Photonic switch system and method

Abstract

A system comprises a photonic circuit and control logic. The photonic circuit comprises a number of input paths and output paths, each output path associated with a corresponding frequency bin. A photonic switch network is coupled between the input paths and the output paths and comprises an arrangement of active optical elements. The control logic is configured to: receive an input signal indicative of the presence of one or more incoming photons in a corresponding one or more of the input paths; select one or more output paths; and generate one or more control signals to configure the active optical elements such that, for each of the one or more selected output paths, a photon of the one or more incoming photons is coupled to that selected output path. Methods and computer readable media are also described.

Description

Photonic Switch System and MethodTechnical Field

[0001] The present disclosure relates to photonic systems, and related controllers and methods.Background

[0002] Photonic qubits are a promising candidate for scalable universal quantum computing. Photonic platforms provide many advantages including the ability to facilitate quicker gate operations compared to the decoherence time of quantum information, fast read-out measurements, and efficient qubit transfer. Linear optical quantum computing tends to use only beam splitters, phase shifters and photon detectors to process the quantum information encoded in light. Furthermore, photonic systems can largely operate at room temperature. Measurement-based quantum computing (MBQC), in which a large entangled multiqubit state known as a computational lattice is generated and then adaptive single-qubit measurements are performed on said computational lattice to carry out an algorithm, is one of the most promising paths towards universal quantum computing.

[0003] However, photonic quantum computing is hindered by the fact that many of the underlying processes are inherently probabilistic. As a first example, single-photon sources typically rely on non-linear processes such as spontaneous parametric down conversion (SPDC) or spontaneous four-wave mixing (SFWM). These non-linear processes are probabilistic and so it is challenging to simultaneously achieve a high probability of producing a photon and a high single-photon fidelity. To improve the single-photon fidelity of the heralded photon, such sources are often operated in the regime of low -excitation probability, which in turn means that the time at which a single heralded photon will be produced is highly unpredictable. As a second example, constructing a computational lattice reliably is hindered by a lack of deterministic entangling operations. Instead, probabilistic measurement-based operations must be performed in order to entangle photonic qubit states.

[0004] One way to mitigate the effects of probabilistic photon production and probabilistic entangling operations is to scale the number of resources (single -photon sources, photon detectors etc.) accordingly. However, this can be large and costly.

[0005] The present disclosure mitigates one or more problems of the prior art.Summary

[0006] According to an aspect of the present disclosure a system is provided. The system comprises a photonic circuit. The photonic circuit comprises a number of input paths. The photonic circuit further comprises a number of output paths, each output path associated with a corresponding frequency bin. The photonic circuit further comprises a photonic switch network coupled between the input paths and the output paths, the photonic switch network comprising an arrangement of active optical elements to selectively couple a photon from any one of the input paths to any one of the output paths based on a frequency of that photon. The system further comprises control logic coupled to the photonic switch network. The control logic is configured to receive an input signal indicative of the presence of one or more incoming photons in a corresponding one or more of the input paths. The control logic is further configured to select, based on the received input signal, one or more output paths, each of the one or more selected output paths associated with a corresponding frequency bin that includes a frequency of the one or more incoming photons. The controller is further configured to generate one or more control signalsfor configuring the active optical elements such that, for each of the one or more selected output paths, a photon of the one or more incoming photons is coupled to that selected output path.

[0007] Advantageously, the system enables one to couple photons from any input path to any output path based on an input signal and the frequency of the photon(s) in the input paths. In circumstances in which the input paths hold photons of different frequencies, the system accordingly is operable as a non-blocking switching fabric, capable of coupling multiple inputs to multiple outputs at the same time. This provides switching rate improvements over, for example, blocking switching fabrics.

[0008] The system may further comprise a number of outbound frequency converters, each frequency converter arranged to receive a photon from a respective output path, and to output a photon having a predefined frequency, wherein the frequency converters are configured to output photons having the same predefined frequency. The term “outbound” here is understood to be a label indicating that a frequency converter is coupled to an output path, and is not otherwise limiting. Advantageously, the output photons may accordingly be indistinguishable from one another, which is a useful property for many applications in cryptography, computational lattice generation, and quantum computing applications. Further advantageously, the frequency converters may be used to output photons having a predefined frequency that is compatible with the materials used in downstream photonic circuitry.

[0009] The one or more frequencies of the one or more incoming photons may be from a predefined set of input frequencies. For example, each of the incoming photons may have an input frequency from a fixed alphabet of input frequencies. In some examples, different input paths may be associated with different input frequencies. The incoming photons in different input paths may have different input frequencies. For example, any incoming photon(s) in each input path of a plurality of K input paths may have an input frequency different from the input frequencies of any incoming photon(s) in the other (K - 1) input paths.

[0010] The frequency bins associated with at least two of the selected output paths may be disjoint. Advantageously, this enables the system to be operable in a non-blocking manner.

[0011] The input signal may be indicative of a respective one or more frequencies of the one or more incoming photons. The input signal may be further indicative of, for example, photon number.

[0012] The photonic switching network may comprise a generalised Mach-Zehnder interferometer.

[0013] Each input path of the photonic circuit may be coupled to an output path of a different one of a set of heralded multi-spectral single-photon sources. The input signal may comprise heralding signals from one or more of the multi-spectral single photon sources. The system may comprise the set of heralded multi-spectral singlephoton sources. Each multi-spectral single-photon source may be configured to output a heralding signal and a signal photon.

[0014] As described further above, heralded single photon generation is a probabilistic phenomenon and so it is difficult to predict when a particular single photon source will emit a single photon. A multi -spectral single-photon source is capable of randomly emitting single photons having a frequency from among a plurality of resonant frequencies, and accordingly can boost the rate of single photon emission in comparison to single frequency mode sources. A suitable set of multi-spectral single-photon sources may reliably generate enough photons to ensure that at least one photon of each resonant frequency is generated in each time interval. By coupling each input path of the photonic circuit to an output path of a different one of a set of heralded multi-spectral single-photon sources,and by providing enough heralded multi-spectral single-photon sources, the system may be used to reliably produce single photons in multiple known output paths of the photonic switch network in every time interval.

[0015] Each multi-spectral single-photon source may comprise a photon pair source for generating frequency- correlated photon pairs across a range of frequencies, each photon pair comprising a signal photon and a heralding photon. For example, a photon pair source may comprise a cavity parametric down conversion photon pair source. For example, a photon pair source may comprise a spontaneous four wave mixing (SFWM) source. Each multi- spectral single-photon source may further comprise a spectral demultiplexer for guiding heralding photons along a plurality of frequency -dependent optical paths / lightpaths. Each multi-spectral single-photon source may further comprise a plurality of photon detectors, wherein each of the photon detectors is coupled to a respective frequency dependent optical path / lightpath of the spectral demultiplexer such that a particular photon detector is associated with the detection of a heralding photon having a frequency within a particular frequency range.

[0016] The system may further comprise a plurality of inbound frequency converters, each frequency converter located on an input path of the photonic circuit. The term “inbound” here is understood to be a label indicating that a frequency converter is coupled to an input optical path (as opposed to an output optical path) of the photonic switching network. Each inbound frequency converter is configurable to receive an inbound photon having an inbound frequency (the term “inbound” here being used as a label only), and to output a photon having a corresponding input frequency as an incoming photon to the photonic switch network. The “inbound” frequency (frequency of photon received by the frequency converter) is different to the “input frequency” (frequency of photon received by the photonic switch network). Advantageously, the plurality of inbound frequency converters may be used to ensure that incoming photons in different input paths have different frequencies and can accordingly all be coupled to output paths at the same time.

[0017] The control logic may be further coupled to the plurality of inbound frequency converters. The control logic may be further configured to, based on the received input signal, generate one or more control signals to configure at least one of the frequency converters to, upon receiving an inbound photon having the inbound frequency, output a photon having the corresponding input frequency as an incoming photon to the photonic switch network. Advantageously, the control logic may accordingly dynamically adjust the frequencies of incoming photons in order to ensure that as many photons can be passed through the photonic switch network in a given time interval as possible.

[0018] The input frequencies may all be different from one another.

[0019] Each input path and each output path may comprise a plurality of waveguides. The system may further comprise a linear optical circuit configured to probabilistically generate an entangled resource state comprising a plurality of dual-rail encoded photonic qubits. The input signal may be further indicative of the successful generation of an entangled resource state.

[0020] The photonic circuit may be implemented on-chip.

[0021] At least a portion of the system may be implemented in bulk optics and optical fibre.

[0022] According to an aspect of the present disclosure, a method is provided. The method comprises receiving an input signal indicative of the presence of one or more incoming photons in a corresponding one or more input paths of a photonic circuit. The method further comprises selecting, based on the received input signal, one or more output paths of the photonic circuit, each of the one or more selected output paths associated with a corresponding frequency bin that includes a frequency of the one or more incoming photons. The method furthercomprises generating one or more control signals for configuring a photonic switch network of the photonic circuit such that, for each of the one or more selected output paths, a photon of the one or more incoming photons is coupled to that selected output path.

[0023] In some examples, the input signal may be indicative of a respective one or more frequencies of the one or more incoming photons.

[0024] In some examples, the method may further comprise, based on the received input signal, generating one or more control signals to configure at least one frequency converter to, upon receiving an inbound photon having an inbound frequency, output a photon having a corresponding input frequency as an incoming photon to the photonic switch network.

[0025] According to an aspect of the present disclosure, a controller is provided. The controller is configured to implement the functionality of control logic as described herein.

[0026] According to an aspect of the present disclosure, a computer-readable storage medium is provided. The computer-readable storage medium has stored thereon a computer-readable circuit description of a controller for implementing the functionality of control logic as described herein. The computer-readable circuit description, when processed in a controller generation system, causes the controller generation system to manufacture or otherwise generate an implementation of the controller.

[0027] Many modifications and other embodiments of the disclosure will come to mind to a person skilled in the art to which these disclosures pertain in the light of the teachings herein. Therefore, it will be understood that the present disclosure herein is not limited to the specific embodiments disclosed herein. Moreover, although the description provided herein provides several example embodiments in the context of certain combinations of elements, steps and / or functions may be provided by alternative embodiments without departing from the scope of the disclosure.Brief Description of the Figures

[0028] Illustrative embodiments of the present disclosure will now be described, by way of example only, with reference to the drawings. In the drawings:Fig. 1 illustrates a block diagram of a system according to an example of the present disclosure;Fig. 2A shows a photonic switch network according to an example;Fig. 2B shows an active phase modulation circuit of the photonic switch network of Fig. 2A according to an example;Fig. 3 shows a diagram of a system according to an example;Fig. 4 shows a multi-spectral single-photon source according to an example;Fig. 5 shows a diagram of a system according to an example;Fig. 6 shows a diagram of a system according to an example; andFig. 7 shows a linear optical circuit for producing an entangled resource state according to an example;Fig. 8 shows an active phase modulation circuit of a photonic switch network according to an example; andFig. 9 shows an active phase modulation circuit of a photonic switch network according to an example.

[0029] Throughout the description and the drawings, like reference numerals refer to like parts.Detailed Description

[0030] Embodiments of the disclosure are described with reference to the accompanying drawings. However, it should be appreciated that the disclosure is not limited to the embodiments, and all changes and / or equivalents or replacements thereto also belong to the scope of the disclosure. The same or similar reference denotations may be used to refer to the same or similar elements throughout the specification and the drawings.

[0031] As used herein, the terms “have”, “may have”, “include”, or “may include” a feature (e.g. a number, function, operation, or a component such as a part) indicate the existence of the feature and do not exclude the existence of other features. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0032] As used herein, the terms “A or B”, “at least one of A and / or B”, or “one or more of A and / or B” may include all possible combinations of A and B. For example, “A or B”, “at least one of A or B”, “at least one of A and B” may indicate all of (1) including at least one A, (2) including at least one B, or (3) including at least one A and at least one B.

[0033] As used herein, the terms “first” and “second” may modify various components regardless of importance and do not limit the components. These terms are only used to distinguish one component from another. For example, reference to a first component and a second component may indicate different components from each other regardless of the order or importance of the components.

[0034] It will be understood that when an element (e.g. a first element) is referred to as being (physically, operatively or communicatively) “coupled with / to”, or “connected with / to” another element (e.g. a second element), it can be coupled with / to the other element directly or via a third element. In contrast, it will be understood that when an element (e.g. a first element) is referred to as being “directly coupled with / to” or “directly connected with / to” another element (e.g. a second element), no element (e.g. a third element) intervenes between the element and the other element.

[0035] The terms as used herein are provided merely to describe some embodiments thereof, but not to limit the scope of other embodiments of the disclosure. It is to be understood that the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. All terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of the disclosure belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealised or overly formal sense unless expressly so defined herein.

[0036] Fig. 1 shows a block diagram of a system 100 according to an embodiment of the present disclosure. The system 100 comprises a photonic circuit 102 comprising a number K of input paths 104-1, 104-2,..., IO4- / < and a number L of output paths 106-1, 106-2, ... , 106-L. The photonic circuit 102 further comprises a photonic switch network 108 coupled between the input paths 104 and the output paths 106, the photonic switch network 108comprising an arrangement of active optical elements (not shown in Fig. 1) for selectively coupling a photon from any one of the input paths 104 to any one of the output paths 106 based on the frequency of the photon.

[0037] The term “photonic circuit” as used herein is intended to be interpreted broadly as any technology capable of generating, transporting, processing and / or detecting light. A photonic circuit may accordingly comprise any number of passive or active photonic elements / components such as waveguides, phase shifters, beam splitters, photon sources, photon detectors and so on. For example, the photonic circuit 102 may comprise a photonic integrated circuit, with each input path 104 and each output path 106 respectively comprising one or more waveguides etched or otherwise defined in the photonic integrated circuit. The photonic integrated circuit may be fabricated in a suitable combination of layers on a substrate using materials including, for example, indium phosphide, silicon, silicon nitride and / or thin film lithium niobate. As another example, the photonic circuit 102 may be implemented in bulk optics as an optical assembly / optical train utilising free space or fibre interconnects.

[0038] The term active optical element as used herein is intended to be interpreted broadly as any feature in the photonic circuit that may require some type of external energy either to perform their functions or to be used over a wider operating range than a passive device. For example, an active optical element may comprise an active phase shifter, a variable optical attenuator, tuneable optical filter, dynamic gain equalizer, optical add / drop multiplexer, polarization controller, or dispersion compensators.

[0039] The input paths 104 and output paths 106 may be suitable for carrying one or more photons. For example, each input (output) path may comprise a single waveguide suitable for carrying a single photon, Alternatively, each input (output) path may comprise a plurality of waveguides suitable for carrying a multi-photon quantum state.

[0040] Each of the output paths is associated with a corresponding frequency / wavelength bin. This is indicated in Fig. 1 by the symbols { / / } with j = 1,2, ... , L next to the corresponding output path 106- / , which indicates that the output path is associated in some way with a small frequency band that includes the frequency / ). In some examples, the association between each output path and its corresponding frequency / wavelength bin may be embodied physically in the photonic circuit 102, for example, the photonic circuit 102 may include filters for each output path such that only a photon having a frequency in the corresponding (unfiltered) frequency range may pass along that output path. In some examples, the association between each output path and its corresponding frequency / wavelength bin may be defined by the control logic applied to the photonic circuit - for example, the control logic may define that photons of a particular frequency range are to be directed towards a particular output path and configure the active photonic elements of the photonic switch network accordingly. As depicted in Fig. 1, the frequency bins may be disjoint i.e. each output path may be associated with different, non-overlapping, frequency ranges. Generally speaking, the number L of output paths 106 is fewer than or equal to the number K of input paths 104.

[0041] The photonic switch network 108 is coupled between the input paths 104 and the output paths 106 and comprises a plurality of active optical elements for selectively coupling a photon from any one of the input paths to any one of the output paths based on the frequency of that photon.

[0042] The system 100 further comprises control logic 110. The control logic 110 may be implemented in any suitable form as a controller, for example in hardware such as an integrated circuit. While a controller may be any kind of general or dedicated processor, such as a central processing unit (CPU), a graphics processing unit (GPU), or an integrated circuit, due to the speeds required it may be preferable to implement the control logic 110 in adedicated, application-specific processing unit. For example, the control logic 110 may be implemented in an application-specific integrated circuit (ASIC) or an application-specific standard product (ASSP) or another domain-specific architecture (DSA). Alternatively, the control logic 110 may be implemented in adaptive computing hardware (that is, hardware comprising configurable hardware blocks / configurable logic blocks) that has been configured to perform the required functions, for example in a configured field programmable gate array (FPGA).

[0043] The control logic 110 is coupled to the photonic switch network 108. The control logic is configured to receive an input signal 112 indicative of the presence of one or more incoming photons in one or more of the input paths 104. In some examples, the indication of the one or more frequencies (referred to herein as “input frequencies”) of the one or more incoming photons may be contained within the input signal, for example the input signal may contain information defining the frequency of each photon. In some examples, the indication of the one or more input frequencies may be inferable from the input signal, for example each input path may be associated with a particular input frequency and the input signal 112 may contain only information as to which input path 104 a photon is in. As will be appreciated by the skilled person, the indication of the frequency of a photon may comprise an indication of a small bandwidth within which lies the frequency of the photon. In some examples, as described further below, the control logic 110 may be configured to control the input frequencies of the incoming photons.

[0044] The control logic 110 is further configured to select, based on the received input signal 112, one or more output paths 106, each of the one or more selected output paths 106 associated with a corresponding frequency bin that includes a frequency of the one or more incoming photons. The control logic 110 is further configured to generate one or more control signals 114 for configuring the active optical elements such that, for each of the one or more selected output paths, a photon of the one or more incoming photons is coupled through the photonic switch network 108 to that selected output path.

[0045] Advantageously, the system 100 may be utilised as a non-blocking photonic switching fabric.

[0046] Figs. 2A and 2B illustrate an example of a photonic switch network 200 according to an example, although the skilled person would appreciate that other architectures may be used. In this example, the photonic switch network 200 comprises a generalised Mach-Zehnder interferometer.

[0047] The photonic switch network 200 comprises a first multimode interference device 210 having a number K of input ports 205 and a number K of output ports 215, a second multimode interference device 230 having a number K of input ports 225 and a number L of output ports 235, and a number K of active phase modulation photonic circuits 220 arranged therebetween.

[0048] In this example, it shall be assumed that each input path 104 / output path 106 comprises one waveguide. Each input port (205, 225) / output port (215, 235) is coupled to a waveguide (indicated with dotted lines in the figure) for carrying a photon. Accordingly, the waveguide entering each input port 205 of the first multimode interference device 210 may correspond to an input path 104 of the system 100. Similarly, the waveguide coupled to each output port 235 of the second multimode interference device 230 may correspond to an output path 106 of the system 100.

[0049] The first multimode interference device 210 is configured to receive an incoming photon in at least one of the input ports 205, each of the incoming photons having a different frequency . The first multimode interferencedevice 210 is further configured transform each input photon into a superposition state across all output paths, in the form of a discrete Fourier transform (DFT), in the spatial mode basis.

[0050] A multimode interference device may be implemented in any suitable manner. For example, when the photonic circuit is implemented in integrated photonics, a multimode interference device may comprise a multimode interferometer (MMI) or multimode interference coupler comprising an optical waveguide. The input paths 205-1 to 205-K and output paths 215-1 to 215-K may each comprise a (narrow) optical waveguide, and the multimode interference coupler may comprise a (broader) optical waveguide, the size and shape of which has been designed to perform a Fourier transform.

[0051] Each of the output ports 215 of the first multimode interference device 210 is coupled to an active phase modulation circuit 220, an example of which is illustrated in Fig. 2B.

[0052] For the purposes of the present discussion, it is assumed that the incoming photons may have an input frequency from a fixed alphabet of S input frequencies. The number S of input frequencies may be the same as the number L of output paths 106.

[0053] Each active phase modulation circuit 220 comprises a spectral demultiplexer 240 for guiding the different frequency components of a quantum superposition state along different frequency -dependent optical paths, indicated by the S waveguides in Fig. 2B. This enables the different frequency components of each quantum superposition state to be acted upon independently. The spectral demultiplexer 240 may comprise a diffraction grating, a virtually imaged phased array, an arrayed waveguide grating (AWG), a dense wavelength division multiplexer (DWDM) or any other component or element capable of spatially separating the different frequency components.

[0054] Each of the frequency -dependent optical paths is coupled to an active optical element in the form of an electro-optical phase modulator 250 configured to impart a phase shift on the received frequency component in response to a control signal 114 generated by the control logic 110. Each phase modulator 250 may comprise, for example, non-linear optical material between electrodes that apply a controllable electric field to impart a phase shift.

[0055] Each of the frequency -dependent optical paths are also coupled to a spectral multiplexer 260 for recombining the different frequency components and outputting the recombined state to an input port 225 of the second multimode interference device 230. The spectral multiplexer 260 may be implemented using any element or component capable of spectral multiplexing, such as a virtually imaged phase array or dense wavelength division multiplexer.

[0056] The second multimode interference device 230 is configured to receive the phase-adjusted quantum superposition states at the input ports 225, to interfere the states, and to accordingly couple different spectral modes to different output ports 235. The second multimode inference device 230 may accordingly implement a (inverse) discrete Fourier transform.

[0057] By tuning the phase shifts imparted by the phase modulators 250, the interference in the second multimode interference device 230 can be influenced. In use, the control logic 110 is configured to generate one or more control signals 114 to control the phase shifts applied to each spectral mode within each active phase modulation circuit 220. Accordingly, with a suitable choice of phase shift parameters, the control logic 110 is able to output to each output path 106 light having the frequency associated with that output port 106.

[0058] Fig. 3 shows a block diagram of a system 300 according to an embodiment of the present disclosure.

[0059] For many purposes it is desirable to produce a plentiful supply of single photons contemporaneously - that is, substantially simultaneously and within the same photon pair generation clock cycle (e.g within the pump pulse duration for the photon pair source module) such that the photons may be interfered in downstream photonic circuitry. For example, in photonic quantum computing it is desirable to synchronously produce many single photons in known spatial and frequency modes and then to input these photons into one or more downstream entangling circuits in order to construct a computational lattice. Due to the inherently probabilistic nature of most single-photon sources, if one were to require a fixed number P of photons per clock cycle and only had that fixed number P of single-photon sources, then one may need to wait a long time for all P of the single-photon sources to produce a single photon at the same time, which in turn would drastically slow down the wall clock time of any calculation.

[0060] The system 300 enables single photons that are generated in a particular clock cycle to be coupled to output paths based on a frequency of those photons. Advantageously, with a suitable number of photon sources, the system can be used to ensure that the fixed number L of photons are produced in known output paths 106.

[0061] As with the system 100 of Fig. 1, the system 300 of Fig. 3 comprises a number K of input paths 104, a number L of output paths 106, and a photonic switch network 108 coupled therebetween. The input paths 104 and output paths 106 of system 300 each comprise a single waveguide for carrying a photon. In the specific example illustrated in Fig. 3, the number K of input paths 104 is eight and the number L of output paths 106 is four, although the skilled person would appreciate that this is a non-limiting example only. As with the system 100 of Fig. 1, the system 300 further comprises control logic 110 coupled to the photonic switch network 108 and configured to: receive an input signal 112 indicative of the presence of one or more incoming photons in a corresponding one or more of the input paths 104 and a respective one or more frequencies of the one or more incoming photons; select, based on the received input signal 112, one or more output paths 106, each of the one or more selected output paths associated with a corresponding frequency bin that includes a frequency of the one or more incoming photons; and generate one or more control signals 114 to configure the active optical elements of the photonic switch network 108 such that, for each of the one or more selected output paths, a photon of the one or more incoming photons is coupled to that selected output path.

[0062] In system 300, each input path 104 is coupled to an output path of a different one of a set 302 of heralded multi-spectral single photon sources (302-1, 302 -2, ... ,302-8). Each of the heralded multi-spectral single photon sources 302 is configured to probabilistically produce single photons having a frequency from among a plurality of different frequencies and to produce a heralding signal indicative of the production of that single photon and its frequency.

[0063] Referring to Fig. 4, an example of a heralded multi-spectral single photon source 400 is shown, that may perform the function of the multi-spectral single photon source(s) 302 of Fig. 3. The skilled person will appreciate that the features illustrated in Fig. 4 are not drawn to scale. The heralded multi-spectral single photon source 400 comprises a photon pair source 402 and a detection module 414.

[0064] The photon pair source 402 is configured to probabilistically generate pairs of frequency -entangled photons across a range of frequencies, each photon pair comprising a first photon 412 and a second photon 410. As used herein, the words “first” and “second” in relation to the photons of a photon pair, are intended to act as labels of the photons of a photon pair and are not intended to be limiting. For example, a photon pair source may generate a plurality of photon pairs, and therefore generate a plurality of first photons and a corresponding pluralityof second photons. Sometimes herein, the first photon 412 of a photon pair that is sent to a detector module 414 is referred to as a “heralding photon” and the second photon 410 is referred to as a “signal photon”.

[0065] In the example shown in Fig. 4, the photon pair source 402 comprises a cavity photon pair source. In particular, the cavity photon pair source comprises a non-linear material 404 provided inside a photonic cavity having optically reflective elements 406, 408. The optically reflective elements 406, 408 may comprise mirrors, for example bulk optic mirrors, or Bragg gratings. As an alternative, the cavity may comprise a non-linear crystal waveguide with end facets covered with a reflective coating.

[0066] The non-linear material 404 may have a second order (z(2)) non-linearity or a third order (z(3)) nonlinearity. The non-linear photonic material 404 may use spontaneous four wave mixing (SFWM) or spontaneous parametric down conversion (SPDC) to convert pump light into photon pairs. For SPDC the resulting photon pairs have lower frequencies and longer wavelength than the pump light. Examples of materials that may be used as the non-linear material 404 include any of, but not limited to: lithium iodate (LiIO3), fybcta barium borate (BaB2O4), bismuth triborate (BiB3O6), potassium titanyl phosphate (KTP / KTiOPO4), potassium titanyl arsenate (KT A), periodically -poled crystals, such as periodically poled lithium niobate (PPLN).

[0067] In use, a pump beam is applied to the photon pair source 402. The pump beam passes through a first of the optically reflective elements 406, transmissive at the frequency of the pump beam, and enters the cavity. The non-linear material 404 probabilistically converts pump light into photon pairs via a non-linear process. The second of the optically reflective elements 408 is transmissive at the frequencies of the photon pairs produced by the non-linear photonic material 404 and is reflective at the frequency of the pump light. Accordingly, the pump beam is reflected by the second optically reflective element 408 back into the non-linear photonic material 404, while the photon pairs produced by the non-linear photonic material 404 are emitted. The first optically reflective element 406 is reflective at the frequencies of the photon pairs to direct the photon pairs back through the cavity and out through the second optically reflective element 408. Due to conservation of energy and momentum, the resultant photon pairs are entangled and the wavelengths / frequencies of the photons of each photon pair will be strongly correlated - if one were to measure the frequency of a first photon 412 of the photon pair, one is able to determine the spectral properties of the (heralded) second photon 410 of the photon pair without the need to directly measure the frequency of the second photon 410.

[0068] The optical cavity (406, 408) enables the confinement of light with frequencies such that the round-trip distance is equal to an integer number of wavelengths. Accordingly, there is a plurality of different frequency modes that satisfy this constraint. This set of frequency modes can be grouped as modal pairs that are equidistant in frequency about some central frequency. Each of these modal pairs can support the generation of a photon pair with frequencies equal to those of the associated modes. Accordingly, the cavity photon pair source is capable of producing a plurality of photon pairs across a range of frequencies.

[0069] The pump system that generates the pump beam may comprise a laser, such as a distributed feedback (DFB) laser. The pump system may output light that is continuous wave or pulsed. A pulsed pump beam may be advantageous as the pulses help to naturally define time bins / time intervals in which a photon pair may be generated. The wavelength of operation of the pump system may be any wavelength, for example between 700- 1700 nm, or more of the following bands: the O-band (original band: 1260-1360 nm); the C-band (conventional band: 1530-1565 nm), the L-band (long-wavelength band: 1565-1625 nm); the S-band (short-wavelength band: 1460-1530 nm); the E-band (extended-wavelength band: 1360-1460 nm). The wavelength of operation of thepump system may be a telecommunications wavelength, for example between 1300nm and 1600nm. The pump system may be wavelength tuneable.

[0070] The heralded multi-spectral single photon source 400 may further comprise, for example, a beam separator for separating the first photon 412 from the second photon 410 and guiding the first photon 412 towards the detection module 414 and guiding the second photon 410 towards an input path 104.

[0071] The detection module 414 is configured to cause the generation of a heralding signal 422 in response to a detection of a first photon 412 of an entangled photon pair. To this end, the detection module 414 is configured to place the first photon 412 in a particular spatial mode based on a frequency of that first photon 412, and accordingly a detection event in a particular spatial mode is indicative of a frequency of that first photon 412, and by extension is indicative of a frequency of the corresponding second photon 410.

[0072] The detection module 414 comprises a spectral demultiplexer 416 for guiding photons along frequencydependent optical paths. The spectral demultiplexer 416 is configured to receive a first photon 412 of a photon pair and to guide the first photon 412 along an optical path based on the frequency of the first photon 412 towards a corresponding detector. The spectral demultiplexer 416 may be implemented in a number of ways. For example, the spectral demultiplexer 416 may comprise at least one of a prism, a fibre Bragg grating, a wavelength division multiplexer (WDM) or dense wavelength division multiplexer (DWDM), or a dichroic filter, or the spectral demultiplexer 416 may be implemented using any other component(s) capable of spatially separating light of different frequencies.

[0073] The detection module 414 further comprises a detector arrangement 418, the detector arrangement comprising a plurality of photon detectors 418-1 to 418-S.

[0074] Each of the photon detectors is coupled to a respective frequency -dependent optical path of the spectral demultiplexer 416 such that a detection event at a particular photon detector (e.g. 418-1) is associated with the detection of a photon having a specific frequency or having a frequency within a particular frequency range. The detection module 414 is accordingly capable of heralding that a signal photon 410 has been produced by the photon pair source 402 and is capable of indicating the frequency of that signal photon 410 based on which of the S detectors 418 registers a detection event. The photon detectors may be photon number resolving (PNR) detectors, capable of determining how many photons are received. For example, the detectors 418 may comprise superconducting nanowire detectors that generate an output signal intensity proportional to the (discrete) number of photons that strike a detector. PNR detectors are useful in circumstances where it is useful to know whether multiple photons are being generated in the same spectral mode at the same time. Alternatively, the photon detectors may not have PNR capabilities. For example, the detectors may comprise avalanche photodiodes. For example, the detectors may comprise transition edge sensors (TESs).

[0075] The detection module 414 further comprises control logic 420 communicatively coupled to each of the photon detectors 418-1 to 418-S. The control logic 420 is configured to generate a heralding signal 422 in response to a detection event at one of the S detectors 418, the heralding signal 422 indicative of a frequency of the second photon 410 corresponding to the detected first photon 412 of the photon pair. The control logic 420 may be implemented in any suitable low -latency architecture, for example in a Field Programmable Gate Array (FPGA) or an Application Specific Integrated Circuit (ASIC).

[0076] In some examples, the number S of distinct frequencies detectable by the detection module 414 may be the same as the number K of input paths 104 to the system 300. In some examples, the number S of distinctfrequencies detectable by the detection module 414 may be the same as the number L of output paths 106 of the system 300.

[0077] The heralding signal 422 may comprise an analogue or digital signal. For example, the heralding signal 422 may comprise one of an electrical signal, a magnetic signal or an optical signal.

[0078] The skilled person will appreciate that the control logic 420 of the detection module 414 may be implemented in the same controller that implements the control logic 110 of the system 300. In other words, the input signal 112 may comprise a heralding signal directly generated by a photon detector 418 in response to a detection event.

[0079] In some examples, the control logic 420 may be configured to further generate a feedback signal for controlling the rate of generation of photon pairs produced by the photon pair source 402, for example by controlling the power of a pump system that pumps the photon pair source. For example, if detection events are occurring at multiple photon detectors 418 simultaneously, then the control logic 420 may cause the rate of generation of photon pairs produced by the photon pair source 402 to be decreased, while if no detection events are registered for a number of time intervals / a predetermined duration, then the control logic 420 may cause the rate of generation of photon pairs produced by the photon pair source 402 to be increased.

[0080] The skilled person will appreciate that the detection module 414 may comprise more components. Furthermore, the detection module 414 may be provided on-chip (with on-chip nanowire detectors for example) or may be provided in a distributed architecture (for example, in which the detector arrangement 418 is provided in one or more units in a server rack and the control logic 420 is provided in another one or more units in the server rack).

[0081] The heralded multi-spectral single photon source 400 may comprise additional components or modules. For example, the photons of the photon pair may be propagated in optical fibre, in free space using bulk optic components, in integrated optic waveguides, in any other suitable waveguide device or in any combinations thereof.

[0082] In examples, the heralded multi-spectral single photon source 400 may be provided in whole or in part as an integrated circuit (having photonic elements). For example, many of the features of a heralded multi-spectral single photon source 400 may be provided as a photonic integrated circuit.

[0083] In some examples, the heralded multi-spectral single photon source 400 might be provided as a distributed system. For example, the heralded multi-spectral single photon source 400 may be formed of a plurality of units for or installation in a server / computer rack (such as a standard 19-inch server rack), with the modules interconnected with various optical fibre interconnects and / or control lines. For example, the photon pair source 402 may be embodied in one or more units in a server rack. The detection module 414 may be embodied in one or more units in a server rack. A pump module (for generating a pump beam for stimulating the production of photon pairs) may be embodied in one or more units in a server rack.

[0084] Referring again to Fig. 3, the input signal 112 may comprise heralding signals 422 generated by the respective control logic 420 from one or more of the heralded multi-spectral single photon sources 302. For example, the input signal 112 may comprise an analogue or digital signal from one or more of the eight heralded multi-spectral single photon sources 302. In other embodiments, the control logic 420 of each of the heralded multi-spectral single-photon sources 302 and the control logic 110 may be implemented in the same controller, and the input signal 112 may comprise detection signals directly received from each of the K x S photon detectors418 of the photon sources 302. Accordingly, the input signal 112 received by the control logic 110 indicates when a photon is incoming in an input path 104 and the frequency of that photon.

[0085] The system 300 of Fig. 3, further comprises a number M of frequency converters 304. In the example shown in Fig. 3, the number M of frequency converters 304 is the same as the number L of output paths 106. Each frequency converter 304-1 to 304-M is arranged to receive a photon from a respective output path, and may be referred to herein as an outbound frequency converter. Each outbound frequency converter 304 is fiirther arranged to output a photon having a predefined frequency (shown as f0in the figure). Each of the outbound frequency converters 304 is configured to output photons having the same predefined frequency f0. Accordingly, the frequency converter 304-j is arranged to receive a photon having a frequency fj and output a photon predefined frequency f0.

[0086] A frequency converter may be formed in any suitable way.

[0087] In one example, a frequency converter may be implemented using an electro-optic modulator (EOM) and an applied electric control field. For example, the EOM may comprise a non-linear photonic element such as a lithium niobate (LiNbO3) crystal, the refractive index of which is a function of the strength of the local electric field. By driving a suitable control field across the crystal, the refractive index experienced by a photon passing through the crystal can be modulated to cause the EOM to output a photon having the predefined frequency f0.

[0088] In another example, a frequency converter may comprise a non-linear photonic element and an applied control field in the form of a pump laser beam. The non-linear photonic element may comprise a material having a second order (z(2)) non-linearity, such as periodically -poled lithium niobate (PPLN) or potassium titanyl phosphate (KTP). The non-linear photonic element may use sum-frequency generation (SFG), in which the received photon from the output path 106- / and a photon of the applied pump field are annihilated and an output photon is generated having a frequency that is the sum of the frequencies of the annihilated received photon and the annihilated photon of the applied pump field. The non-linear photonic element may use difference -frequency generation (DFG) in which the received photon and a photon of the applied pump field are annihilated and an output photon is generated having a frequency that is the difference of the frequencies of the annihilated received photon and the annihilated photon of the applied pump field. In either case, the pump laser is selected to complement the frequency fj that is associated with the output path 106- / .

[0089] As each output path 106 is associated with a particular frequency of photon, the control fields applied to each frequency converter can be fixed accordingly.

[0090] As explained further above, single photon production from the heralded multi-spectral single photon sources is inherently probabilistic. To demonstrate the ftmctionality of the system 300, in Fig. 3, three time intervals t4, t2, and t3are indicated. These time intervals may correspond to, for example, the pulse separation of a pulsed pump beam used to drive the photon pair source 402 to generate photon pairs. For example, some of the photon sources 302 may emit photons contemporaneously - in other words, substantially simultaneously and within the same photon pair generation clock cycle (e.g within the pump pulse duration for the photon pair source module). In each time interval, a heralded multi-spectral single photon source may produce a single photon having a frequency fj from among a plurality of possible frequencies, and in the example illustrated in Fig. 3, each single photon may have one of four frequencies f4, f2, f3or f4.

[0091] In the first time interval t4, shown in Fig. 3, the control logic 110 receives an input signal 112 indicating that the first photon source 302-1 has produced a single photon having a first frequency f4, the fourth photonsource 302-4 has produced a single photon having a second frequency f2, the fifth photon source 302-5 has produced a single photon having a third frequency f3and the seventh photon source 302-7 has produced a single photon having a fourth frequency f4. Accordingly, the control logic 110 generates one or more control signals for configuring the active optical elements of the photonic switch network such that the first input path 104-1 is coupled to the first output path 106-1, the fourth input path 104-4 is coupled to the second output path 106-2, the fifth input path 104-5 is coupled to the third output path 106-3 and the seventh input path 104-7 is coupled to the fourth output path 106-4. In this way, for each of the output paths 106, a photon of the one or more incoming photons is coupled to that selected output path. Each of the outbound frequency converters 304 is arranged to receive a photon from its respective output path, and to output a photon having the predefined frequency f0.

[0092] In the second time interval t2, shown in Fig. 3, the control logic 110 receives an input signal 112 indicating that the first photon source 302-1 has produced a single photon having the fourth frequency f4, the second photon source 302-2 has produced a single photon having the first frequency f4, the fifth photon source 302-5 has produced a single photon having the fourth frequency f4, the seventh photon source 302-7 has produced a single photon having the third frequency f3, and the eighth photon source 302-8 has produced a single photon having the second frequency f2. Accordingly, the control logic 110 generates one or more control signals for configuring the active optical elements of the photonic switch network such that the second input path 104-2 is coupled to the first output path 106-1, the seventh input path 104-7 is coupled to the third output path 106-3, and the eighth input path 104-8 is coupled to the second output path 106-2. As the input signal 112 indicates that the first photon source 302-1 and the fifth photon source 302-5 have both produced a single photon having the fourth frequency, the control logic 110 selects one of the first input path 104-1 or fifth input path 104-5 to couple to the fourth output path 106-4. The selection may be based on one or more criteria such as a known quality of each of the different photon sources 302 or a shortest optical path between the input paths and output path. Each of the outbound frequency converters 304 is arranged to receive a photon from its respective output path, and to output a photon having the predefined frequency f0.

[0093] In the third time interval t3, shown in Fig. 3, the control logic 110 receives an input signal 112 indicating that the second photon source 302-2 has produced a single photon having the second frequency f2, the third photon source 302-3 has produced a single photon having the first frequency f4, the fourth photon source 302-4 has produced a single photon having the fourth frequency f4, the sixth photon source 302-6 has produced a single photon having the second frequency f2, and the eighth photon source 302-8 has produced a single photon having the third frequency f3. Accordingly, the control logic 110 generates one or more control signals for configuring the active optical elements of the photonic switch network such that the third input path 104-3 is coupled to the first output path 106-1, the fourth input path 104-4 is coupled to the fourth output path 106-4, and the eighth input path 104-8 is coupled to the third output path 106-3. As the input signal 112 indicates that the second photon source 302-2 and the sixth photon source 302-6 have both produced a single photon having the second frequency, the control logic 110 selects one of the second input path 104-2 or sixth input path 104-6 to couple to the second output path 106-2. Each of the outbound frequency converters 304 is arranged to receive a photon from its respective output path, and to output a photon having the predefined frequency f0.

[0094] Fig. 5 shows a block diagram of a system 500 according to an embodiment of the present disclosure. The system 500 has many similar components to system 300, and further comprises a number of (inbound) frequency converters 502. Each of the inbound frequency converters 502 is located on an input path 104 and coupled to anoutput path of a different one of the set 302 of heralded multi-spectral single photon sources (302-1, 302-2, ..., 302- 8). Each inbound frequency converter 502 is arranged to receive an inbound photon from its corresponding photon source 302 and, based on one or more control signals 504 generated by control logic 110, to output a photon possibly having a different frequency as an incoming photon to the photonic switch network 108. The control logic 110 is coupled to the inbound frequency converters 502 and is configured to, based on a received input signal 112 indicative of the presence of one or more incoming photons in a corresponding one or more of the input paths 104, generate one or more control signals 504 to configure the frequency converters 502 such that the frequencies of the photons input into the photonic switch network 108 are all different from one another.

[0095] As with the outbound frequency converters 304, the inbound frequency converters 502 may be implemented in any of a number of ways, the difference being that the control signal(s) 504 affect the control field(s) applied to the inbound frequency converters 502 such that the inbound frequency converters are tunable. For example, an inbound frequency converter 502 may be implemented using an electro -optic modulator (EOM) and an applied electric or optical control field that is controlled by the control signal(s) 504.

[0096] Advantageously, the use of the inbound frequency converters 502 means that fewer photons may be “wasted” than with system 300. As an example, consider the situation in which in a particular time interval the input signal 112 indicates that (without loss of generality) the first photon source 302-1 produces a single photon having the first frequency / j, the second photon source 302-2 produces a single photon having the second frequency f2, the third and fourth photon sources 302-3, 302-4 both produce single photons having the third frequency f3, while the fifth, sixth, seventh and eighth photon sources fail to produce a photon. The control logic 110 in this example generates one or more control signals to configure the photonic switch network 108 such that the first input path 104-1 is coupled to the first output path 106-1, the second input path 104-2 is coupled to the second output path 106-2, and the third input path 104-3 is coupled to the third output path 106-3. Without intervention, the photon having frequency f3generated by the fourth photon source 302-4 is not coupled to an output path and so is wasted. Accordingly, the control logic 110 may generate one or more control signals 504 to configure the inbound frequency converter 504-4 on the fourth input path 104-4 to receive the heralded photon from the photon source 302-4 and to output a photon having the frequency f4. The control signal 114 configures the photonic switch network such that the fourth input path 104-4 is coupled to the fourth output path 106-4.

[0097] The skilled person would appreciate that the multi-spectral photon sources may be replaced with singlefrequency photon sources (photon sources that produce photons in the same single spectral mode).

[0098] The examples described above in relation to Fig. 3 to Fig. 5 are useful for more reliably producing multiple single photons at the same time and in known output paths. However, the ideas set out herein may also be used in downstream processes for ensuring that entangled resource states are more reliably produced in known output paths.

[0099] Just as a classical bit has a state - a computational basis state 0 or a computational basis state 1 - a quantum bit (a “qubit”) also has a state. A qubit may be in either of the computational basis states, in Dirac notation written as 10)Land 11)Lrespectively, or may be in a linear combination - a superposition - of those states e.g. |T = a 10)L+ b |l)L. Abstractly, a qubit can be understood as a normalized vector in a two-dimensional Hilbert space. For the purposes of this document, such a qubit will often be referred to as a logical qubit, and the subscript L is used throughout this specification to denote a logical state. A measurement of the qubit in the computational basis will typically project the qubit onto either the 0 state or the 1 state with a probability dependenton the real or complex valued parameters a and b. A logical qubit may be realized using one or more physical qubits - physical quantum systems, the quantum properties of which can be interpreted as qubit states.

[0100] In some examples, qubit states can be “dual-rail encoded”, in which the logical value of a qubit can be expressed in terms of which of two modes a photon resides in. That is, the logical state of the qubit may be considered to be a computational basis state 10)Lif the photon is in a first mode, while the logical state of the qubit may be considered to be a computational basis state 11)Lif the photon is in a second mode orthogonal to the first mode. The logical state of the qubit may be described by a superposition state if the photon is in a superposition of the first and second modes. Without loss of generality, in the dual-rail encoding, the logical states 10)Land 11)Lof a qubit can be expressed as:|0)L= | 1I> 02)DR (EQ- 1)I DL=|0i, 12)DR (EQ. 2) where the subscripts 1 and 2 refer to first and second orthogonal modes respectively, and the corresponding 0 or 1 value represents the number of photons in that mode. The subscript “DR” identifies a dual-rail encoded state. Each mode may be a spatial mode. For example, an input path 104 may comprise a pair of waveguides acting as two spatial modes.

[0101] One type of state that is of interest in quantum computing is the so-called Bell state. A Bell state can be understood to mean (up to normalisation and global phase) a state of the form provided in (EQ. 3) or (EQ. 4):|0,l)L± |l,0)L= Hi, 02, 03, 14)DR± |01(12, 13, 04)DR(EQ. 3)|0,0)L± |1,1)L= |l1, 02, l3, 04)DR± |01(12, 03, l4)Dfi(EQ. 4)

[0102] The production of entangled resource states such as Bell states is typically a probabilistic process. Accordingly, as with single photon production, it is desirable to more reliably produce entangled states in known output paths at the same time.

[0103] Referring now to Fig. 6, a system 600 according to an embodiment of the present disclosure is illustrated. The system 600 of Fig. 6 may be used to more reliably ensure that entangled resource states are in known output paths at the same time. The system includes a plurality of entangling circuits 602, a plurality of inbound frequency converters 604 and an outbound frequency converter 608.

[0104] The number of input paths 104 shown in Fig. 6 is five (K = 5) and the number of output paths 106 shown in Fig. 6 is two (L = 2), although the skilled person would appreciate that the number of input paths 104 and output paths 106 may be varied. In the system 600, each input path 104 / output path 106 comprises a plurality of waveguides, for example four waveguides. The presence or absence of photons in each plurality of waveguides (and accordingly each input path or output path) can be used to define a multi-photon quantum state.

[0105] The system 600 further comprises a plurality of entangling circuits 602. An entangling circuit is a linear optical circuit configured to probabilistically generate an entangled state comprising a plurality of dual -rail encoded photonic qubits. In the example shown in Fig. 6, each entangling circuit 602 probabilistically generates Bell states, and accordingly, the outputs of each entangling circuit 602 are encoded in the four waveguides that make up an input path. The skilled person would appreciate that the architecture of 600 may be more generally used for other entangled states, for example GHZ states.

[0106] The input signal 112 is indicative of the successful generation of an entangled resource state, and accordingly is indicative that a photonic superposition state is present in the input path to which an entangling circuit 602 is coupled.

[0107] An example of an entangling circuit is shown in Fig. 7, that may perform the function of the entangling circuit 602 of Fig. 6. The entangling circuit 700 can be used to probabilistically generate a dual-rail encoded Bell state. The skilled person would appreciate that alternative entangling circuits may be used to generate a dual -rail encoded Bell state. The skilled person would further appreciate that other entangling circuits may be used to, for example, produce GHZ states or other entangled states.

[0108] The entangling circuit 700 comprises a spatial interferometer 710 and a detector assembly 760.

[0109] The interferometer 710 of Fig. 7 comprises eight input ports 720. The interferometer 710 further comprises eight output ports 730 for outputting the eight output modes. The interferometer 710 is arranged to receive, as eight input spatial modes, four dual-rail encoded photonic qubits, each photonic qubit encoded as probability amplitudes corresponding to the photon occupation of two spatial modes.

[0110] The interferometer 710 comprises a plurality of waveguides, shown as dotted lines in Fig. 7, that are arranged to pass through the interferometer to couple the eight input ports 720 to the eight output ports 730. The plurality of waveguides are arranged so as to provide coupling locations 740 between pairs of the plurality of waveguides. At each coupling location the two modes of electromagnetic radiation modes of light carried by the two respective waveguides are capable of coupling with each other in a 50 / 50 beam splitter-like interaction. For example, if the interferometer 710 is implemented in bulk optics then a 50 / 50 beamsplitter may be arranged at each coupling location to couple the two modes. Alternatively, if the interferometer is implemented in a photonic integrated circuit then the beamsplitter interactions may be implemented by a directional waveguide coupler, for example in a small coupling region two waveguides may be situated close enough to one another that the evanescent fields between the waveguides couple, the length of the coupling region and the separation of the waveguides selected in manufacture to provide a desired 50 / 50 coupling ratio. Several crossing points 750 are also indicated in the figure, and indicate locations at which the respective waveguides cross without interference. For example, a crossing point 750 may indicate fibre optic cables passing each other in three dimensional space, or may indicate non-interacting integrated waveguides in different layers of a photonic integrated circuit.

[0111] The interferometer 710 may be designed and manufactured in any suitable and desired way e.g. depending on the modes of electromagnetic radiation to be transformed by the interferometer. Thus, for example, when the electromagnetic radiation has an optical or infrared wavelength (e.g. between 400nm and 700nm or between 700nm and 1600nm), the waveguides may comprise optical fibres. In some examples, the interferometer may be implemented in bulk optics. However, in a preferred embodiment the interferometer comprises a photonic integrated circuit, with the plurality of waveguides and plurality of coupling locations arranged in the photonic integrated circuit. The photonic integrated circuit may be implemented in silicon nitride (Si3N4), thin film lithium niobate, or any other suitable material.

[0112] The detector assembly 760 comprises a detector arrangement 770 and control logic 780. The detector arrangement 770 comprises four photon detectors which in Fig. 7 are labelled 770-A, 770-B, 770-C, 770-D and are aligned with the output ports 730-3, 730-4, 730-5, 730-6 respectively. The detectors 770 are capable of resolving the number of photons detected. The photon number resolving detectors 770 may comprise intrinsicphoton number resolving detectors such as superconducting nano wire -based detectors. The photon number resolving detectors 770 may comprise multiplexed photon number resolving detectors.

[0113] Control logic 780 is configured to receive detection signals from the detectors 780, for example electrical signals indicative of a number of photons incident on each detector at a particular point in time or within a particular time window. The control logic 780 is further configured to determine whether or not a Bell state has successfully been generated. That is, the control logic 780 is further configured to determine whether or not the detection signals from the detectors 770 imply that the photon configuration input into the interferometer correspond to a Bell state. The control logic 780 is further configured to, in the event that a Bell state is successfully generated, provide a heralding signal to that effect (not shown in the figure). In some examples, the control logic 780 may generate an output signal (electrical, audible, visual or otherwise) only when the detection signals imply that the photon configuration input into the interferometer correspond to a Bell state. In other examples, the control logic 780 may generate an output signal for each point in time / time window in which detection signals are collected, and that output signal may contain information indicating whether or not a Bell state was successfully generated at that point in time / within that time window. The control logic 780 may be implemented using any suitable electronic controller or other computing system. In some examples, the controller may comprise an application specific integrated circuit (ASIC). In some examples, the controller may comprise a field programmable gate array (FPGA).

[0114] The control logic 780 is configured to generate a heralding signal based on the pattern of detection events indicated by the detector arrangement 770. For example, if the control logic 780 receives a detection pattern indicative of a successful Bell state generation, then the control logic 780 outputs a heralding signal as an input signal 112 to control logic 110. In some examples, the control logic 110 and control logic 780 may be implemented in the same controller.

[0115] In use, a photon is provided to every other input port. For example, a single photon may be provided to input ports 720-1, 720-3, 720-5 and 720-7 which may be thought of as providing a dual-rail encoded product state 10000)Lto the entangling circuit 700. The photons interfere within the interferometer 710 and detection events at any of the four detectors 770-A to 770-D partially collapse the quantum state, influencing what is output from the four output ports that are not coupled to a detector. The presence or absence of photons in the four output ports 730-1, 730-2, 730-7 and 730-8 that are not coupled to a detector can be used to define a dual-rail encoded two qubit state. The outputs from the detectors 770 indicate whether or not a Bell state has been produced. For example, when the control logic 780 receives an indication that two detectors have registered a detection event while the other two detectors do not, it can be inferred that a Bell state has been produced. The probability of successfully generating a Bell state in this way is 3 / 16. When the control logic 780 determines that a Bell state has been successfully produced, it causes a heralding signal to be generated.

[0116] Each input path 104 in system 600 comprises four waveguides. The output ports 730-1, 730-2, 730-7, 730-8 are coupled into the four waveguides of an input path of the system 600. Accordingly, an input path 104 of the system 600 may comprise a multiphoton entangled state, and in this particular example a Bell state.

[0117] Returning to Fig. 6, it is assumed that each entangling circuit 602 is capable of producing a two-qubit entangled photonic state with photons having a predefined wavelength f0. When entangling circuit 602-1 produces a two-qubit entangled state, that state may be passed to the photonic switch network 108 without modification. When entangling circuits 602-2 to 602-5 produce a two-qubit entangled state, those states may be acted upon byan inbound frequency converter (604-1, 604-2, 604-3, 604-4). Each inbound frequency converter 604 is arranged to receive an inbound photon from its corresponding entangling circuit 602 and, based on one or more control signals 606 generated by control logic 110, to output a photon possibly having a different frequency as an incoming photon to the photonic switch network 108. The frequency converters 604 may be implemented in the same way as described above in relation to Fig. 3 and Fig. 5.

[0118] As explained further above, even with reliable sources of single photons, the production of entangled resource states is inherently probabilistic. In Fig. 6, three time intervals tT, t2, and t3are indicated. In each time interval, an entangling circuit 602 may produce a two -qubit entangled state with photons having a predefined frequency f0.

[0119] In the first time interval t , shown in Fig. 6, the control logic 110 receives an input signal 112 indicating that the first entangling circuit 602-1 and the fourth entangling circuit 602-4 have produced two-qubit entangled states. Accordingly, the control logic 110 generates one or more control signals for configuring the active optical elements of the photonic switch network such that waveguides of the first and fourth input paths 104-1, 104-4 are coupled to respective waveguides of the first and second output paths 106-1,106-2 respectively. As the entangling circuits 602-1 and 602-4 have both produced photonic states having the same frequency, the control logic generates a control signal 606 to configure the frequency converter 604-3 such that the frequency of the photons input into the photonic switch network 108 are all of a different frequency to the predefined frequency. In this way, the photonic switch network 108 is able to couple an entangled state on a pair of input paths to an entangled output state on a pair of output paths. The outbound frequency converter 608 is arranged to receive any photons of frequency output on output path 106-2 and to output photons having the predefined frequency f0.

[0120] In the second time interval t2, shown in Fig. 6, the control logic 110 receives an input signal 112 indicating that the second, third and fifth entangling circuits 602-2, 602-3, and 602-5 have produced two-qubit entangled states. Accordingly, the control logic 110 selects two of the three entangled states (for example the entangled state output from entangling circuit 602-2 and the entangled state output from entangling circuit 602-5) to couple to the output ports 106. Accordingly, the control logic 110 generates one or more control signals for configuring the active optical elements of the photonic switch network such that waveguides of the second input path 104-2 are coupled to respective waveguides of the first output path 106-1, and waveguides of the fifth input path 104-5 are coupled to respective waveguides of the second output path 106-2. The control logic further generates a control signal 606 to control the frequency converter 604-4 such that an entangled state formed of photons having frequency is provided to the photonic switch network 108. The outbound frequency converter is arranged to receive any photons of frequency output on output path 106-2 and to output photons having the predefined frequency f0.

[0121] In the third time interval t3, shown in Fig. 6, the control logic 110 receives an input signal 112 indicating that the second and fourth entangling circuits 602-2, 602-4 have produced two-qubit entangled states. Accordingly, the control logic 110 generates one or more control signals for configuring the active optical elements of the photonic switch network such that waveguides of the second input path 104-2 are coupled to respective waveguides of the first output path 106-1, and waveguides of the fourth input path 104-5 are coupled to respective waveguides of the second output path 106-2. The control logic fiirther generates a control signal 606 to control the frequency converter 604-3 such that an entangled state formed of photons having frequency is provided to the photonic switch network 108.

[0122] Many variations and alternatives to the described examples are envisaged, Such as those described below in relation to Fig. 8 and Fig. 9.

[0123] Fig. 8 shows an active phase modulation circuit 800 that may be used in place of the active phase modulation circuits 220 of Fig. 2 and is well suited for integrated photonics, for example for implementation in thin film lithium niobate. The active phase modulation circuit 800 comprises a waveguide 802 that couples an output port 215 of a first multimode interference device 210 to an input port 225 of a second multimode interference device 230. Situated adjacent to the waveguide 802 are coupled resonator systems, and there is a coupled resonator system for each of the S possible photon frequencies that may be provided to the photonic switch network 108a.

[0124] Each coupled resonator system comprises a first ring resonator 804a-j (j between 1 and S) and a second ring resonator 804b-j, positioned to enable evanescent coupling of light therebetween. The doublet splitting of each pair of resonators accordingly produces a two level photonic system. Each coupled resonator system further comprises a plurality of active optical elements 806a-j, 806b-j, 808a-j, 808b-j, namely electro-active optical phase modulators, that are tuneable by the control logic 110.

[0125] A phase modulator 806a-j of the first resonator 804a-j and the phase modulator 806b-j of the second ring resonator 804b-j are driven using coherent control signals, for example coherent sinusoidal microwave signals, to couple the two levels of the two level photonic system and induce Rabi oscillations between the two levels of the coupled photonic system. The Rabi oscillations are defined by the power of the coherent control signal, which is tuneable to selectively impart a pi phase shift.

[0126] A further phase modulator 808a-j of the first resonator 804a-j and a further phase modulator 808b-j of the second ring resonator 804b-j are used to couple each coupled resonator system to a corresponding frequency component j.

[0127] By tuning the phase shifts imparted by the resonators, the interference in the second multimode interference device 230 can be influenced.

[0128] The skilled person would appreciate that different resonators may be used in place of ring resonators.

[0129] Fig. 9 shows an active phase modulation circuit 900 that may be used in place of the active phase modulation circuits 220 of Fig. 2. The active phase modulation circuit 900 comprises a waveguide 910 that couples an output port 215 of a first multimode interference device 210 to an input port 225 of a second multimode interference device 230 via components 904, 906, 908.

[0130] The active phase modulation circuit 900 comprises a first dispersive element 904 (group velocity dispersion element), a second dispersive element 908, and an active optical element (in the form of a phase shifting element 906) provided therebetween. The first dispersive element 904 is suitable for performing a frequency -to- time mapping of the light output from an output port 215 of the first multimode interference device, such that the different frequency components are spread out in time. The phase modulator 906 is configured to phase shift a received frequency component output from the first dispersive element and output the phase-shifted frequency component towards the second dispersive element 908. The second dispersive element 908 is matched to the first dispersive element 904. In particular, the second dispersive element 908 may have equal length to the first dispersive element 904 but opposite group velocity dispersion to the group velocity dispersion of the first dispersive element 904. Non-limiting examples of the first and second dispersive elements include dispersion compensating fibres and channelised arrayed waveguide gratings.

[0131] The control logic described throughout the examples of the present disclosure may be embodied in hardware such as an integrated circuit. The controller(s) described herein may be configured to generate one or more signals to control hardware components to perform any of the methods described herein. A controller may be any kind of general or dedicated processor, such as a central processing unit (CPU), a graphics processing unit (GPU), or an integrated circuit. Due to the speeds required to perform one or more of the methods described herein, it may be preferable to implement a controller for performing the method(s) in a dedicated, applicationspecific processing unit. For example, the controller may comprise an application-specific integrated circuit (ASIC) or an application-specific standard product (ASSP) or another domain-specific architecture (DSA). Alternatively, the control logic may be implemented in adaptive computing hardware (that is, hardware comprising configurable hardware blocks / configurable logic blocks) that has been configured to perform the required functions, for example in a configured field programmable gate array (FPGA).

[0132] Also described herein is a computer-readable storage medium having stored thereon a computer-readable circuit description of a controller. The circuit description, when processed by a controller generation system, causes the controller generation system to manufacture or otherwise generate an implementation of the controller.

[0133] The computer-readable circuit description may be in the form of computer code defining an implementation of the controller at any level. For example, the circuit description may comprise a hardware description language (HDL) description of the controller and / or a netlist. The circuit description may comprise (but is in no way limited to) one or more of (i) register transfer level (RTL) code, (ii) a high-level circuit representation such as Verilog or VHDL, and / or (iii) a low -level circuit representation such as OASIS®, GDSII, a bitfile or other configuration file for configuring adaptive computing hardware to implement the controller. High-level representations which logically define an integrated circuit (such as RTL) may be processed at a computer system configured for generating a manufacturing definition of an integrated circuit in the context of a software environment comprising definitions of circuit elements and rules for combining those elements in order to generate the manufacturing definition of an integrated circuit so defined by the representation. As is typically the case with software executing at a computer system so as to define a machine, one or more intermediate user steps (e.g. providing commands, variables etc.) may be required in order for a computer system configured for generating a manufacturing definition of an integrated circuit to execute code defining an integrated circuit so as to generate the manufacturing definition of that integrated circuit.

[0134] The computer-readable circuit description may further include software which runs on the controller defined by the circuit description or in combination with the controller defined at the circuit description. For example, the circuit description, when processed by a controller generation system, may cause the controller generation system to, on manufacturing or otherwise generating the controller, load firmware onto that controller in accordance with program code defined at the circuit description or otherwise provide program code with the controller for use with the controller.

[0135] The controller generation system may be any system suitable for generating a controller (or at least an implementation thereof) by processing the computer-readable circuit description. As an example, if the controller is to be implemented in adaptive computing hardware, then the controller generation system may comprise a desktop or laptop computer or other computing hardware (i.e. having processing capability such that it can execute instructions), that is capable of configuring / loading a configuration file onto the adaptive computing hardware to thereby manufacture / generate an implementation of the controller. For example, manufacturing or otherwisegenerating an implementation of the controller may comprise configuring adaptive computing hardware by preparing and loading a configuration file provided as a part of or otherwise derived from the circuit description.

[0136] As another example, the controller generation system may comprise an integrated circuit manufacturing system (ICMS). The ICMS may comprise a layout processing subsystem configured to receive and process the computer-readable circuit description to determine a circuit layout. Methods of determining a circuit layout from a computer-readable circuit description are known in the art, and for example may involve synthesising RTL code to determine a gate level representation of a circuit to be generated, e.g. in terms of logical components. This may be done automatically or with user involvement in order to optimise the circuit layout. When the layout processing system has determined the circuit layout it may output a circuit layout definition to a manufacturing subsystem. The manufacturing subsystem may manufacture an integrated circuit embodying the controller by utilizing a semiconductor device fabrication process to generate the integrated circuit, which may involve a multi-step sequence of photo lithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of semiconducting material. For example, manufacturing or otherwise generating an implementation of the controller may comprise producing a circuit layout and utilizing a semiconductor device fabrication process to generate an integrated circuit based on that circuit layout.

[0137] A computer-readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or any suitable combination thereof. More specific examples of the computer-readable storage medium would include the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read only memory (EPROM or Flash memory), a portable compact disc read-only memory (CDROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer-readable storage medium may be any tangible medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device.

[0138] All the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of the features and / or steps are mutually exclusive.

[0139] Each feature disclosed in this specification (including any accompanying claims, abstract or drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0140] The disclosure is not restricted to the details of any foregoing embodiments. The disclosure extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be constmed to cover merely the foregoing embodiments, but also any embodiments which fall within the spirit and scope of the claims.

Claims

CLAIMS1. A system comprising: a photonic circuit comprising: a number of input paths; a number of output paths, each output path associated with a corresponding frequency bin; and a photonic switch network coupled between the input paths and the output paths, the photonic switch network comprising an arrangement of active optical elements to selectively couple a photon from any one of the input paths to any one of the output paths; and control logic coupled to the photonic switch network, and configured to: receive an input signal indicative of the presence of one or more incoming photons in a corresponding one or more of the input paths; select, based on the received input signal, one or more output paths, each of the one or more selected output paths associated with a corresponding frequency bin that includes a frequency of the one or more incoming photons; and generate one or more control signals to configure the active optical elements such that, for each of the one or more selected output paths, a photon of the one or more incoming photons is coupled to that selected output path.

2. A system according to claim 1, further comprising: a number of outbound frequency converters, each frequency converter arranged to receive a photon from a respective output path, and to output a photon having a predefined frequency, wherein the frequency converters are configured to output photons having the same predefined frequency.

3. A system according to any preceding claim, wherein the one or more frequencies of the one or more incoming photons are from a predefined set of input frequencies.

4. A system according to any preceding claim, wherein the frequency bins associated with at least two of the selected output paths are disjoint.

5. A system according to any preceding claim, wherein the photonic switching network comprises a generalised Mach-Zehnder interferometer.

6. A system according to any preceding claim, wherein each input path of the photonic circuit is coupled to an output path of a different one of a set of heralded multi -spectral single photon sources, and wherein the input signal comprises heralding signals from one or more of the multi-spectral single photon sources.

7. A system according to claim 6, wherein the system comprises the set of heralded multi-spectral single-photon sources, wherein each multi-spectral single-photon source is configured to output a heralding signal and a signal photon, and wherein each multi-spectral single-photon source comprises:a photon pair source for generating frequency -correlated photon pairs across a range of frequencies, each photon pair comprising a signal photon and a heralding photon; a spectral demultiplexer for guiding heralding photons along a plurality of frequency -dependent lightpaths; and a plurality of photon detectors, wherein each of the photon detectors is coupled to a respective frequency - dependent lightpath of the spectral demultiplexer such that a particular photon detector is associated with the detection of a heralding photon having a frequency within a particular frequency range.

8. A system according to claim 7, wherein a photon pair source comprises a cavity parametric down conversion photon pair source.

9. A system according to claim 7, wherein a photon pair source comprises a spontaneous four wave mixing (SFWM) source.

10. A system according to any preceding claim, wherein the input signal is indicative of a respective one or more frequencies of the one or more incoming photons.

11. A system according to any of claims 1 to 6, wherein the system further comprises a plurality of inbound frequency converters, each frequency converter located on an input path and configurable to: receive an inbound photon having an inbound frequency; and output a photon having a corresponding input frequency as an incoming photon to the photonic switch network; wherein at least two of the outbound frequencies are different from one another.

12. A system according to claim 11, wherein the control logic is further coupled to the plurality of inbound frequency converters and is further configured to: based on the received input signal, generate one or more control signals to configure at least one of the inbound frequency converters to, upon receiving an inbound photon having the inbound frequency, output a photon having the corresponding input frequency as an incoming photon to the photonic switch network.

13. A system according to claim 11 or claim 12, wherein the outbound frequencies are all different from one another.

14. A system according to any of claims 11 to 13, wherein: each input path and each output path comprise a plurality of waveguides; the system further comprises a linear optical circuit configured to probabilistically generate an entangled resource state comprising a plurality of dual-rail encoded photonic qubits; and the input signal is further indicative of the successful generation of an entangled resource state.

15. A system according to any preceding claim, wherein the photonic circuit is implemented on-chip.

16. A system according to any preceding claim, wherein at least a portion of the system is implemented in bulk optics and optical fibre.

17. A method comprising: receiving an input signal indicative of the presence of one or more incoming photons in a corresponding one or more input paths of a photonic circuit; selecting, based on the received input signal, one or more output paths of the photonic circuit, each of the one or more selected output paths associated with a corresponding frequency bin that includes a frequency of the one or more incoming photons; and generating one or more control signals for configuring a photonic switch network of the photonic circuit such that, for each of the one or more selected output paths, a photon of the one or more incoming photons is coupled to that selected output path.

18. A method according to claim 17, wherein the input signal is indicative of a respective one or more frequencies of the one or more incoming photons.

19. A method according to claim 17, wherein the method further comprises: based on the received input signal, generating one or more control signals to configure at least one frequency converter to, upon receiving an inbound photon having an inbound frequency, output a photon having a corresponding input frequency as an incoming photon to the photonic switch network.

20. A controller configured to perform the method of any of claims 17 to 19.

21. A computer-readable storage medium having stored thereon a computer-readable circuit description of a controller for performing the method of any of claims 17 to 19; wherein the computer-readable circuit description, when processed in a controller generation system, causes the controller generation system to manufacture or otherwise generate an implementation of the controller.

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