ‘Example 2’, but is better because it removes any relative delay between the first and second optical pathway parts and because the phase imparted across the photon spectrum in the two pathway parts is equal. In addition, the spectral widths of the transmission spectra of the ring optical resonators on the first and second optical pathway parts are (as in ‘Example 2’) optimised by being approximately equal to twice the available spectral shift applied to the modulated ring optical resonator. Figure 13 shows two spectra of the optical phase shift imposed upon the frequency components of a photon wave packet optical signal as transmitted by each of the two ring optical resonators of the optical switch of Fig. 11. The spectrum is plotted as a function of “photon frequency” which refers to a value of the frequency measured relative to a reference frequency having a value corresponding to the mid-point of the frequency spectrum 62 of the input optical wave packet shown in Fig. 14. Thus, a “photon frequency” of “0” on the x-axis of the graph of Fig. 13 denotes an optical frequency matching the midpoint of the frequency spectrum 62 of the input optical wave packet. Note that the wave packet 62 comprises a weighted distribution of frequency components itself (e.g., a Gaussian function of frequency). This means that different frequency components of a wave packet may experience different optical phase shifts imposed upon them by passage through a given ring resonator, and a relative difference in imposed optical phase between the two ring resonators in one of two switching states of the optical switch. The optical phase shift spectrum 76 of the second ring resonator 34 is identical in shape to the optical phase shift spectrum 78 of the ring resonator 14 upon the first waveguide 8. However, whereas the spectral position of the optical phase shift spectrum 76 of the second ring resonator 34 is static, the spectral position of the optical phase shift spectrum 86 of the ring resonator 14 upon the first waveguide 8 is variable according to the modulation of the refractive index of that ring resonator as applied by the modulator 28. In a first switching state (Fig. 11) in which “no photon” is detected by the single-photon detector 24, the refractive index of the ring resonator 14 upon the first waveguide 8 is such that the resonance frequency of that ring resonator is detuned from the mid-point of the frequency spectrum 62 of the input optical wave packet by about: A = -1 / zeF while the resonance frequency of the second ring resonator 34 remains detuned from the mid-point of that spectrum by about: A = +1 [ieV. In this state, the difference 80 between the optical phase shift spectra of the two ring resonators is suitably close in value to n radians across a suitable broad spectral range centred upon the mid-point of the frequency spectrum 62 of the input optical wave packet. This means that the mid-point of the frequency spectrum of the input optical wave packet experiences a relative phase shift of n radians (i.e., tt / 2 - (— tt / 2) = rr), as between the two ring resonators, and to a good approximation (or ‘good enough’ for useful practical purposes) so too does the majority of the spectrum of the optical wave packet. The result is that the necessary optical interferences take place in the optical switch so as to appropriately route the optical wave packet, as shown in Fig. 11. By contrast, in a second switching state (Fig. 12) in which a photon is detected by the single-photon detector 24, the refractive index of the ring resonator 14 upon the first waveguide 8 is modulated such that the resonance frequency of that ring resonator is detuned from the mid-point of the frequency spectrum 62 of the input optical wave packet by about: A = +1 while the resonance frequency of second ring resonator 34 remains detuned from that mid-point by about: A = +1 ueV. In this state, the optical phase shift spectra of the two ring resonators coincide such that the difference between them is suitably close in value to 0 (zero) radians across their respective spectral ranges. This means that the mid-point of the frequency spectrum of the input optical wave packet experiences substantially no difference in phase shifts as between the two ring resonators (i.e., each phase shift being -rr / 2), and similarly for the rest of the spectrum of the wave packet. The result is that the necessary optical interferences take place in the optical switch so as to appropriately route the optical wave packet, as shown in Fig. 12. Fig. 14 shows an optical frequency distribution 62 of the input Gaussian wave packet optical pulse 3 together with the relative spectral position of the transmitted power spectrum (82a, 82b, 82c, 82d) of the optical output port 18 of the second optical pathway part 10 of the optical switch of Fig. 11, and the transmission optical phase spectra (78, 76) of the two ring optical resonators (14, 34) thereof. Four graphs are shown with each one corresponding to one of four different (increasing) values of spectral detuning A (refractive index modulation) of the ring optical resonator upon the first optical pathway part 8 of the optical switch, whereby A = -l^eiz -> A = +I^e7 in three steps of equal size, as the switch transitions from one switching state to another switching state. In the first switching state (leftmost graph; no photon detected) the centre of the optical frequency distribution 62 of the input Gaussian wave packet optical pulse 3 is coincident with the point on the optical phase spectrum curve 78 of the ring resonator upon the first optical pathway part, corresponding to an optical phase shift of +tt / 2 radians. It is also coincident with the point on the optical phase spectrum curve 76 of the second ring resonator corresponding to an optical phase shift of -tt / 2 radians. As a result, the optical wave packet 17 is instead routed for output from the second optical pathway 18. In the second switching state (rightmost graph; photon detected) the centre of the optical frequency distribution 62 of the input Gaussian wave packet optical pulse 3 is coincident with the point on the optical phase curves (76, 78) of both ring resonators, corresponding to an optical phase shift of -tt / 2 radians. As a result, the optical wave packet 17 is instead routed for output from the first optical pathway 20. Figure 15(a) shows a transmission spectrum (82a, 82b, 82c, 82d) of the optical output port 20 of the first optical pathway part 8 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the ring optical resonator 14 as the optical switch transitions from the first switch state (82a; Fig. 11) to the second switch state (82d; Fig. 12). Figure 15(b) shows a transmission spectrum (84a, 84b, 84c, 84d) of the optical output port 18 of the second optical pathway part 10 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the ring optical resonator 14 as the optical switch transitions from the first switch state (84a; Fig. 11) to the second switch state (84d; Fig. 12). Fig. 16(c) shows a spectral power distribution 85 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 11. Fig. 16(a) shows the optical transmission spectrum 82d of the optical output port 18 of the second optical pathway 10 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 11), together with a spectral power output distribution 81 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10. Fig. 16(d) shows an optical transmission spectrum 84a of the optical output port 20 of the first optical pathway 8 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 12), together with a spectral power output distribution 88 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8. The relative output power as between the optical output port 18 of the second optical pathway 10 and the optical output port 20 of the first optical pathway 8 is, about 0.32% and about 99.68% respectively. A switching fidelity of 0.9966 Is obtained. Fig. 16(b) shows a graph of the photon coincidence rate 86 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10 and as output from the optical output port 20 of the first optical pathway 8 by the switch as between the switching states shown in Fig. 11 and of Fig. 12. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a relative time delay of about 0ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway 8 relative to the arrival time at the output port 18 of the second optical pathway 10. An indistinguishability of 0.997 is achieved between the wave packet with this relative delay between the two routing states. In another example, parameters of the ring optical resonator 14 and modulator 28, for an operating optical wavelength of 1550 nm and assuming the wave packet (photon) spectrum is Gaussian with FWHM spectral width of 1 peV or 0.7ns temporal width (similar to a quantum dot photon), were as follows: (1) Ring-waveguide coupling constant: k = 4.0 jieV (2) Ring quality factor: Q = 200,000 (3) Detuning 0) of the ring resonance frequency of the ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum: No photon detection: A = -1 fieV With photon detection: zl = +1 fieV (4) Second ring resonator coupling constant: k = 4.0 [ieV With these parameters, the following switch characteristics were found to arise. Fig. 17(a) shows a spectral power distribution 91 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 11. Fig. 17(c) shows the optical transmission spectrum 65 of the optical output port 18 of the second optical pathway 10 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 11), together with a spectral power output distribution 93 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10. Fig. 17(b) shows an optical transmission spectrum 83a of the optical output port 20 of the first optical pathway 8 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 12), together with a spectral power output distribution 90 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8, and a spectral power output distribution 92 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10. The relative output power as between the optical output port 18 of the second optical pathway 10 and the optical output port 20 of the first optical pathway 8 is, about 13.49% and about 86.51% respectively, with a switching fidelity of 0.8651. Fig. 17(d) shows a graph of the photon coincidence rate 94 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10 and as output from the optical output port 20 of the first optical pathway 8 by the switch as between the switching states shown in Fig. 11 and of Fig. 12. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a relative time delay of about 0ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway 8 relative to the arrival time at the output port 18 of the second optical pathway 10. A distinguishability of 0.9066 is achieved between the wave packet with this relative delay between the two routing states. Thus, good figures of merit may be achieved with spectrally broader photons. Here, as noted above, the wave packet (photon) spectrum is Gaussian with FWHM spectral width of 1 peV or 0.7ns temporal width (similar to a quantum dot photon). Note that the optimal parameters for the two ring resonators do not depend on input photon spectral width, and are instead determined by the spectral shift / modulation available and applicable to the modulated ring resonator. Phase Considerations Consideration of the phases incurred demonstrates the role played by the modulated ring optical resonator in the optical swich configurations of Fig. 6 and Fig. 7 and of Fig. 11 and Fig. 12. Here, the modulated ring resonator acts as a phase shifter switchably applying a phase shift of either +n / 2 radians or -n / 2 radians to light traversing the modulated ring, in either switch state. Light couples equally (in magnitude) into ring resonator in both switch states. The effect of the optical circuit of the optical swich, in these examples, is schematically illustrated in Fig. 18. The effects shown are purely in terms of changes it makes to the optical phase of input optical signals, E± and E2, upon each of the two input ports (2, 4) of the switch, as apparent in the resulting output optical signals, Er and E2, upon each of the two output ports (18, 20) of the switch, may be approximately modelled by the following matrix equation: ©=- © „ _ 1 rl h . p - fexP [—0 A V2M 1' K 0 exp [—10O] / Here, i = V-l, and U is a unitary scattering matrix representing the action of the input coupler 6, and the action of the output coupler 16. The matrix R represents the action of the modulating optical ring resonator in adding an optical phase <f)R to an optical signal transmitted by it, and also represents the action of any phase-shifting unit (34, 60) upon the second pathway part in adding an optical phase <p0 to an optical signal transmitted by it. This gives: = VexP + ^¾) + iexp [-1001(¾ + iEi)\ \E2J 2 \exp [-1001(¾ + 1¾) + iexp [-i0«] (¾ + if2) / Applying this to the optical switch of Fig. 6 or Fig. 11, let: E1 = 0, < / >R = -tt / 2,0O = + tt / 2. This gives: (ea (eA - If + (0(-0¾ w M \Ej \eJ 2 \(—0¾ + (0(0(0¾)^ Alternatively, applying this to the optical switch of Fig. 7 or Fig. 12, let: Er = 0, = +n / 2,0O = + 77: / 2. This gives: (EA (EA = 1 / (-1)(1)¾ + (1)(-1)¾ \ (iE2\ \Ej 2 ((-1)¾ + (1)(-1)(1)¾)^ 10 / Thus, the action of the input coupling part 6 is to add a + n / 2 phase shift (i.e., multiply by exp [i tt / 2] = i) to the optical signal passed by it to the first optical pathway part 8, relative to the phase of the optical signal passed by it to the second optical pathway part 10. The action of the ring optical resonator is either to add a +n / 2 phase shift (i.e., multiply by exp[in / 2] = i), or to add a -tt / 2 phase shift (i.e., multiply by exp[-i7T / 2] = -i), to the optical signal passed by it to the output coupling part 16, the choice being conditional on detection of a photon by the single-photon detector 24 causing modulation of the refractive index of the ring 14 by the modulator 28. The action of the phase shifter unit (60, 34) is to add a + tt / 2 phase shift (i.e., multiply by exp[i ?r / 2] = i) to the optical signal passed by it to the output coupling part 16. The action of the output coupling part 16 is to add a +77- / 2 phase shift (i.e., multiply by exp[i7r / 2] = i) to the optical signal passed by it to the output port 20 of the first optical pathway part 8, relative to the phase of the optical signal passed by it to the output port 18 of the second optical pathway part 10. The appropriate choice of phase shift applied by the optical ring resonator 14 thereby controls the routing of the optical signal. Similarly, applying this to the optical switch of Fig. 2, let: = 0, 0R = tt, 0o = 0. This gives: m (ea=y + (0(1)¾ v / o \ E2) \eJ 2 \(1)E2 + (1)(-1)(1)¾)^ W Whereas, applying this to the optical switch of Fig. 1, let: = 0, <pR = 2tt, 0o = 0. Noting that $0 = 0 corresponds to the absence of the phase shifter unit (34, 60), and that <pR = 2n is equivalent to = 0, this gives: (1)(0¾ + (0(1)¾ \ / 1¾\ (i)s2+(0(1)(0¾)^ Uv EXAMPLE 4 Figure 19 shows a schematic diagram of an interferometer providing an optical switch in the first switch state in which no photon, 19a, is detected by the single-photon detector 24 of the optical switch. Figure 20 shows a schematic diagram of the optical switch of Fig. 19 in second switch state in which a photon, 19b, is detected by the single-photon detector of the optical switch. In this example of this invention, there is provided an optical switch for receiving an optical signal 3 from an optical signal source 30 and for outputting the optical signal 17. The optical switch comprises an optical router configured for routing the optical signal conditional on the detection of a single photon, 19b. The optical router comprises, a first ring optical resonator 104 formed from an optical waveguide ring, a first optical pathway part 106 comprising a linear optical waveguide and also comprising the first ring optical resonator 104, and a second optical pathway part 109. The optical switch comprises an optical input coupling part 102 is in the form of a second ring optical resonator for receiving the optical signal and for coupling the received optical signal 7 into the first optical pathway part 106 and for coupling the received optical signal 5 into the second optical pathway part 109. The optical switch comprises a distributed optical output coupling part (114, 110) for receiving the optical signal 11 via the first optical pathway part 106 and for receiving the optical signal 5 via the second optical pathway part 109. The optical signal (13, 15) as received via the first optical pathway part 106 optically interferes with the optical signal 15 as received via the second optical pathway part 109. The result of this optical interference is output, by the optical switch, to the first optical pathway part 106 (at output port 20) and to the second optical pathway part 109 (at output port 18). The optical switch comprises an optical modulator 28b coupled to a single-photon detector unit 24 configured to output a detection signal 100b in response to the detection of a single photon (19b, Fig. 20). The optical modulator 28b is configured, in response to the detection signal 100b, to modulate the refractive index of the first ring optical resonator 104 to change an optical phase shift incurred by the optical signal 13 upon passage through the first ring optical resonator such that the optical interference results in a routing of said output optical signal to one of the first and second optical pathway parts (at output port 18 or output port 20) selected conditional on the detection of a single photon 19b. It is to be noted, however, that for some examples, the optical modulator may comprise two separate modulator parts (28a, 28b) comprising the optical modulator 28b described above, serving as a first modulator part, and further comprising a second optical modulator 28a serving as a second modulator part. The second modulator 28a may be coupled to the single-photon detector unit 24 which may be further configured to output a second detection signal 100a, as well as a first detection signal 100b, in response to the detection of a single photon (19b, Fig. 20). The second optical modulator 28a may be configured, in response to the second detection signal 100a, to modulate the refractive index of the optical resonator forming the optical input coupling part 102 to change an optical phase shift incurred by the optical signal 7 upon passage through that optical resonator. This is shown in Fig. 19 and Fig. 20 for completeness and clarity. However, in some examples, the second optical modulator 28a may be omitted entirely, or controlled such that it does not respond to the second detection signal 100a and does not modulate the refractive index of the second ring resonator 102 forming the optical input coupling part 102, with the result that, in either case, an optical modulation is only applied to the ring resonator 104 in response to the first detection signal 100b. The distributed optical output coupling part (114,110) optically couples the first ring resonator 104 evanescently to the optical waveguide forming the first optical pathway part 106 at a first coupling region 114, and optically couples the first ring resonator 104 evanescently to the optical waveguide forming the second optical pathway part 109 at a second coupling region 110. Similarly, a further distributed optical output coupling part (112, 108) optically couples the optical input coupling part (second ring resonator) 102 evanescently to the optical waveguide forming the first optical pathway part 106 at a third coupling region 112, and optically couples the optical input coupling part (second ring resonator) 102 evanescently to the optical waveguide forming the second optical pathway part 109 at a fourth coupling region 108. In the following discussion, with reference to Fig. 21, Fig. 22 and Fig. 23, the results in question were obtained with the second optical modulator 28a omitted entirely, or is controlled such that it does not respond to the second detection signal 100a and does not modulate the refractive index of the second ring resonator forming the optical input coupling part 102 (i.e., it remains in the “OFF” state in Fig. 20). In an example, parameters of the ring optical resonators 102, 104 and modulator 28b, for an operating optical wavelength of 1550 nm, were as follows: (1) Modulated ring-waveguide coupling constant (110, 114): k1 = 1.9 geK (2) Modulated ring 104 quality factor: Q = 200,000 (3) Detuning 01) of the ring resonance frequency of the ring resonator 104 from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: Al = -1 / ieV With photon detection: Al = +1 / ieV (4) Second (non-modulated) ring resonator 102: coupling constant (108, 112): k1 = 50.0 / / eV (5) Second (non-modulated) ring resonator 102: detuning 0) of the ring resonance frequency of the ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum: 2)2 = -26 ^.eV (fixed). With these parameters, the following switch characteristics were found to arise. Figure 21 (a) shows a transmission spectrum (116a, 116b, 116c, 116d) of the optical output port 20 of the first optical pathway part 106 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the first ring optical resonator 104 as the optical switch transitions from the first switch state (116a; Fig. 19) to the second switch state (116d; Fig. 20). Figure 21(b) shows a transmission spectrum (118a, 118b, 118c, 118d) of the optical output port 18 of the second optical pathway part 109 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the ring first optical resonator 104 as the optical switch transitions from the first switch state (118a; Fig. 19) to the second switch state (118d; Fig. 20). Fig. 22(a) shows a spectral power distribution 120 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 19. Fig. 22(b) shows the optical transmission spectrum 124 of the optical output port 20 of the first optical pathway 106 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 19), together with a spectral power output distribution 122 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 106, and a spectral power output distribution 126 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 109. The relative output power as between the optical output port 20 of the first optical pathway 106 and the optical output port 18 of the second optical pathway 109 is, about 97.51% and about 2.49% respectively. Fig. 22(c) shows an optical transmission spectrum 132 of the optical output port 18 of the second optical pathway 109 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 20), together with a spectral power output distribution 128 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 109. The relative output power as between the optical output port 18 of the second optical pathway and the optical output port 20 of the first optical pathway is, about 97.93% and about 2.07% respectively, with a switching fidelity of 0.9549. Fig. 22(d) shows a graph of the photon coincidence rate 134 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway and as output from the optical output port 20 of the first optical pathway by the switch as between the switching states shown in Fig. 19 and of Fig. 20. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a relative time delay of about 0ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway relative to the arrival time at the output port 18 of the second optical pathway. An indistinguishability of 0,9968 is achieved between the wave packet with this relative delay between the two routing states. Fig. 23 shows an optical frequency distribution 120 of the input Gaussian wave packet optical pulse 3 together with the relative spectral position of the transmitted power spectrum (124a, 124b, 124c, 124d) of the optical output port 20 of the first optical pathway part of the optical switch of Fig. 19, and the transmission spectra (136 for ring 102; 138a, 138b, 138c, 138d for ring 104) of the two ring optical resonators (102, 104) thereof. Four graphs are shown with each one corresponding to one of four different (increasing) values of spectral detuning Al (refractive index modulation) of the first (modulated) ring optical resonator 104 upon the first optical pathway part 8 of the optical switch, whereby 41 = -l^teV -> Al = +l(ieV in three steps of equal size, as the switch transitions from one switching state to another switching state. The second ring 102 has a broad spectrum far detuned from resonance so as to impart an approximately constant transmission and optical phase shift across the bandwidth of the wave packet. It acts as a beam splitter with the necessary +n / 2 phase. EXAMPLE 5 Figure 24 shows a schematic diagram of an interferometer providing an optical switch in first switch state in which no photon, 19a, is detected by the single-photon detector 24 of the optical switch. Figure 25 shows a schematic diagram of the optical switch of Fig. 24 in second switch state in which a photon, 19b, is detected by the single-photon detector of the optical switch. In this example of this invention, there is provided an optical switch for receiving an optical signal 3 from an optical signal source 30 and for outputting the optical signal 17. The optical switch comprises an optical router configured for routing the optical signal conditional on the detection of a single photon, 19b. The optical router comprises, a first ring optical resonator 144 formed from an optical waveguide ring, a first optical pathway part 106 comprising a linear optical waveguide and also comprising the first ring optical resonator 144, and a second optical pathway part 109. The optical switch comprises a distributed optical input coupling part (140, 143) which is in the form of a second ring optical resonator 142 for receiving the optical signal 3 and for coupling the received optical signal 7, via a coupling region 143, into the first optical pathway part (106, 144) of which the first ring optical resonator 144 forms a part, and for coupling the received optical signal (15, 7) into the second optical pathway part 109 (of which the second ring optical resonator 142 forms a part) via a coupling region 140. The optical switch comprises a distributed optical output coupling part (145, 143, 140) for receiving the optical signal 15 via the first optical pathway part (144), of which the first ring optical resonator 144 forms a part, and for receiving the optical signal 7 via the second optical pathway part 142. The optical signal 15 as received via the first optical pathway part 144 optically interferes with the optical signal 7 as received via the second optical pathway part 142. The result of this optical interference is output, by the optical switch, to the first optical pathway part 106 (at output port 20) and to the second optical pathway part 109 (at output port 18). The optical switch comprises an optical modulator 28b coupled to a single-photon detector unit 24 configured to output a detection signal 100b in response to the detection of a single photon (19b, Fig. 25). The optical modulator 28b is configured, in response to the detection signal 100b, to modulate the refractive index of the first ring optical resonator 144 to change an optical phase shift incurred by the optical signal 15 upon passage through the first ring optical resonator such that the optical interference results in a routing of said output optical signal to one of the first and second optical pathway parts (at output port 18 or output port 20) selected conditional on the detection of a single photon 19b. The optical modulator comprises two separate modulator parts (28a, 28b) comprising the optical modulator 28b described above, serving as a first modulator part, and further comprising a second optical modulator 28a serving as a second modulator part. The second modulator 28a is coupled to the singlephoton detector unit 24 which is further configured to output a second detection signal 100a, as well as a first detection signal 100b, in response to the detection of a single photon (19b, Fig. 25). The second optical modulator 28a is configured, in response to the second detection signal 100a, to modulate the refractive index of the second optical ring resonator 142 to change an optical phase shift incurred by the optical signal 7 upon passage through that second optical ring resonator. The distributed optical output coupling part (143, 140) optically couples the first ring resonator 144 evanescently to the optical waveguide forming the second ring optical resonator at a first coupling region 143, and optically couples the second ring resonator 142 evanescently to the optical waveguide forming the second optical pathway part 109 at a second coupling region 140. Similarly, a third coupling region 145 optically couples the first ring resonator 144 evanescently to the optical waveguide forming the first optical pathway part 106. In an example, parameters of the ring optical resonators 102, 104 and modulator 28b, for an operating optical wavelength of 1550 nm, were as follows: (1) First ring-waveguide coupling constant (145): kI = 1.15 geV (2) Second ring-waveguide coupling constant (140): k2 = 1.15 geV (3) First-to-second ring coupling constant (143): g = 0.35 ueV (4) First and second ring (142, 144) quality factor (each ring): Q = 200,000 (5) Detuning 01) of the resonance frequency of the first ring resonator 144 from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 211 = 0 geV With photon detection: Al = +1 geV (6) Detuning 02) of the resonance frequency of the second ring resonator 142 from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 212 = 0 geV With photon detection: 212 = +1 geV With these parameters, the following switch characteristics were found to arise. Figure 26(a) shows a transmission spectrum (146a, 146b, 146c, 146d) of the optical output port 20 of the first optical pathway part 106 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the first and second ring optical resonators (144,142) in concert as the optical switch transitions from the first switch state (146a; Fig. 24) to the second switch state (146d; Fig. 25). Figure 26(b) shows a transmission spectrum (148a, 148b, 148c, 148d) of the optical output port 18 of the second optical pathway part 109 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the first and second ring optical resonators (144, 142) in concert as the optical switch transitions from the first switch state (148a; Fig. 24) to the second switch state (148d; Fig. 25). Fig. 27(a) shows a spectral power distribution 150 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 24. Fig. 27(b) shows the optical transmission spectrum 154 of the optical output port 20 of the first optical pathway 106 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 24), together with a spectral power output distribution 152 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 106, and a spectral power output distribution 156 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 109. The relative output power as between the optical output port 18 of the second optical pathway 109 and the optical output port 20 of the first optical pathway 106 is, about 11.47% and about 88.53% respectively. Fig. 27(c) shows an optical transmission spectrum 160 of the optical output port 18 of the second optical pathway 109 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 25), together with a spectral power output distribution 158 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 109, and a spectral power output distribution 162 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 106. The relative output power as between the optical output port 18 of the second optical pathway and the optical output port 20 of the first optical pathway is, about 90.46% and about 9.54% respectively, with a switching fidelity of 0.8008. Fig. 27(d) shows a graph of the photon coincidence rate 164 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway and as output from the optical output port 20 of the first optical pathway by the switch as between the switching states shown in Fig. 24 and of Fig. 25. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a relative time delay of about 0ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway relative to the arrival time at the output port 18 of the second optical pathway. An indistinguishability of 0.9796 is achieved between the wave packet with this relative delay between the two routing states. In another example, parameters of the ring optical resonators 102, 104 and modulator 28b, for an operating optical wavelength of 1550 nm, were as follows: (1) First ring-waveguide coupling constant (145): «1 = 1.45 geV (2) Second ring-waveguide coupling constant (140): k2 = 1.45 geV (3) First-to-second ring coupling constant (143): g = 0.43 geV (4) First and second ring (142, 144) quality factor (each ring): Q = 200,000 (5) Detuning (211) of the resonance frequency of the first ring resonator 144 from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 21 = 0 geV With photon detection: Al = +1 peV (6) Detuning (212) of the resonance frequency of the second ring resonator 142 from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 42 = 0 / ieV With photon detection: 42 = -1 ^eV With these parameters, the following switch characteristics were found to arise. Figure 28(a) shows a transmission spectrum (166a, 166b, 166c, 166d) of the optical output port 20 of the first optical pathway part 106 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the first and second ring optical resonators (144, 142) in concert as the optical switch transitions from the first switch state (166a; Fig. 24) to the second switch state (166d; Fig. 25). Figure 28(b) shows a transmission spectrum (168a, 168b, 168c, 168d) of the optical output port 18 of the second optical pathway part 109 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the first and second ring optical resonators (144, 142) in concert as the optical switch transitions from the first switch state (168a; Fig. 24) to the second switch state (168d; Fig. 25). Fig. 29(a) shows a spectral power distribution 170 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 24. Fig. 29(b) shows the optical transmission spectrum 174 of the optical output port 20 of the first optical pathway 106 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 24), together with a spectral power output distribution 172 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 106, and a spectral power output distribution 176 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 109. The relative output power as between the optical output port 18 of the second optical pathway 109 and the optical output port 20 of the first optical pathway 106 is, about 6.68% and about 93.32%respectively. Fig. 29(c) shows an optical transmission spectrum 180 of the optical output port 18 of the second optical pathway 109 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 25), together with a spectral power output distribution 178 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 109, and a spectral power output distribution 182 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 106. The relative output power as between the optical output port 18 of the second optical pathway and the optical output port 20 of the first optical pathway is, about 93.76% and about 6.24% respectively, with a switching fidelity of 0.875. Fig. 29(d) shows a graph of the photon coincidence rate 184 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway and as output from the optical output port 20 of the first optical pathway by the switch as between the switching states shown in Fig. 24 and of Fig. 25. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a relative time delay of about 0ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway relative to the arrival time at the output port 18 of the second optical pathway. An indistinguishability of 0.9892 is achieved between the wave packet with this relative delay between the two routing states. Fig. 30 shows an optical frequency distribution 170 of the input Gaussian wave packet optical pulse 3 together with the relative spectral position of the transmitted power spectrum (186a, 186b, 186c, 186d) of the optical output port 20 of the first optical pathway part of the optical switch of Fig. 24, and the transmission spectra (190a, 190b, 190c, 190dfor ring 142; 188a, 188b, 188c, 188d for ring 144) of the two ring optical resonators (142, 144) thereof. Four graphs are shown with each one corresponding to one of four different (Increasing) values of spectral detuning, 211,212, (refractive index modulation) of the first and second ring optical resonators 144 upon the first and second optical pathway parts of the optical switch, whereby Al = 212 = OgeV -> 211 = -IgeV ; A2 = +l / zeiz in three steps of equal size, as the switch transitions from one switching state to another switching state. The transmitted power spectrum |T21|2 (see: 186a, 186b, 186c, 186d) of the optical switch as seen at the optical output port 20 of the first optical pathway part has the form of two overlapping and mutually interfering component Lorentzian transmission functions, a respective one for each of the two rings, expressed as follows: J j ..... ~;------------------------------------------------~ -• A -- 4( / ) -■ -■ A + 4g).....^) Note that: (at - |2)1| - 4g) = (at - {|2ll| + 4g}) and (w + |2l2| - 4g) = (at - {|zl21 - 4g}). Thus, applying ring detunings of equal magnitude, |2ll| = \A2\ = A, but opposite sign, means that varying the modulation (detuning A) has the effect of separating (or bringing together) the two respective component Lorentzian transmission profiles as shown, to achieve the desired optical routing. This example illustrates an example of the optical switch for receiving an optical signal from an optical signal source and for outputting the optical signal, the optical switch comprising an optical router configured for routing the optical signal conditional on the detection of a single photon. The optical router comprises: an optical resonator 144; a first optical pathway part comprising the optical resonator, and a second optical pathway part comprising a further (second) optical resonator 142 wherein the optical resonator and the further optical resonator are optically coupled to form a pair of optically coupled optical resonators. An optical input coupling part (140, 143) is provided for receiving the optical signal and for coupling the received optical signal into the first optical pathway part (144, 106) and into the second optical pathway part (109, 142). An optical modulator (28a, 28b) is coupled to the single-photon detector unit configured to output the detection signal (100a, 100b) in response to said detection of a single photon, the optical modulator being configured, in response to the detection signal, to modulate the refractive index of the optical resonator and the further optical resonator to change an optical phase shift incurred by the optical signal upon passage through the optical resonator and the further optical resonator such that an optical interference results in a routing of the output optical signal to one of the first and second optical pathway parts (106, 109) selected conditional on the detection of a single photon. An optical coupling 143 between the optical resonator and the further optical resonator may serve the purpose of the optical input coupling part. The optical switch comprises an optical output coupling part for receiving the optical signal via the first optical pathway part and for receiving the optical signal via the second optical pathway part such that the optical signal as received via the first optical pathway part optically interferes with the optical signal as received via the second optical pathway part, and for outputting the result of the optical interference to the first optical pathway part and to the second optical pathway part. An optical coupling 143 between the optical resonator and the further optical resonator serves the purpose of the optical output coupling part. This is because, due to the ability of the two coupled ring resonators to circulate the optical signal repeatedly through the coupling region, the optical coupling in question is able to input the optical signal to either one of the two optical resonators it couples, and to is also able to output the optical signal from either one of the two optical resonators it couples. The same analysis applies to the optical switch disclosed herein under “Example 8” with reference to Fig. 39, Fig. 40, Fig. 41 and Fig. 42. The optical resonator 144 and the further optical resonator 142 may each be formed from a respective pair of optically coupled optical resonators, such as is disclosed herein under “Example 7” with reference to Fig. 36, Fig. 37 and Fig. 38. The optical signal may comprise a given optical signal frequency bandwidth, and the (or each, if more than one) optical resonator may be configured to resonate at a respective resonant optical frequency within a resonance bandwidth determined by a refractive index of the optical resonator. The optical modulator may be configured to modulate the refractive index of the optical resonator and the further optical resonator so as to modulate the respective resonance bandwidths thereof selectively and individually to either: change from being a resonance bandwidth centred at an optical frequency lower than a centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than or lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that excludes the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that excludes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth. EXAMPLE 6 Figure 31 shows a schematic diagram of an interferometer providing an optical switch in first switch state in which no photon, 19a, is detected by the single-photon detector 24 of the optical switch. Figure 32 shows a schematic diagram of the optical switch of Fig. 31 in second switch state in which a photon, 19b, is detected by the single-photon detector of the optical switch. In this example, the optical switch is configured for receiving an optical signal 3 from an optical signal source 30 as a pulse of laser light (or a photon). The optical switch is arranged for routing the input signal by passing it through an optical router of the switch configured for routing the optical signal conditional on the detection of a single photon. Figure 31 shows a schematic diagram of an optical switch comprising a Mach-Zehnder interferometer in first one of two switching states. Figure 32 shows a schematic diagram of the optical switch of Fig. 31 in second one of two switching states. Fig. 31 and Fig. 32 each illustrate the optical switch according to an embodiment of the invention employing an optical interferometer including a first waveguide 8 forming a first optical pathway part comprising an optical ring resonator (14a, 14b), and a second waveguide 10 forming a second optical pathway part. The ring resonator is controllable for routing an input optical signal 3 (e.g., an optical pulse) received into an input optical port 2 of the second waveguide part 10 from the optical signal source (e.g., a laser unit) in optical communication with the optical switch, for onward propagation along the second waveguide part 10 for input to the optical interferometer. The switching state (routing) is put into effect by the action of the ring resonator, as described herein, conditional on the detection of a single photon (19a / 19b). The optical switch provides a single-photon triggered optical signal switch / router. In particular, the optical router comprises an optical input coupling part in the form of an input directional coupler 6 formed by an evanescent optical coupling of respective adjacent sections of the first waveguide 8 and the second waveguide 10 being in suitably close physical proximity. The coupling constants of the input directional coupler 6 are such that the directional coupler redistributes substantially 50% of the input signal 7 from the second waveguide 10 into the first waveguide 8 with substantially 50% of the input signal 5 remaining in the second waveguide 10. As a result, a substantially equal redistribution of the initial optical signal 3 is shared amongst the first optical pathway part and the second optical pathway part. The optical input coupling part 6 is thereby configured for receiving the optical signal 3 and for coupling the received optical signal into the first optical pathway part 8 and into the second optical pathway part 10. It may accurately be described by, and have the effect of, a unitary scattering matrix. Accordingly, a redistribution (5, 7) of the intensity, or probability density, of the input optical signal is formed as between the first and second optical pathways parts. The ring optical resonator (14a, 14b) is directly optically coupled to the first waveguide part 8, such that the first optical pathway part comprises the optical resonator. The optical ring resonator comprises a pair of optically coupled ring optical resonators including primary ring optical resonator 14b formed from an optical waveguide and a secondary ring optical resonator 14a formed from an optical waveguide. The secondary ring optical resonator 14a is directly optically coupled evanescently to the first waveguide of the first optical pathway 8 and is also directly optically coupled evanescently to the waveguide of the primary ring optical resonator 14b. However, the primary ring optical resonator 14b is only indirectly optically coupled to the first waveguide of the first optical pathway 8 via the secondary ring optical resonator. Consequently, an optical signal 9 upon the first optical pathway 8 may pass between the first waveguide of the first optical pathway 8 and the primary ring optical resonator 14b only via the secondary ring optical resonator 14a. The optical switch comprises an optical modulator 28b coupled to a single-photon detector unit 24 configured to output a detection signal 100b in response to the detection of a single photon (19a / 19b). The optical modulator 28b is configured, in response to the detection signal, to modulate the refractive index of the primary optical resonator 14b to change an optical phase shift incurred by the optical signal 9 upon passage through the primary optical resonator, and therefore through the optical resonator (14a, 14b) as a whole, collectively comprising the primary and secondary ring resonators, such that the optical interference occurring at the optical output coupling part 16 results in a routing of the output optical signal 17 to one of said first and second optical pathway parts selected conditional on said detection of a single photon. It is to be noted, however, that for some examples, the optical modulator may comprise two separate modulator parts (28a, 28b) comprising the optical modulator 28b described above, serving as a first modulator part, and further comprising a second optical modulator 28a serving as a second modulator part. The second modulator 28a may be coupled to the single-photon detector unit 24 which may be further configured to output a second detection signal 100a, as well as a first detection signal 100b, in response to the detection of a single photon (19b, Fig. 32). The second optical modulator 28a may be configured, in response to the second detection signal 100a, to modulate the refractive index of the secondary optical resonator forming a part of the collective optical resonator, to change an optical phase shift incurred by the optical signal 7 upon passage through that secondary optical resonator. This is shown in Fig. 31 and Fig. 32 for completeness and clarity. However, in some examples, the second optical modulator 28a may be omitted entirely, or controlled such that it does not respond to the second detection signal 100a and does not modulate the refractive index of the secondary ring resonator 14a with the result that, in either case, an optical modulation is only applied to the primary ring resonator 14b in response to the first detection signal 100b. The primary optical ring resonator 14b is configured to resonate at resonant optical frequencies within a resonance bandwidth determined by a refractive index, n, of that optical resonator. The optical waveguide forming the primary ring resonator 14b is directly evanescently optically coupled to the optical waveguide forming the secondary ring resonator 14a at a ring-to-ring coupling region 300, and is only indirectly optically coupled to the waveguide forming the first waveguide part 8 via the secondary ring resonator 14a. A second coupling region 301 is provided where the secondary ring resonator 14a is itself directly evanescently optically coupled to the first waveguide part 8. An optical input port of the single-photon detector is coupled to the optical output end of a feed waveguide 22 for conveying photons to the single photon detector. An optical input end of the feed waveguide 22 is optically coupled to a source (not shown) of single photons, for receiving single photons therefrom and for guiding the single photons to the optical input port of the single-photon detector for detection. This source of single photons may be any suitable source readily apparent and available to the person skilled in the art. The single-photon detector unit 24 is electrically coupled to an electro-optical modulator (28a, or 28a and 28b) which is configured to receive the electrical detection signal and to output an electrical modulation signal (100a, or 100a and 100b) in response to the electrical detection signal. An optional electrical contact part 26 electrically links the single-photon detector 24 and the electro-optical modulator 28a, 28b. This electrical contact part serves to provide signal processing or amplification of the electrical detection signal produced by single-photon detector unit 24 and may comprise signal filtering elements or capacitive elements. It may be used to generate a larger output voltage pulse, such as by passing the signal through a cascading array of nanowire detectors to amplify the output pulse from a single detector. Alternatively, or in addition, the electrical contact part 24 may comprise an impedance-matching taper, or / and may comprise a compact low power signal amplifier. The electro-optical modulator 28 is configured to modulate the refractive index, n, of the material forming the waveguide defining the primary optical ring resonator 14a (or, in some examples, both the primary and secondary optical ring resonators 14a, 14b) by the electrical modulation signal, in response to detection of a single photon by the single-photon detector unit. The effect of the electrical modulation signal is to modulate (Aa»res) the spectral position, wres0, of the centre of the resonance bandwidth of the optical ring resonator being modulated. The effect of the modulation in spectral position is to modulate the amount of an optical phase accumulated by an optical signal 9 within the ring resonator thereby to modulate the optical interference occurring at the optical output coupling part 16 when the redistributed optical signal 11 as received via the first optical pathway part optically interferes with the redistributed optical signal 13 as received via the second optical pathway part. As shown in Fig. 31, in the absence 19a of a photon in the feed waveguide 22 to the single-photon detector 24, no detection of a photon takes place and, consequently no electrical detection signal is generated. This means that the electro-optical modulator 28b at the primary ring resonator is not driven to modulate, An, the refractive index of the material of the waveguide forming the primary ring resonator (i.e., the modulator is “OFF”). Consequently, no modulation, Ao»res, of the resonance frequency (or resonance wavelength modulation, AAres) takes place. An optical phase shift will be accumulated by an optical signal 9 within the primary ring resonator. In some examples where the optical modulator 28a of the secondary ring resonator is dormant or omitted entirely, then an optical phase shift will also be accumulated by the optical signal 9 within the secondary ring resonator. The total optical phase shift accumulated by the optical signal 9 within the primary and secondary ring resonators collectively, is a first pre-set optical phase shift. Alternatively, in some other examples, the electro-optical modulator 28a of the secondary ring resonator may be present and active (i.e., not dormant) but controlled to be not driven to modulate, An, the refractive index of the material of the waveguide forming the secondary ring resonator (i.e., the modulator 28a is “OFF”) in the absence of detection of the single photon 19a. Consequently, again, no modulation, &a)res, of the resonance frequency (or resonance wavelength modulation, AAres) takes place. By contrast, as shown in Fig. 32, upon detection of one or more photons 19b via the feed waveguide 22, the single-photon detector 24 produces a photon detection electrical signal which drives the electro-optical modulator 28b (i.e., switches the modulator “ON”) to change the resonant frequency (wavelength) of the primary ring resonator 14b, and the spectral position of the bandwidth of the resonance profile thereof. The result is that a second pre-set optical phase shift will be accumulated by an optical signal 9 within the primary and secondary ring resonators collectively. In some examples where the optical modulator 28a of the secondary ring resonator is dormant or omitted entirely, then an optical phase shift accumulated by the optical signal 9 within the secondary ring resonator when the electro-optical modulator 28b is switched “ON” will be the same as that accumulated when the electro-optical modulator 28b was switched “OFF”. Alternatively, in some other examples, the electro-optical modulator 28a of the secondary ring resonator may be present and active (i.e., not dormant) and controlled to be driven to modulate, An, the refractive index of the material of the waveguide forming the secondary ring resonator (i.e., the modulator 28a is “ON”) in the presence of detection of the single photon 19b. Consequently, again, a modulation, A<yres, of the resonance frequency (or resonance wavelength modulation, AAres) takes place. The optical router also comprises an optical output coupling part 16 configured for receiving the redistributed optical signal 11 via the first optical pathway part and for receiving the redistributed optical signal 13 via the second optical pathway part such that the redistributed optical signal as received via the first optical pathway part optically interferes with the redistributed optical signal as received via the second optical pathway part. The output coupling part 16 is arranged to output the result 17 of the optical interference to an optical output port 20 the first optical pathway part and to an optical output port 18 of the second optical pathway part. The output coupling part 16 defines an output directional coupler formed by an evanescent optical coupling of respective adjacent sections of the first waveguide 8 and the second waveguide 10 being in suitably close physical proximity. The coupling constants of the output directional coupler 6 are configured such that the directional coupler may redistribute substantially 50% of the optical signal 11 received from the first waveguide 8 into the second waveguide 10 and substantially 50% of the optical signal 11 remaining in the first waveguide. The coupling constants of the output directional coupler 6 are configured such that, simultaneously, the directional coupler may redistribute substantially 50% of the optical signal 13 received from the second waveguide 10 into the first waveguide 8 and substantially 50% of the optical signal 13 remaining in the second waveguide. As a result, a substantially equal redistribution of the optical signal (11,13) from the first optical pathway part and the second optical pathway part is shared amongst the first optical pathway part and the second optical pathway part. The first optical pathway part 8 thereby defines a first interferometer arm of the Mach-Zehnder interferometer and the second optical pathway part 10 defines a second interferometer arm of the Mach-Zehnder interferometer. Accordingly, a final redistribution 17 of the intensity, or probability density, of the input optical signal is formed as between the first and second optical pathways parts, according to the nature of the optical interference that occurs within the output directional coupler. That optical interference is modulated according to the modulation of the refractive index of the primary ring optical resonator 14b (or both the primary and secondary ring optical resonators 14a, 14b) and the optical phase shift it applies to the optical signal 9 passing through it. In the first switching state of the optical switch, as shown in Fig.31, the optical phase shift applied to the optical signal 9 passing through the primary and secondary ring resonators 14a, 14b, is such as to control the optical interference within the output directional coupler to fully, or at least mostly, redistribute the intensity, or probability density, of the input optical signal (11, 13) to the second optical pathway part 10 for output via the optical output port 18 thereof. Little of, or a negligible amount of, the intensity, or probability density, of the input optical signal is redistributed to the optical output port 20 of the first optical pathway 8. In the second switching state of the optical switch, as shown in Fig. 32, the optical phase shift applied to the optical signal 9 passing through the ring resonator 14 differs from the optical phase shift applied to the optical signal 9 when in the first switching state, by a phase difference of -n and is such as to control the optical interference within the output directional coupler to fully, or at least mostly, redistribute the intensity, or probability density, of the input optical signal (11, 13) to the first optical pathways part 8 for output via the optical output port 20 thereof. Little of, or a negligible amount of, the intensity, or probability density, of the input optical signal is redistributed to the optical output port 18 of the second optical pathway 10. The optical input coupling part 6 forms a 50 / 50% beam-splitting directional optical coupler and the optical output coupling part 16 also forms a 50 / 50% beam-splitting directional optical coupler, each configured to split an optical signal input to any one input port thereof, into two equal parts output on a respective one of two output ports. The input and output directional couplers may each accurately be described by, and have the effect of, a respective unitary scattering matrix. In the following discussion, with reference to Fig. 33, Fig. 34 and Fig. 35, the results in question were obtained with the second optical modulator 28a omitted entirely, or is controlled such that it does not respond to the second detection signal 100a and does not modulate the refractive index of the second ring resonator forming the optical input coupling part 102 (i.e., it remains in the “OFF” state in Fig. 32). In an example, parameters of the ring optical resonators and modulator, at an operating wavelength of A = 1550nm, were as follows: (1) Secondary ring-waveguide coupling constant (301): kI = 100 fieV. (2) Secondary (non-modulated) ring resonator 14a: detuning (212) of the ring resonance frequency of the ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum: 42 = 24 fieV (fixed). The secondary ring resonator spectral bandwidth centre frequency is fixed and imparts a constant phase shift to the optical signal. (3) Secondary ring 14a quality factor: Q = 8,000. (4) Detuning (41) of the primary ring resonance frequency of the ring resonator 14b from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 41 = 2 neV With photon detection: 41 = 0 fieV (5) Primary-to-secondary ring resonator coupling constant (300): g = 6.9 geV. In this example, the bandwidth of the transmission spectrum of the primary resonator is determined by the ring-ring coupling constant, g. (6) Primary ring 14b intrinsic quality factor: Q >1,000,000. With these parameters, the following switch characteristics were found to arise. Figure 33(a) shows a transmission spectrum (192a, 192b, 192c, 192d) of the optical output port 20 of the first optical pathway part 8 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the primary ring optical resonator 14b as the optical switch transitions from the first switch state (192a; Fig. 31) to the second switch state (192d; Fig. 32). Figure 33(b) shows a transmission spectrum (194a, 194b, 194c, 194d) of the optical output port 18 of the second optical pathway part 10 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the primary ring optical resonator 14b as the optical switch transitions from the first switch state (194a; Fig. 31) to the second switch state (194d; Fig. 32). Fig. 34(a) shows a spectral power distribution 196 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 31. Fig. 34(b) shows the optical transmission spectrum 198 of the optical output port 18 of the second optical pathway 10 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 31), together with a spectral power output distribution 200 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10, and a spectral power output distribution 202 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8. The relative output power as between the optical output port 20 of the first optical pathway 8 and the optical output port 18 of the second optical pathway 10 is, about 2.36% and about 97.64% respectively. Fig. 34(c) shows an optical transmission spectrum 204 of the optical output port 20 of the first optical pathway 8 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 32), together with a spectral power output distribution 206 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8. The relative output power as between the optical output port 20 of the first optical pathway and the optical output port 18 of the second optical pathway is, about 97.8% and about 2.2% respectively, with a switching fidelity of 0.9548. Fig. 34(d) shows a graph of the photon coincidence rate 210 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway and as output from the optical output port 20 of the first optical pathway by the switch as between the switching states shown in Fig. 31 and of Fig. 32. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a nonzero relative time delay of about 0ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway relative to the arrival time at the output port 18 of the second optical pathway. An indistinguishability of 0.9968 is achieved between the wave packet with this relative delay between the two routing states. Fig. 35 shows an optical frequency distribution 212 of the input Gaussian wave packet optical pulse 3 together with the relative spectral position of the transmitted power spectrum (214a, 214b, 214c, 214d) of the optical output port 18 of the second optical pathway part 10 of the optical switch of Fig. 31, and the collective optical phase transmission spectra (216a, 216b, 216c, 216d) of the pair of ring optical resonators (primary ring 14b and secondary ring 14a, as one combined resonator). Four graphs are shown with each one corresponding to one of four different (increasing) values of spectral detuning 41 (refractive index modulation) of the primary (modulated) ring optical resonator 14b upon the first optical pathway part 8 of the optical switch, whereby 41 = Q / ieV -> 41 = 2[ieV in three steps of equal size, as the switch transitions from one switching state to another switching state. In the first switch state (Fig. 31) the spectral centre frequency of the frequency spectrum 212 of the optical wave packet has a maximum coinciding with the frequency at which the collective optical phase transmission spectrum 216a causes an accumulation (net) of 0 x n (i.e. zero) radians of optical phase shift. The result is to maximise the transmitted power 214a from the optical output port 18 of the second optical pathway part 10 at the same centre frequency. In the second switch state (Fig. 32) the spectral centre frequency of the frequency spectrum 212 of the optical wave packet has a maximum coinciding with the frequency at which the collective optical phase transmission spectrum 216a causes an accumulation (net) of +tt (i.e. one equivalent to the other) radians of optical phase shift. The result is to minimise the transmitted power 214a from the optical output port 18 of the second optical pathway part 10 at the same centre frequency, the transmitted power instead being routed from the optical output port 20 of the first optical pathway part 8. EXAMPLE 7 Figure 36 shows a schematic diagram of an interferometer providing an optical switch in first switch state in which no photon, 19a, is detected by the single-photon detector 24 of the optical switch. Figure 37 shows a schematic diagram of the optical switch of Fig. 36 in second switch state in which a photon, 19b, is detected by the single-photon detector of the optical switch. In this example of the invention, there is provided an optical switch for receiving an optical signal 3 from an optical signal source 30 and for outputting the optical signal 17. The optical switch comprises an optical router configured for routing the optical signal conditional on the detection of a single photon, 19b. The optical router comprises, a first optical resonator formed from a first pair of optically-coupled ring optical resonators 314c, 314d, each one of the two ring optical resonators of the first pair being formed as a respective ring-shaped optical waveguide. The optical router provides a first optical pathway part comprising the first optical resonator (314c, 314d) and further comprising a linear optical waveguide 8 to which the first optical resonator is directly evanescently optically coupled. A second optical pathway part (314a, 314b, 10) also forms a part of the optical router. In particular, the second optical pathway part comprises an optical input coupling part (314a, 314b) and further comprises a linear optical waveguide 10 to which the first optical resonator is directly evanescently optically coupled. The first optical resonator (314c, 314d) is also directly evanescently optically coupled at a coupling region 304 to the optical input coupling part (314a, 314b). The optical input coupling part is provided in the form of a second optical resonator comprising a second pair of optically-coupled ring optical resonators 314a, 314b, with each one of the two ring optical resonators of the second pair being formed as a respective ring-shaped optical waveguide. The optical input coupling part is configured for receiving the optical signal 3 and, via a coupling region 302 between the optical input coupling part and the linear waveguide part 10 and, via the coupling region 304 between the optical input coupling part and the first optical resonator 314c, 314d, for coupling the received optical signal 7 into the first optical pathway part (314c, 314d, 8) of which the first optical resonator (314c, 314d) forms a part. The optical switch further comprises an optical output coupling part provided by the coupling region 304 between the optical input coupling part and the first optical resonator which is configured for coupling the received optical signal 15 from the first optical resonator of the first optical pathway part (314c, 314d, 8) back into the second pair of optically-coupled ring optical resonators 314a, 314b of the second optical pathway part (314a, 314b, 10). In this way, the input coupling part and the output coupling part comprise common components of the optical switch which serve the dual purpose of acting as coupling elements and resonant structures permitted by the optical circulating action of the optical ring resonators of the switch. A distributed coupling part is thereby formed by the combination of: the coupling region 304 between the optical input coupling part and the first optical resonator, a coupling region 306 between the first optical resonator and the linear optical waveguide 8 of the first optical pathway; and the coupling region 302 between the optical input coupling part and the linear waveguide part 10 of the second optical pathway. The received the optical signal 7, 15 may be distributed between the first optical pathway part and the second optical pathway part for output from an output port 20 the first optical pathway part, and for output from an output port 18 the second optical pathway part, depending on the optical interferences that take place at the coupling region 304 between the optical input coupling part and the first optical resonator, between the optical signal 7 as received there via the second optical pathway part 314a, 314b, 10, and the optical signal 15 as received via the first optical pathway part 314c, 314d. In this way, the result of this optical interference is output, by the optical switch, to the first optical pathway part 8 (at output port 20) and to the second optical pathway part 109 (at output port 18). The optical switch comprises an optical modulator comprising four separate modulator parts (328a, 328b, 328c, 328d) each of which is coupled to a common single-photon detector unit 24 configured to output a detection signal (100a, 100b, 100c, 100d) to a respective one of the four separate modulator parts, in response to the detection of a single photon (19b, Fig. 37). Each one of the four separate modulator parts is configured, in response to the respective detection signal received by it, to modulate the refractive index of the ring optical resonator to which it is coupled to change an optical phase shift incurred by the optical signal upon passage through the modulated ring optical resonator in question. The optical interferences that occur within the optical switch are modulated by the action of the four separate modulators applying respective modulations simultaneously, which results in a routing of the output optical signal to one of the first and second optical pathway parts (at output port 18 or output port 20) selected conditional on the detection of a single photon 19b. The optical modulator comprises a first modulator part 328a, a second modulator part 328b, a third modulator part 328c, and a fourth modulator part 328d. The first, second, third and fourth modulator parts are respectively coupled to the single-photon detector unit 24 which is configured to output a first, second, third and fourth detection signal 100a, 100b, 100c, 100d, respectively to the first, second, third and fourth modulator parts, in response to the detection of a single photon (19b, Fig. 37). The first optical modulator 328a is configured, in response to the first detection signal 100a, to modulate the refractive index of a first one, 314a, of the two ring optical resonator parts (314a, 314b) of the input optical coupling part to change an optical phase shift incurred by the optical signal 7 upon passage through that first optical ring resonator part. The first ring optical resonator part is directly evanescently optically coupled at a coupling region 302 to the linear waveguide part 10 of the second optical pathway part. The second optical modulator 328b is configured, in response to the second detection signal 100b, to modulate the refractive index of a second one, 314b, of the two ring optical resonator parts (314a, 314b) of the input optical coupling part to change an optical phase shift incurred by the optical signal 7 upon passage through that second ring optical resonator part. The second ring optical resonator part is directly evanescently optically coupled at a coupling region 303 to the first ring optical resonator part. The third optical modulator 328c is configured, in response to the third detection signal 100c, to modulate the refractive index of a third one, 314c, of the two ring optical resonator parts (314c, 314d) collectively forming the first optical resonator, to change an optical phase shift incurred by the optical signal 15 upon passage through that third ring optical resonator part. The third ring optical resonator part is directly evanescently optically coupled at a coupling region 304 to the second ring optical resonator part. The fourth optical modulator 328d is configured, in response to the fourth detection signal 100d, to modulate the refractive index of a fourth one, 314d, of the two ring optical resonator parts (314c, 314d) collectively forming the first optical resonator, to change an optical phase shift incurred by the optical signal 15 upon passage through that fourth ring optical resonator part. The fourth ring optical resonator part is directly evanescently optically coupled at a coupling region 305 to the third ring optical resonator part, and is also directly evanescently optically coupled at a coupling region 306 to the linear waveguide part 8 of the first optical pathway part. The first optical modulator 328a and the fourth optical modulator 328d are each configured to impart to the respective ring optical resonator to which they are coupled, a respective modulation of refractive index of the material of the respective ring that is of the same size (magnitude) and the same sign (polarity), such as both being an increase in refractive index or both being a decrease, of the same amount. Similarly, the second optical modulator 328b and the third optical modulator 328c are each configured to impart to the respective ring optical resonator to which they are coupled, a respective modulation of refractive index of the material of the respective ring that is of the same size (magnitude) and the same sign (polarity), such as both being a decrease in refractive index or both being an increase, of the same amount. The size (magnitude) of the refractive index change imparted to the second and third ring optical resonators is the same as the size (magnitude) of the refractive index change imparted to the first and fourth ring optical resonators, whereas the sign (polarity) of the refractive index change imparted to the second and third ring optical resonators is the opposite to as the sign (polarity) of the refractive index change imparted to the first and fourth ring optical resonators. In an example, parameters of the four ring optical resonators (314a, 314b, 314c, 314d) and the four modulator parts (328a, 328b, 328c, 328d), at an operating wavelength of A = 1550nm, were as follows: (1) First ring-waveguide coupling constant (302): k1 = 1.8 geV (2) Fourth ring-waveguide coupling constant (306): k2 = 1.8 geV (3) First-to-second ring coupling constant (303): g = 0.41 geV (4) Third-to-fourth ring coupling constant (305): g = 0.41 geV (5) Second-to-third ring coupling constant (304): g = 0.32 geV (6) First and fourth ring (314a, 314d) quality factor (each ring): Q = 445,000. This value is set by having k1 = k2 = 1.8 geV. (7) Second and third ring (314b, 314c) intrinsic quality factor (each ring) preferably Q >1,000,000 (8) Detuning (41,44) of the resonance frequency of, respectively, the first and fourth ring resonators from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 41 = 44 = 0 geV With photon detection: 41=44 = +0.5 geV (9) Detuning (42,43) of the resonance frequency of, respectively, the first and fourth ring resonators from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 42 = 43 = 0 geV With photon detection: 42 = 43 = -0.5 geV With these parameters, the following switch characteristics were found to arise. However, it is to be noted that the relative signs of the four different detunings discussed in this example are just one option (i.e. relative detunings: 41 = -42 = -43 = 44 (i. e.: +, -, -, +), and another option is to employ alternating signs to the detunings: 41 = -42 = 43 = -44 (i.e.: +,-, +,-). The same figures of merit can also be achieved using this alternative option, albeit with different switch characteristics to those discussed below. Fig. 38(a) shows a spectral power distribution 330 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 36. Fig. 38(b) shows the optical transmission spectrum 332 of the optical output port 20 of the first optical pathway 8 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 36), together with a spectral power output distribution 332 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8, and a spectral power output distribution 334 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10. The relative output power as between the optical output port 18 of the second optical pathway and the optical output port 20 of the first optical pathway is, about 4.55% and about 95.45% respectively. Fig. 38(c) shows an optical transmission spectrum 336 of the optical output port 18 of the second optical pathway 10 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 37), together with a spectral power output distribution 338 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway, and a spectral power output distribution 340 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8. The relative output power as between the optical output port 18 of the second optical pathway and the optical output port 20 of the first optical pathway is, about 93.31% and about 6.69% respectively, with a switching fidelity of 0.8906. Fig. 38(d) shows a graph of the photon coincidence rate 342 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway and as output from the optical output port 20 of the first optical pathway by the switch as between the switching states shown in Fig. 36 and of Fig. 37. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a nonzero relative time delay of about 2 ns as between arrival of the wave packet at the output port 20 of the first optical pathway relative to the arrival time at the output port 18 of the second optical pathway. A distinguishability of 0.9733 is achieved between the wave packet with this relative delay between the two routing states. This example illustrates an example of the optical switch for receiving an optical signal from an optical signal source and for outputting the optical signal, the optical switch comprising an optical router configured for routing the optical signal conditional on the detection of a single photon. The optical router comprises: an optical resonator (314c, 314d), a first optical pathway part comprising the optical resonator, and a second optical pathway part comprising a further (second) optical resonator (314a, 314b) wherein the optical resonator and the further optical resonator are optically coupled to form a pair of optically coupled optical resonators. The optical resonator (314c, 314d) and the further optical resonator (314a, 314b) are each formed from a respective pair of optically coupled optical resonators. An optical input coupling part (302, 304) is provided for receiving the optical signal and for coupling the received optical signal into the first optical pathway part (314c, 314d and 8) and into the second optical pathway part (314a, 314b and 10). An optical modulator (328a, 328b, 328c, 328d) is coupled to the single-photon detector unit configured to output the detection signal (100a, 100b, 100c, 10Od) in response to said detection of a single photon, the optical modulator being configured, in response to the detection signal, to modulate the refractive index of the optical resonator and the further optical resonator to change an optical phase shift incurred by the optical signal upon passage through the optical resonator and the further optical resonator such that an optical interference results in a routing of the output optical signal to one of the first and second optical pathway parts selected conditional on the detection of a single photon. An optical coupling 304 between the optical resonator and the further optical resonator may serve the purpose of the optical input coupling part. The optical switch comprises an optical output coupling part for receiving the optical signal via the first optical pathway part and for receiving the optical signal via the second optical pathway part such that the optical signal as received via the first optical pathway part optically interferes with the optical signal as received via the second optical pathway part, and for outputting the result of the optical interference to the first optical pathway part and to the second optical pathway part. An optical coupling 304 between the optical resonator and the further optical resonator serves the purpose of the optical output coupling part. This is because, due to the ability of two coupled ring resonators to circulate the optical signal repeatedly through the coupling region, the optical coupling in question is able to input the optical signal to either one of the two optical resonators it couples, and to is also able to output the optical signal from either one of the two optical resonators it couples. The optical signal may comprise a given optical signal frequency bandwidth, and the (or each, if more than one) optical resonator may be configured to resonate at a respective resonant optical frequency within a resonance bandwidth determined by a refractive index of the optical resonator. The optical modulator may be configured to modulate the refractive index of the optical resonator and the further optical resonator, and each component ring optical resonator thereof, so as to modulate the respective resonance bandwidths thereof selectively and individually to either: change from being a resonance bandwidth centred at an optical frequency lower than a centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than or lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that excludes the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that excludes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth. EXAMPLE 8 Figure 39 shows a schematic diagram of an interferometer providing an optical switch in first switch state in which no photon, 19a, is detected by the single-photon detector 24 of the optical switch. Figure 40 shows a schematic diagram of the optical switch of Fig. 39 in second switch state in which a photon, 19b, is detected by the single-photon detector of the optical switch. In this example of this invention, there is provided an optical switch for receiving an optical signal 3 from an optical signal source 30 and for outputting the optical signal 17. The optical switch comprises an optical router configured for routing the optical signal conditional on the detection of a single photon, 19b. The optical router comprises, a first ring optical resonator 346b formed from an optical waveguide ring, a first optical pathway part 106 comprising a linear optical waveguide and also comprising the first ring optical resonator 346b, and a second optical pathway part 109. The optical switch comprises a distributed optical input coupling part (344, 348) which is in the form of a second ring optical resonator 346a for receiving the optical signal 3 and for coupling the received optical signal 7, via a coupling region 348, into the first optical pathway part (106, 346b) of which the first ring optical resonator 346b forms a part, and for coupling the received optical signal (15, 7) into the second optical pathway part 109 (of which the second ring optical resonator 346a forms a part) via a coupling region 344. The optical switch further comprises a third ring optical resonator 346c formed from a ring optical waveguide which is optically coupled to a first linear optical waveguide which forms a portion of the first optical pathway part 106 to which the first ring optical resonator 346b is also optically coupled 351 at an optically upstream location. The result is that the optical signal received by the first ring optical resonator is directed by the first linear optical waveguide to the third ring optical resonator 346c, for subsequent output from output port 20 of the first optical pathway part 106. The optical switch also comprises a fourth ring optical resonator 346d formed from a ring optical waveguide which is optically coupled to a second linear optical waveguide which forms a portion of the first optical pathway part 109 to which the second ring optical resonator 346a is also optically coupled 345 at an optically upstream location. The result is that the optical signal received by the second ring optical resonator is directed by the second linear optical waveguide to the fourth ring optical resonator 346d, for subsequent output from output port 18 of the second optical pathway part 109. The optical switch comprises a distributed optical output coupling part (348, 344) for receiving the optical signal 15 via the first optical pathway part (346b), of which the first ring optical resonator 346b forms a part, and for receiving the optical signal 7 via the second optical pathway part 346a. The optical signal 15 as received via the first optical pathway part 346b optically interferes with the optical signal 7 as received via the second optical pathway part 346a. The result of this optical interference is output, by the optical switch, to the first optical pathway part 106 (at output port 20) and to the second optical pathway part 109 (at output port 18). The optical switch comprises an optical modulator 260b coupled to a single-photon detector unit 24 configured to output a detection signal 261b in response to the detection of a single photon (19b, Fig. 40). The optical modulator 260b is configured, in response to the detection signal 261b, to modulate the refractive index of the first ring optical resonator 346b to change an optical phase shift incurred by the optical signal 15 upon passage through the first ring optical resonator such that the optical interference results in a routing of said output optical signal to one of the first and second optical pathway parts (at output port 18 or output port 20) selected conditional on the detection of a single photon 19b. The optical modulator comprises four separate modulator parts (260a, 260b, 260c, 260d) comprising a first modulator 260b serving as a first modulator part, a second optical modulator 260a serving as a second modulator part, a third optical modulator 260c serving as a third modulator part, and a fourth optical modulator 260d serving as a fourth modulator part. The first modulator 260b, the second modulator 260a, the third modulator 260c and the fourth modulator 260d are each coupled to the single-photon detector unit 24 which is configured to output to the four modulator parts, respectively, a first detection signal 261b, a second detection signal 261a, a third detection signal 261c, and a fourth detection signal 261 d in response to the detection of a single photon (19b, Fig. 40). The first optical modulator 260b is configured, in response to the first detection signal 261b, to modulate the refractive index of the first optical ring resonator 346b to change an optical phase shift incurred by the optical signal 15 upon passage through that optical ring resonator. The second optical modulator 260a is configured, in response to the second detection signal 261a, to modulate the refractive index of the second optical ring resonator 346a to change an optical phase shift incurred by the optical signal 7 upon passage through that optical ring resonator. Similarly, the third optical modulator 260c is configured, in response to the third detection signal 261c, to modulate the refractive index of the third optical ring resonator 346c to change an optical phase shift incurred by the optical signal 17 upon passage through that optical ring resonator. Finally, the fourth optical modulator 260d is configured, in response to the fourth detection signal 261 d, to modulate the refractive index of the fourth optical ring resonator 346d to change an optical phase shift incurred by the optical signal 17 upon passage through that optical ring resonator. The distributed optical output coupling part (348, 344) optically couples the first ring resonator 346b evanescently to the optical waveguide forming the second ring optical resonator 346a at a first coupling region 348, and optically couples the second ring resonator 346b evanescently to the optical waveguide forming the second optical pathway part 109 at a second coupling region 344. Similarly, a third coupling region 350 optically couples the first ring resonator 346a evanescently to the optical waveguide forming the first optical pathway part 106. A fourth coupling region 351 optically couples the third ring resonator 346c evanescently to the optical waveguide forming the first optical pathway part 106. A fifth coupling region 345 optically couples the fourth ring resonator 346d evanescently to the optical waveguide forming the second optical pathway part 109. In an example, parameters of the four ring optical resonators (314a, 314b, 314c, 314d) and the four modulator parts (328a, 328b, 328c, 328d), at an operating wavelength of A = 1550nm, were as follows: (1) First ring-waveguide coupling constant (344): k1 = 0.9 geV (2) Second ring-waveguide coupling constant (350): «2 = 0.9 geV (3) First-to-second ring coupling constant (348): g = 0.3 geV (4) Third ring-waveguide coupling constant (351): k3 = 1.49 geV (5) Fourth ring-waveguide coupling constant (345): «4 = 1.49 geV (6) First, second, third and fourth ring (346a, 346b, 346c, 346d) quality factor (each ring): Q >200,000. The loaded Q-factor of each ring is determined by its respective coupling constant, k, value, each of which gives Q >200000. (7) Detuning (2! 1,42,43,44) of the resonance frequency of, respectively, the first, second, third and fourth ring resonators from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 41 = 42 = 0 peV ;43 = 44 = 0.5 peV With photon detection: 41 = -42 = 0.5 / ieV ; 43 = 44 = 0 / ieV The effect of this is that the light coupled into the larger first and second rings (346a, 346b) in the “no photon detection” state of the optical switch, is transferred predominantly between waveguides and accumulates a given delay in doing so, and in the “photon detection” state of the optical switch, light coupled into the smaller fourth ring 346d accumulates the same given delay. With these parameters, the following switch characteristics were found to arise. Figure 41(a) shows a transmission spectrum (360a, 360b, 360c, 360d) of the optical output port 20 of the first optical pathway part 106 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the first, second, third and fourth ring optical resonators in concert as the optical switch transitions from the first switch state (360a; Fig. 39) to the second switch state (360d; Fig. 40). Figure 41(b) shows a transmission spectrum (362a, 362b, 362c, 362d) of the optical output port 18 of the second optical pathway part 109 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the first, second, third and fourth ring optical resonators in concert as the optical switch transitions from the first switch state (360a; Fig. 39) to the second switch state (360d; Fig. 40). Fig. 42(a) shows a spectral power distribution 330 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 39. Fig. 42(b) shows the optical transmission spectrum 332 of the optical output port 20 of the first optical pathway 106 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 39), together with a spectral power output distribution 334 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 106, and a spectral power output distribution 335 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 109. The relative output power as between the optical output port 18 of the second optical pathway 109 and the optical output port 20 of the first optical pathway 106 is, about 17.18% and about 82.82% respectively. Fig. 42(c) shows an optical transmission spectrum 336 of the optical output port 18 of the second optical pathway 109 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 40), together with a spectral power output distribution 338 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 109, and a spectral power output distribution 340 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 106. The relative output power as between the optical output port 18 of the second optical pathway and the optical output port 20 of the first optical pathway is, about 84.56% and about 15.44% respectively, with a switching fidelity of 0.7004. Fig. 42(d) shows a graph of the photon coincidence rate 342 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway and as output from the optical output port 20 of the first optical pathway by the switch as between the switching states shown in Fig. 39 and of Fig. 40. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a relative time delay of about 0ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway relative to the arrival time at the output port 18 of the second optical pathway. An indistinguishability of 0.8248 is achieved between the wave packet with this relative delay between the two routing states. This example illustrates an example of the optical switch for receiving an optical signal from an optical signal source and for outputting the optical signal, the optical switch comprising an optical router configured for routing the optical signal conditional on the detection of a single photon. The optical router comprises: an optical resonator 346b; a first optical pathway part comprising the optical resonator, and a second optical pathway part comprising a further (second) optical resonator 346a wherein the optical resonator and the further optical resonator are optically coupled to form a pair of optically coupled optical resonators. An optical input coupling part (348, 344) is provided for receiving the optical signal and for coupling the received optical signal into the first optical pathway part (346b, 106) and into the second optical pathway part (109, 346a). An optical modulator (260a, 260b) is coupled to the single-photon detector unit configured to output the detection signal (261 a, 261 b) in response to said detection of a single photon, the optical modulator being configured, in response to the detection signal, to modulate the refractive index of the optical resonator and the further optical resonator to change an optical phase shift incurred by the optical signal upon passage through the optical resonator and the further optical resonator such that an optical interference results in a routing of the output optical signal to one of the first and second optical pathway parts (106, 109) selected conditional on the detection of a single photon. An optical coupling 348 between the optical resonator and the further optical resonator may serve the purpose of the optical input coupling part. The optical switch comprises an optical output coupling part for receiving the optical signal via the first optical pathway part and for receiving the optical signal via the second optical pathway part such that the optical signal as received via the first optical pathway part optically interferes with the optical signal as received via the second optical pathway part, and for outputting the result of the optical interference to the first optical pathway part and to the second optical pathway part. An optical coupling 348 between the optical resonator and the further optical resonator serves the purpose of the optical output coupling part. This is because, due to the ability of the two coupled ring resonators to circulate the optical signal repeatedly through the coupling region, the optical coupling in question is able to input the optical signal to either one of the two optical resonators it couples, and to is also able to output the optical signal from either one of the two optical resonators it couples. The optical signal may comprise a given optical signal frequency bandwidth, and the (or each, if more than one) optical resonator may be configured to resonate at a respective resonant optical frequency within a resonance bandwidth determined by a refractive index of the optical resonator. The optical modulator may be configured to modulate the refractive index of the optical resonator and the further optical resonator so as to modulate the respective resonance bandwidths thereof selectively and individually to either: change from being a resonance bandwidth centred at an optical frequency lower than a centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than or lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency lower than the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that includes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that excludes the centre of the optical signal frequency bandwidth; or, change from being a resonance bandwidth that excludes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth. EXAMPLE 9 Figure 43 shows a schematic diagram of an interferometer providing an optical switch in first switch state in which no photon, 19a, is detected by the single-photon detector 24 of the optical switch. Figure 44 shows a schematic diagram of the optical switch of Fig. 43 in second switch state in which a photon, 19b, is detected by the single-photon detector of the optical switch. In this example, the optical switch is configured for receiving an optical signal 3 from an optical signal source 30 as a pulse of laser light (or a photon). The optical switch is arranged for routing the input signal by passing it through an optical router of the switch configured for routing the optical signal conditional on the detection of a single photon. Figure 43 shows a schematic diagram of an optical switch comprising a Mach-Zehnder interferometer in first one of two switching states. Figure 44 shows a schematic diagram of the optical switch of Fig. 43 in second one of two switching states. Fig. 43 and Fig. 44 each illustrate the optical switch according to an embodiment of the invention employing an optical interferometer including a first waveguide 8 forming a first optical pathway part comprising a first ring optical resonator (14a, 14b), and a second waveguide 10 forming a second optical pathway part comprising a second ring optical resonator (514a, 514b). The first and second ring optical resonators are each controllable for routing an input optical signal 3 (e.g., an optical pulse) received into an input optical port 2 of the second waveguide part 10 from the optical signal source (e.g., a laser unit) in optical communication with the optical switch, for onward propagation along the second waveguide part 10 for input to the optical interferometer. The switching state (routing) is put into effect by the action of the ring resonator, as described herein, conditional on the detection of a single photon (19a / 19b). The optical switch provides a single-photon triggered optical signal switch / router. In particular, the optical router comprises an optical input coupling part in the form of an input directional coupler 6 formed by an evanescent optical coupling of respective adjacent sections of the first waveguide 8 and the second waveguide 10 being in suitably close physical proximity. The coupling constants of the input directional coupler 6 are such that the directional coupler redistributes substantially 50% of the input signal 7 from the second waveguide 10 into the first waveguide 8 with substantially 50% of the input signal 5 remaining in the second waveguide 10. As a result, a substantially equal redistribution of the initial optical signal 3 is shared amongst the first optical pathway part and the second optical pathway part. The optical input coupling part 6 is thereby configured for receiving the optical signal 3 and for coupling the received optical signal into the first optical pathway part 8 and into the second optical pathway part 10. It may accurately be described by, and have the effect of, a unitary scattering matrix. Accordingly, a redistribution (5, 7) of the intensity, or probability density, of the input optical signal is formed as between the first and second optical pathways parts. The first optical resonator (14a, 14b) is directly optically coupled, at a coupling region 352, to the first waveguide part 8, such that the first optical pathway part comprises the optical resonator. The first optical resonator (14a, 14b) comprises a pair of optically coupled ring optical resonators including primary ring optical resonator 14b formed from an optical waveguide and a secondary ring optical resonator 14a formed from an optical waveguide. The secondary ring optical resonator 14a is directly optically coupled evanescently to the first waveguide of the first optical pathway 8 and is also directly optically coupled evanescently to the waveguide of the primary ring optical resonator 14b. However, the primary ring optical resonator 14b is only indirectly optically coupled to the first waveguide of the first optical pathway 8 via the secondary ring optical resonator. Consequently, an optical signal upon the first optical pathway 8 may pass between the first waveguide of the first optical pathway 8 and the primary ring optical resonator 14b only via the secondary ring optical resonator 14a. The second optical resonator (514a, 514b) comprises a pair of optically coupled ring optical resonators including primary ring optical resonator 514b formed from an optical waveguide and a secondary ring optical resonator 514a formed from an optical waveguide. The secondary ring optical resonator 514a is directly optically coupled evanescently to the second waveguide 10 of the second optical pathway and is also directly optically coupled evanescently to the waveguide of the primary ring optical resonator 514b of the second optical resonator. However, the primary ring optical resonator 514b is only indirectly optically coupled to the second waveguide of the second optical pathway 10 via the secondary ring optical resonator. Consequently, an optical signal upon the second optical pathway 10 may pass between the second waveguide of the second optical pathway 10 and the primary ring optical resonator 514b only via the secondary ring optical resonator 514a. The optical switch comprises an optical modulator 344a, 344b, coupled to a single-photon detector unit 24 configured to output a detection signal 343a, 343b, in response to the detection of a single photon (19a / 19b). It is to be noted that the optical modulator comprises two separate modulator parts (344a, 344b) comprising a first optical modulator 344b serving as a first modulator part, and further comprising a second optical modulator 344a serving as a second modulator part. The optical modulator is configured, in response to the detection signal, to modulate the refractive index of the primary ring optical resonator 14b of the first optical resonator, and also to modulate the refractive index of the primary ring optical resonator 514b of the second optical resonator, to change an optical phase shift incurred by the optical signal upon passage through the two primary ring optical resonators, and therefore through the first and second optical resonators (14a, 14b ; 514a, 514b) as a whole, collectively each comprising a respective primary and secondary ring resonator. The effect is such that the optical interference occurring at the optical output coupling part 16 results in a routing of the output optical signal 17 to one of the first and second optical pathway parts 20, 18, selected conditional on the detection of a single photon. The optical waveguide forming the primary ring resonator 14b of the first optical resonator is directly evanescently optically coupled to the optical waveguide forming the secondary ring resonator 14a of the first optical resonator at a ring-to-ring coupling region 350 and is only indirectly optically coupled to the waveguide forming the first waveguide part 8 via the secondary ring resonator 14a. A second coupling region 352 is provided where the secondary ring resonator 14a is itself directly evanescently optically coupled to the first waveguide part 8. Similarly, the optical waveguide forming the primary ring resonator 514b of the second optical resonator is directly evanescently optically coupled to the optical waveguide forming the secondary ring resonator 514a of the second optical resonator at a further ring-to-ring coupling region 348 and is only indirectly optically coupled to the waveguide forming the second waveguide part 10 via the secondary ring resonator 514a. A second coupling region 346 is provided where the secondary ring resonator 514a is itself directly evanescently optically coupled to the second waveguide part 10. An optical input port of the single-photon detector is coupled to the optical output end of a feed waveguide 22 for conveying photons to the single photon detector. An optical input end of the feed waveguide 22 is optically coupled to a source (not shown) of single photons, for receiving single photons therefrom and for guiding the single photons to the optical input port of the single-photon detector for detection. This source of single photons may be any suitable source readily apparent and available to the person skilled in the art. An optional electrical contact part 26 electrically links the single-photon detector 24 and the optical modulator 344a, 344b. This electrical contact part serves to provide signal processing or amplification of the electrical detection signal produced by single-photon detector unit 24 and may comprise signal filtering elements or capacitive elements. It may be used to generate a larger output voltage pulse, such as by passing the signal through a cascading array of nanowire detectors to amplify the output pulse from a single detector. Alternatively, or in addition, the electrical contact part 26 may comprise an impedancematching taper, or / and may comprise a compact low power signal amplifier. The optical modulator 344a, 344b is configured to modulate the refractive index, n, of the material forming the waveguide defining both of the primary ring optical resonators 14b, 514b, by the electrical modulation signal, in response to detection of a single photon by the single-photon detector unit. The effect of the electrical modulation signal is to modulate (&a)res) the spectral position, a)res0, of the centre of the resonance bandwidth of the optical ring resonator being modulated. The effect of the modulation in spectral position is to modulate the amount of an optical phase accumulated by an optical signal within the ring resonator thereby to modulate the optical interference occurring at the optical output coupling part 16 when the redistributed optical signal 11 as received via the first optical pathway part optically interferes with the redistributed optical signal 13 as received via the second optical pathway part. As shown in Fig. 43, in the absence 19a of a photon in the feed waveguide 22 to the single-photon detector 24, no detection of a photon takes place and, consequently no electrical detection signal is generated. This means that the optical modulators at the primary ring resonators are not driven to modulate, An, the refractive index of the material of the waveguide forming the primary ring resonators (i.e., the modulator is “OFF”). Consequently, no modulation, A<nres, of the resonance frequency (or resonance wavelength modulation, AAres) takes place. An optical phase shift will be accumulated by an optical signal within the primary ring resonators. An optical phase shift will also be accumulated by the optical signal within the secondary ring resonators. The total optical phase shift accumulated by the optical signal within the primary and secondary ring resonators collectively, is a first pre-set optical phase shift. By contrast, as shown in Fig. 44, upon detection of one or more photons 19b via the feed waveguide 22, the single-photon detector 24 produces a photon detection electrical signal which drives the optical modulators 344a, 344b, (i.e., switches the modulators “ON”) to change the resonant frequency (wavelength) of the primary ring resonators 14b, 514b, and the spectral position of the bandwidth of the resonance profile thereof. The result is that a second pre-set optical phase shift will be accumulated by an optical signal within the primary and secondary ring resonators collectively, of the respective optical resonators. The optical router also comprises an optical output coupling part 16 configured for receiving the redistributed optical signal 11 via the first optical pathway part and for receiving the redistributed optical signal 13 via the second optical pathway part such that the redistributed optical signal as received via the first optical pathway part optically interferes with the redistributed optical signal as received via the second optical pathway part. The output coupling part 16 is arranged to output the result 17 of the optical interference to an optical output port 20 the first optical pathway part and to an optical output port 18 of the second optical pathway part. The output coupling part 16 defines an output directional coupler formed by an evanescent optical coupling of respective adjacent sections of the first waveguide 8 and the second waveguide 10 being in suitably close physical proximity. The coupling constants of the output directional coupler 6 are configured such that the directional coupler may redistribute substantially 50% of the optical signal 11 received from the first waveguide 8 into the second waveguide 10 and substantially 50% of the optical signal 11 remaining in the first waveguide. The coupling constants of the output directional coupler 6 are configured such that, simultaneously, the directional coupler may redistribute substantially 50% of the optical signal 13 received from the second waveguide 10 into the first waveguide 8 and substantially 50% of the optical signal 13 remaining in the second waveguide. As a result, a substantially equal redistribution of the optical signal (11,13) from the first optical pathway part and the second optical pathway part is shared amongst the first optical pathway part and the second optical pathway part. The first optical pathway part 8 thereby defines a first interferometer arm of the Mach-Zehnder interferometer and the second optical pathway part 10 defines a second interferometer arm of the Mach-Zehnder interferometer. Accordingly, a final redistribution 17 of the intensity, or probability density, of the input optical signal is formed as between the first and second optical pathways parts, according to the nature of the optical interference that occurs within the output directional coupler. That optical interference is modulated according to the modulation of the refractive index of the primary ring optical resonators 14b, 514b and the optical phase shift it applies to the optical signal passing through them. In the first switching state of the optical switch, as shown in Fig.43, the optical phase shift applied to the optical signal passing through the primary and secondary ring resonators of the first optical resonator, is such as to control the optical interference within the output directional coupler to fully, or at least mostly, redistribute the intensity, or probability density, of the input optical signal (11, 13) to the second optical pathway part 10 for output via the optical output port 18 thereof. Little of, or a negligible amount of, the intensity, or probability density, of the input optical signal is redistributed to the optical output port 20 of the first optical pathway 8. In the second switching state of the optical switch, as shown in Fig. 44, the optical phase shift applied to the optical signal passing through the primary and secondary ring resonators of the first optical resonator differs from the optical phase shift applied to the optical signal when In the first switching state, by a phase difference configured such as to control the optical interference within the output directional coupler to fully, or at least mostly, redistribute the intensity, or probability density, of the input optical signal (11, 13) to the first optical pathway part 8 for output via the optical output port 20 thereof. Little of, or a negligible amount of, the intensity, or probability density, of the input optical signal is redistributed to the optical output port 18 of the second optical pathway 10. The optical input coupling part 6 forms a 50 / 50% beam-splitting directional optical coupler and the optical output coupling part 16 also forms a 50 / 50% beam-splitting directional optical coupler, each configured to split an optical signal input to any one input port thereof, into two equal parts output on a respective one of two output ports. The input and output directional couplers may each accurately be described by, and have the effect of, a respective unitary scattering matrix. In an example, parameters of the ring optical resonators and modulator, at an operating wavelength of A = 1550nm, were as follows: (1) First optical resonator: secondary ring-to-first waveguide coupling constant (352): / cl = 99 iieV. (2) First optical resonator: secondary ring resonator 14a: detuning (41) of the ring resonance frequency of the ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum: 41 = 39 fieV (fixed). The secondary ring resonator spectral bandwidth centre frequency is fixed and imparts a constant phase shift to the optical signal. (3) First optical resonator: secondary ring 14a quality factor: Q = 8,000. This is determined according to: k1 = 99 geV. (4) First optical resonator: primary ring 14b intrinsic quality factor: Q >1000,000. (5) First optical resonator: detuning (42) of the primary ring resonance frequency of the ring resonator 14b from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 41 = 0.7 geV With photon detection: 41 = -0.27 geV (6) First optical resonator: primary-to-secondary ring resonator coupling constant (350): g = 3.58 geV. (7) Second optical resonator: secondary ring-to-second waveguide coupling constant (346): k2 = 45 geV. (8) Second optical resonator: secondary ring resonator 514a: detuning (44) of the ring resonance frequency of the ring resonator from coincidence with the centre frequency of the optical wave packet frequency spectrum: 41 = 2.4 geV (fixed). The secondary ring resonator spectral bandwidth centre frequency is fixed and imparts a constant phase shift to the optical signal. (9) Second optical resonator: secondary ring 514a quality factor: Q = 18,000. This is determined according to: k2 = 45 geV (10) Second optical resonator: primary ring 514b intrinsic quality factor: Q >1,000,000. (11) Second optical resonator: detuning (43) of the primary ring resonance frequency of the ring resonator 514b from coincidence with the centre frequency of the optical wave packet 3 frequency spectrum: No photon detection: 43 = 0.7 geV With photon detection: 43 = -0.27 geV With these parameters, the following switch characteristics were found to arise. The optical transmission spectrum of the optical switch (360, 366; Fig. 46) is found to have a sharp, or rapid transition over a small frequency range, from its maximum transmission value to its minimum transition value. This is highly desirable as it allows a rapid transition in the switch state. Figure 45(a) shows a transmission spectrum (354a, 354b, 354c, 354d) of the optical output port 20 of the first optical pathway part 8 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the primary ring optical resonators 14b, 514b as the optical switch transitions from the first switch state (354a; Fig. 43) to the second switch state (354d; Fig. 44). Figure 45(b) shows a transmission spectrum (356a, 356b, 356c, 356d) of the optical output port 18 of the second optical pathway part 10 as a function of increasing detuning (refractive index modulation) of the resonance frequency of the primary ring optical resonators 14b, 514b as the optical switch transitions from the first switch state (356a; Fig. 43) to the second switch state (364d; Fig. 44). Fig. 46(a) shows a spectral power distribution 358 of the Gaussian wave packet optical pulse 3 input to the optical switch of Fig. 43. Fig. 46(b) shows the optical transmission spectrum 356a of the optical output port 18 of the second optical pathway 10 of the optical switch when in the switching state corresponding to “no photon detected” (Fig. 43), together with a spectral power output distribution 362 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10, and a spectral power output distribution 364 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8. The relative output power as between the optical output port 18 of the second optical pathway 10 and the optical output port 20 of the first optical pathway 8 is, about 94.3% and about 5.7% respectively. Fig. 46(c) shows an optical transmission spectrum 354d of the optical output port 20 of the first optical pathway 8 of the optical switch when in the switching state corresponding to “photon detected” (Fig. 44), together with a spectral power output distribution 368 of the input Gaussian wave packet optical pulse as output from the optical output port 20 of the first optical pathway 8, alongside a spectral power output distribution 370 of the input Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway 10. The relative output power as between the optical output port 18 of the second optical pathway and the optical output port 20 of the first optical pathway is, about 3.64% and about 96.36% respectively, with a switching fidelity of 0.9086. Fig. 46(d) shows a graph of the photon coincidence rate 372 of the Gaussian wave packet optical pulse as output from the optical output port 18 of the second optical pathway and as output from the optical output port 20 of the first optical pathway by the switch as between the switching states shown in Fig. 43 and of Fig. 44. The coincidence rates show a so-called Hong-Ou-Mandel (HOM) ’dip’ centred at a nonzero relative time delay of about 0.1 ns (zero) as between arrival of the wave packet at the output port 20 of the first optical pathway relative to the arrival time at the output port 18 of the second optical pathway. An indistinguishability of 0.8679 is achieved between the wave packet with this relative delay between the two routing states. In some examples, a modulation of an optical ring resonator of the optical switch may be implemented as follows, as appropriate. Consider first the case schematically illustrated in Fig. 47 for which the singlephoton detector 22 produces no electrical signal and the optical modulator 24 has its “rest” default effect on the ring resonator, which in turn will have a resonance frequency at its default frequency value (cores) given by the total coupling strength to all external optical modes, as determined by the manufacturing process used to create the optical switch / router. Figure 47(a) (upper graph) shows a schematic diagram of the transmission spectrum 380 of the ring resonator when in the state in which no photon is detected by the single-photon detector. That is to say, the frequency (wsignai) of the optical signal input to the optical switch corresponds to (or at least falls within the resonance bandwidth 382) of the resonance frequency of the ring resonator (i.e., Wres = Wsignai). This means that the optical signal upon an optical pathway of optical switch optically coupled to the ring resonator may be strongly, resonantly, coupled into the ring resonator. In this way, optical signals within optical modes of the waveguides of the optical pathways of the switch, which impinge on the ring resonator, may be coupled into and out of the resonator as determined by their frequency distribution and the strength of the coupling between the optical modes of the waveguide and resonator. As is shown in Figure 47(b), when the single-photon detector 22 detects a photon it creates an output voltage signal which is used to drive the optical modulator 24, which in turn changes the resonant frequency of the ring resonator. The result of the modulation is to shift the resonance frequency of the ring resonator from its default frequency value (wres) to a shifted resonance frequency (wres —► Wres + Aw) which is shifted by a frequency shift 384 (Aw, which may be positive or negative in value) determined by the modulator 24. Figure 47(b) (lower graph) shows a schematic diagram of the transmission spectrum 386 of the ring resonator when in the state in which a photon is detected by the single-photon detector. That is to say, the frequency (wsignai) of the optical signal input to optical switch is excluded from the resonance bandwidth 388 of the ring resonator. This means that the optical signal upon an optical pathway of optical switch optically coupled to the ring resonator is no longer resonantly coupled into the ring resonator. This may be used to control how incoming optical signals in the switch are redistributed provide a corresponding outgoing signal, thus enacting a single-photon triggered rerouting of optical signals. Alternatively, the transmission spectrum 387 of a ring resonator may be as shown schematically in Fig. 48(a) (upper graph), when in the state for which the single-photon detector 22 produces no electrical signal and the optical modulator 24 has its “rest” default effect on the ring resonator. That is to say, the frequency (wsignai) of the optical signal within the optical switch is excluded from the resonance bandwidth 389 of the ring resonator (i.e., Wres = wSignai+ Aw). This means that the optical signal within the optical switch is not resonantly coupled into the ring resonator. Fig. 48(b) (lower graph) shows a schematic diagram of the transmission spectrum 394 of the ring resonator when in the state when the single-photon detector 22 detects a photon and creates an output voltage signal which is used to drive the optical modulator 24, which in turn changes the resonant frequency of the ring resonator. That is to say, the frequency (wsignai) of the optical signal within the optical switch is included in the resonance bandwidth 393 of the ring resonator (i.e., Wres = Wsignai). This means that the optical signal within optical switch may be resonantly coupled into the ring resonator. The resonant frequency of a ring optical resonator in any example of the invention may be modulated / driven by an electrical signal (e.g., voltage) output from e.g., a superconducting nano-wire single-photon detector (SNSPD) upon the detection of a single photon. When given in terms of a voltage, this electrical signal may be expressed as: V (n - Zn410G / 2°-----7T-7T777- v J v 14. e-2 ln(9K / Tr Here, rd is the decay time, Tr is the rise time, Zo is the SNSPD circuit impedance, G is the SNSPD amplifier gain in dB, and lb is the bias current. Figure 49 shows the temporal shape of the electrical signal output by the conducting nano-wire single-photon detector (SNSPD) upon the detection of a single photon. Figures 50A to 50C show schematic diagrams of different implementations of an optical resonator according to embodiments of the invention. Note that the embodiments disclosed herein each employ a linear waveguide coupled to a looping optical resonator structure (e.g., a circular ring structure in these examples). However, the linear waveguide may, in other embodiments, be coupled to a linear optical resonator structure. Examples, which are not intended to be exhaustive, are shown schematically in figures 50A to 50C. For example, Figure 50A shows a linear optical resonator structure 450A formed within the material of the core of an optical waveguide (or at least a section of one). A resonator optical cavity 453 is formed in the linear space between two fibre Bragg grating structures 452 (e.g., distributed Bragg reflectors) formed within the core of the waveguide and separated by linear separation along the axis of the waveguide to define a linear grating-free region extending along the core of the waveguide between the two separated fibre Bragg grating structures. Light 462 input to the linear optical resonator structure 450A is coupled into the resonator optical cavity 453 via a first one of the two separated Bragg grating structures 452 and, depending upon the parameters of the resonator, may resonate 464 within the resonator optical cavity 453 or may pass through the cavity 453 as output light 462. The electro-optical modulator 426 (e.g., serving the same purpose as item 28 of Fig. 1, Fig. 2, etc.) is arranged to apply an electrical signal generated by the single-photon detector (e.g., item 24 of Fig.1, Fig. 2, etc.) with which to modulate the refractive index of the material of the core of the optical waveguide defining the optical cavity 453 thereby to cause a shift in the frequency at which the resonance spectral profile of the linear optical resonator structure 450A is centred or positioned, as discussed above. The linear optical resonator structure 450A may be formed within the input optical waveguide (8) of the optical switch / router described herein, with the optical ring resonator (e.g., item 14, Fig. 1) omitted. Figure 50B shows an alternative linear optical resonator structure 450B formed within a dielectric stack defining an optical waveguide (or at least a section of one). A resonator optical cavity 454 is formed in the linear space between two distributed Bragg reflectors 456 formed by the dielectric stack and separated by linear separation along the axis of the stack to define a linear grating-free region extending along the axis of the stack between the two separated distributed Bragg reflectors 456. Light 462 input to the linear optical resonator structure 450B is coupled into the resonator optical cavity 454 via a first one of the two separated distributed Bragg reflectors 456 and, depending upon the parameters of the resonator, may resonate 464 within the resonator optical cavity 454 or may pass through the cavity 454 as output light 462. The electro-optical modulator 426 (e.g., serving the same purpose as item 28 of Fig. 1, Fig.2, etc.) is arranged to apply an electrical signal generated by the single-photon detector (e.g., item 24 of Fig.1, Fig. 2, etc.) with which to modulate the refractive index of the material of the core of the dielectric stack defining the optical cavity 454 thereby to cause a shift in the frequency at which the resonance spectral profile of the linear optical resonator structure 450B is centred or positioned, as discussed above. The linear optical resonator structure 450B may be formed within the input optical waveguide 8 of the optical switch / router described herein, with the optical ring resonator (e.g., item 14, Fig. 1) omitted. Figure 50C shows an alternative linear optical resonator structure 450C formed by a photonic crystal defect structure within a dielectric material of an optical waveguide (or at least a section of one). The photonic crystal is formed as a periodic linear array of cylinders of a first dielectric material (e.g., a solid substance, or an airgap, or void) embedded within a surrounding material of the waveguide which is a second dielectric material different the first dielectric material. A defect on the periodicity of the periodic linear array of cylinders is provided by the absence of one or more such cylinders where there would otherwise be such a cylinder(s) according to the periodicity of the array. This defect, namely the absence of one of more such cylinders from the periodic array, defines a resonator optical cavity 460 in the linear space between two separate sub-sections of the linear array of cylinders 458 separated by linear separation along the axis of the waveguide to define a linear cylinder-free region extending along the axis of the photonic crystal between the two separated sub-sections of the linear array of cylinders 458. Light 462 input to the linear optical resonator structure 450C is coupled into the resonator optical cavity 460 via a first one of the two separated sub-sections of the linear array of cylinders 458 and, depending upon the parameters of the resonator, may resonate 464 within the resonator optical cavity 460 or may pass through the cavity 460 as output light 462. The electro-optical modulator 426 (e.g., serving the same purpose as item 28 of Fig. 1, Fig.2, etc.) is arranged to apply an electrical signal generated by the singlephoton detector (e.g., item 24 of Fig. 1, Fig. 2, etc.) with which to modulate the refractive index of the dielectric material of waveguide defining the optical cavity 460 thereby to cause a shift in the frequency at which the resonance spectral profile of the linear optical resonator structure 450C is centred or positioned, as discussed above. The linear optical resonator structure 450C may be formed within an optical waveguide 8 of the optical switch / router described herein, with the optical ring resonator (e.g., item 14, Fig. 1) omitted. It is to be noted that even though the examples provided above are given in terms of a looped optical resonator structures (e.g., a ring), the invention has general applicability to other structures of optical resonator such as a linear waveguide. Figures 51A to 51E each show a respective example of a waveguide structure suitable for use in forming the optical waveguides and resonator structures according to the invention. Figure 51A shows a buried channel waveguide which is formed with a high-refractive index (m) waveguiding core 471, of width “w” and depth “d”, buried in a low-refractive index ¢12) surrounding medium 470. The waveguiding core can have any cross-sectional geometry though it is often a rectangular shape. Figure 51B shows a strip-loaded waveguide 472 formed by loading a planar waveguide comprising a low-refractive index (n?) layer 474 beneath a high-refractive index (m) slab 473, which already provides optical confinement in the depth “d” direction, with a dielectric strip 475 of intermediate refractive index ns<ni or a metal strip to facilitate optical confinement in the width “w” direction. The waveguiding core of a strip waveguide is the high-refractive index (m) region under the loading strip 475, with its thickness determined by the thickness “d” of the high-refractive index (m) region, and its width “w” defined by the width of the loading strip 475. Figure 51C shows a ridge waveguide which has a structure similar to that of a strip waveguide, but in which a strip, or ridge 477 of width “w”, depth “d” and high-refractive index (m), is disposed on top of a planar structure 476 of low refractive index (n?), and this acts as the waveguiding core. A ridge waveguide has strong optical confinement because it is surrounded on three sides by low refractive index air (or cladding material). Figure 51D shows a rib waveguide has a structure similar to that of a strip or ridge waveguide, but differs in that the strip 481, of width “w” and height “h”, has the same high-refractive index (m) as the high index planar layer 480 beneath it and is part of the waveguiding core. The combined thickness of the planar layer 480 and the strip (of height “h”) is denoted as “d”. The planar layer is disposed on top of a planar structure 479 of low refractive index (ns). These four types of waveguides, shown in Figure 51A to 51D, are usually termed rectangular waveguides with a thickness “d” (or height “h”) in the x direction and a width “w” in the y direction, though their shapes are normally not exactly rectangular. Figure 51E shows a diffused waveguide formed by creating a high-index region 483, of depth “d” and width “w”, in a substrate 482 through diffusion of dopants. An example is a LiNbOa waveguide with a core of high-refractive index (m) formed by Ti diffusion into the substrate material of low refractive index (ns). Because of the diffusion process, the core boundaries in the substrate are not sharply defined. A diffused waveguide also has a thickness “d” defined by the diffusion depth of the dopant in the x direction and a width “w” defined by the distribution of the dopant in the y direction. Any of the waveguide structures noted above can be used in the invention. Preferable waveguide structures are rib, ridge and strip-loaded waveguides. Suitable dimensions are a depth or thickness of waveguide core in the range: d = about 200nm to about d = 800nm and a width of waveguide core in the range: w = about 300 to w = about 2000nm. The preferable wavelength of the optical signal (item 3, Fig. 1 etc.) to be guided by the optical waveguides of the invention may be a wavelength in the range of: about 700nm to about 1600nm. The bend radius of a looped resonator structure, such as any ring resonator disclosed herein, may be a bend radius (i.e., radius of curvature) in the range of: about 10pm to about 200pm. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +1-10%. References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein. [1] “Measurement of subpicosecond time intervals between two photons by interference”; C. K. Hong, Z. Y. Ou, and L. Mandel; Phys. Rev. Lett. 59, 2044 - Published 2 November 1987.

Claims

1. An optical switch for receiving an optical signal from an optical signal source and for outputting the optical signal, the optical switch comprising an optical router configured for routing the optical signal conditional on the detection of a single photon, the optical router comprising:an optical resonator;a first optical pathway part comprising the optical resonator, and a second optical pathway part;an optical input coupling part for receiving the optical signal and for coupling the received optical signal into the first optical pathway part and into the second optical pathway part;an optical output coupling part for receiving the optical signal via the first optical pathway part and for receiving the optical signal via the second optical pathway part such that the optical signal as received via the first optical pathway part optically interferes with the optical signal as received via the second optical pathway part, and for outputting the result of the optical interference to the first optical pathway part and to the second optical pathway part; and,an optical modulator coupled to a single-photon detector unit configured to output a detection signal in response to said detection of a single photon, the optical modulator being configured, in response to the detection signal, to modulate the refractive index of the optical resonator to change an optical phase shift incurred by the optical signal upon passage through the optical resonator such that said optical interference results in a routing of said output optical signal to one of said first and second optical pathway parts selected conditional on said detection of a single photon.

2. An optical switch according to any preceding claim wherein the second optical pathway part comprises an optical phase-shifter part configured to impose a pre-set optical phase shift incurred by the optical signal upon passage through the optical phase-shifter part to the optical output coupling part.

3. An optical switch according to claim 2 wherein the pre-set optical phase shift is about (2n + 1)tt / 2 radians, where n is any integer.

4. An optical switch according to any preceding claim wherein the optical phase-shifter part comprises an optical resonator.

5. An optical switch according to any preceding claim wherein the optical input coupling part, the first optical pathway part, the second optical pathway part and the optical output coupling part collectively form a Mach-Zehnder interferometer.

6. An optical switch according to any preceding claim wherein the first optical pathway part defines a first interferometer arm of the Mach-Zehnder interferometer and the second optical pathway part defines a second interferometer arm of the Mach-Zehnder interferometer.

7. An optical switch according to any preceding claim wherein the optical resonator of the first optical pathway part comprises an asymmetric pair of optically coupled optical resonators comprising a primary optical resonator and secondary optical resonator whereby a quality factor of the primary optical resonator differs from a quality factor of the secondary optical resonator.

8. An optical switch according to claim 7 wherein the optical modulator comprises a plurality of separate optical modulator parts wherein, in response to the detection signal, each optical modulator part is configured simultaneously to modulate the refractive index of a respective one of the primary optical resonator and the secondary optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the respective optical resonator.

9. An optical switch according to any preceding claim wherein the optical resonator of the first optical pathway part defines a first optical resonator and the second optical pathway part comprises a second optical resonator, and the optical modulator comprises a first optical modulator part and a second optical modulator part wherein, in response to the detection signal, the first optical modulator part is configured to modulate the refractive index of said first optical resonator and simultaneously the second optical modulator part is configured to modulate the refractive index of said second optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the first and second optical resonators.

10. An optical switch according to claim 9 when dependent upon claim 7 wherein the first optical resonator and the second optical resonator each comprise said asymmetric pair of optically coupled optical resonators, and the optical modulator comprises a plurality of separate optical modulator parts wherein, in response to the detection signal, each optical modulator part is configured simultaneously to modulate the refractive index of the secondary optical resonator of a respective one of the first optical resonator and the second optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the secondary optical resonator thereof.

11. An optical switch according to any preceding claim wherein the optical input coupling part is configured for coupling the received optical signal into the first optical pathway part and into the second optical pathway part so as to impart a pre-set optical phase shift upon the optical signal coupled into the first optical pathway part relative to the optical signal coupled into the second optical pathway part.

12. An optical switch according to any preceding claim wherein the optical input coupling part is configured to impart a pre-set optical phase shift upon the received optical signal and for coupling the result into the first optical pathway part and into the second optical pathway part so as to impartsubstantially no optical phase difference upon the optical signal coupled into the first optical pathway part relative to the optical signal coupled into the second optical pathway part.

13. An optical switch according to any of claims 11 and 12 wherein the pre-set optical phase difference is about (2n + 1)tt / 2 radians, where n is any integer.

14. An optical switch according to any preceding claim wherein the second optical pathway part comprises an optical resonator configured to impose a pre-set optical phase shift incurred by the optical signal upon passage through the optical phase-shifter part to the optical output coupling part.

15. An optical switch according to any preceding claim wherein the optical input coupling part comprises a directional optical coupler and the optical output coupling part comprises a directional optical coupler.

16. An optical switch according to any preceding claim wherein the optical input coupling part comprises a first optical resonator and the optical output coupling part comprises a second optical resonator.

17. An optical switch according to claim 16 wherein the second optical resonator comprises said optical resonator comprising a part of the first optical pathway part.

18. An optical switch according to claim 16 or claim 17 wherein said optical resonator comprising a part of the first optical pathway part is optically coupled to the second optical pathway part.

19. An optical switch according to any of claims 16 to 18 wherein the optical modulator comprises a first optical modulator part and a second optical modulator part wherein, in response to the detection signal, the first optical modulator part is configured to modulate the refractive index of said first optical resonator and simultaneously the second optical modulator part is configured to modulate the refractive index of said second optical resonator to change a respective optical phase shift incurred by the optical signal upon passage through the first and second optical resonators.

20. An optical switch according to claim 19 wherein the first optical modulator part and the second optical modulator part are configured respectively to apply a modulation to change the refractive index of said first optical resonator and said second optical resonator by substantially the same change.

21. An optical switch according to claim 20 wherein the first optical modulator part and the second optical modulator part are configured respectively to apply a modulation to change the refractive index of said first optical resonator and said second optical resonator by opposite respective changes of substantially the same size.

22. An optical switch according to any of claims 16 to 21 wherein the optical resonator comprises a plurality of separate optical resonators including said optical resonator comprising a part of the firstoptical pathway part, and the optical modulator comprises a plurality of separate optical modulator parts wherein, in response to the detection signal, each optical modulator part is configured simultaneously to modulate the refractive index of a respective one of the plurality of optical resonators to change a respective optical phase shift incurred by the optical signal upon passage through the respective optical resonator.

23. An optical switch according to claim 22 wherein the plurality of separate optical resonators comprises one or more optically coupled pairs of optically coupled optical resonators in which each pair of optically coupled optical resonators comprises at least one of said first optical resonator and said second optical resonator.

24. An optical switch according to claim 23 wherein the plurality of separate optical resonators comprises a pair of optically coupled optical resonators in which one optical resonator of the pair of optical resonators comprises said first optical resonator and said second optical resonator, the plurality of separate optical resonators further comprising a third optical resonator and a fourth optical resonator wherein the first optical pathway part comprises the third optical resonator and the second optical pathway part comprises the fourth optical resonator.

25. An optical switch according to any preceding claim wherein the optical signal comprises a given optical signal frequency bandwidth, and wherein one or each said optical resonator is configured to resonate at resonant optical frequencies within a resonance bandwidth determined by a refractive index of the optical resonator, and the optical modulator is configured to modulate the refractive index of the optical resonator so as to modulate the resonance bandwidth to either:change from being a resonance bandwidth centred at an optical frequency lower than a centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth; or,change from being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency higher than or lower than the centre of the optical signal frequency bandwidth; or,change from being a resonance bandwidth centred at an optical frequency higher than the centre of the optical signal frequency bandwidth to being a resonance bandwidth centred at an optical frequency lower than the centre of the optical signal frequency bandwidth.

26. An optical switch according to any preceding claim wherein the optical signal comprises a given optical signal frequency bandwidth, and wherein one or each said optical resonator is configured to resonate at resonant optical frequencies within a resonance bandwidth determined by a refractive index of the optical resonator, and the optical modulator is configured to modulate the refractive index of the optical resonator so as to modulate the resonance bandwidth to either:change from being a resonance bandwidth that includes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that excludes the centre of the optical signal frequency bandwidth; or,change from being a resonance bandwidth that excludes a centre of the optical signal frequency bandwidth to being a resonance bandwidth that includes the centre of the optical signal frequency bandwidth.

27. An optical switch for receiving an optical signal from an optical signal source and for outputting the optical signal, the optical switch having;an optical router configured for routing the optical signal conditional on the detection of a single photon, the optical router comprising a Mach-Zehnder optical interferometer comprising:an Input optical coupling part for receiving the optical signal and splitting the received optical signal into a first optical signal portion and a second optical signal portion;a first optical interferometer arm and a second optical interferometer arm for receiving, respectively, the first and second optical signal portions;an optical resonator optically coupled to the first interferometer arm and configured to resonate at optical frequencies within a resonance bandwidth determined by a refractive index of the optical resonator;an output optical coupler for receiving and recombining the first and second optical signal portions from the first and second optical interferometer arms;an optical modulator coupled to a single-photon detector unit configured to output a detection signal in response to said detection of a single photon, the optical modulator being configured, in response to the detection signal, to modulate the refractive index of the optical resonator to change an optical phase shift incurred by the first optical signal portion by passage through the optical resonator by at least about 3tt / 4 radians such that said recombining of the two signal portions by the output optical coupler results in a routing of said output optical signal to one of two optical output ports thereof selected conditional on said detection of a single photon.

28. An optical switch according to claim 27 wherein the modulation signal is configured to modulate the refractive index of the optical resonator to change an optical phase shift incurred by the first optical signal portion by passage through the optical resonator by about it radians.

29. An optical switch according to any preceding claim wherein each said optical resonator comprises a ring resonator.

30. An optical switch according to any preceding claim comprising the optical signal source.

31. An optical or photonic circuit or chip comprising the optical switch according to any preceding claim.

32. A photonic information processor circuit or chip, or a photonic quantum computer circuit or chip, or a photonic communications circuit or chip, comprising the optical switch according to any preceding claim.

33. An optical switching method for receiving an optical signal from an optical signal source and for outputting the optical signal for routing the optical signal conditional on the detection of a single photon, the method comprising:providing an optical resonator;providing a first optical pathway part comprising the optical resonator, and a second optical pathway part;providing an optical modulator coupled to a single-photon detector unit configured to output a detection signal in response to said detection of a single photon; and,by an optical Input coupling part, receiving the optical signal and coupling the received optical signal into the first optical pathway part and into the second optical pathway part;by an optical output coupling part, receiving the optical signal via the first optical pathway part and receiving the optical signal via the second optical pathway part such that the optical signal as received via the first optical pathway part optically interferes with the optical signal as received via the second optical pathway part, and outputting the result of the optical interference to the first optical pathway part and to the second optical pathway part; and,by the optical modulator, in response to the detection signal, modulating the refractive index of the optical resonator to change an optical phase shift incurred by the optical signal upon passage through the optical resonator such that the optical interference results in a routing of the output optical signal to one of said first and second optical pathway parts selected conditional on said detection of a single photon.

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