photodetection

Abstract

Photodetection apparatus, photon emission system and methods of determining a spectral channel of a photon. A photodetection apparatus may include a first waveguide, a second waveguide, a coupling arrangement and a photodetector. The coupling arrangement couples photons from the first waveguide to the second waveguide, and comprises a plurality of discrete optical components each configured to couple photons in a different spectral channel from the first waveguide to the second waveguide. The photodetector receives photons output from the second waveguide and detects a time of arrival of photons at the photodetector. An optical pathlength between an input port of the first waveguide and an output port of the second waveguide is different for each spectral channel.

Description

PHOTODETECTIONTECHNICAL FIELD

[0001] The present disclosure relates to a photodetection apparatus. A photodetection apparatus may find particular utility in determining a spectral channel of single photons.BACKGROUND

[0002] The detection of single photons has utility in a variety of different applications such as metrology, quantum computing, quantum cryptography, imaging, and random number generation. In at least some applications it may be desirable to determine a frequency of single photons.

[0003] Typical approaches to detection of a frequency of a single photon include spatial separation of different spectral modes. For example, a dispersive element, such as a prism or wavelength division demultiplexer, may be used to spatially separate photons of different frequencies and to guide different photon frequencies into different spatial modes. A plurality of single photon detectors may then be arranged to receive photons of different frequencies. That is, single photon detectors may be arranged at different positions with respect to a dispersive element so as to receive photons of different frequencies. Detection of a photon by a single photon detector (of the plurality of single photon detectors) therefore heralds a photon of a specific frequency, the frequency being determined by virtue of the identity and position of the single photon detector which detects the photon. Such an approach requires as many detectors as different frequencies which can be detected. Furthermore, splitting spectral modes into different spatial modes may lead to a bulky detection arrangement and requires electronic circuitry to manage and process information from each and every detector. When scaled to a large number of frequencies such a detection arrangement can become expensive, bulky, complicated and lossy.

[0004] It is in this context the present disclosure has been devisedSUMMARY

[0005] It has been realised that the detection of a frequency of a photon may be improved by separating different spectral modes into different temporal modes, rather than different spatial modes. In particular, apparatus is described herein which is configured to manipulate photons in different spectral channels to arrive at a photodetector in different time bins. Using this approach a spectral channel (frequency) of a photon may be determined by virtue of a time at which the photon is detected by the photodetector. By converting different spectralmodes into different temporal modes, a spectral channel of a photon may be determined (from a plurality of possible spectral channels) using as few as one single photon detector (photodetector). Such arrangements can significantly reduce complexity, expense, spatial footprint and / or photon loss associated with photon detection.

[0006] Apparatus described herein further utilises a plurality of discrete optical components to couple photons in different spectral channels into different temporal modes (time bins). The use of discrete optical components may provide for a more efficient arrangement and may provide flexibility in prioritising the detection of one or more spectral channels over other spectral channels. Alternative approaches of splitting spectral modes into different temporal modes may operate on a continuous frequency domain. For example, components such as a long length of optical fibre or a chirped fibre Bragg grating may be used to disperse a continuous frequency domain onto a continuous time domain. However, such components may be relatively lossy, may not achieve a level of dispersion sufficient to separate tightly spaced frequency modes for detection in the time domain and / or may not allow for flexible prioritisation of preferred frequency modes.

[0007] According to a first aspect of the present disclosure there is provided a photodetection apparatus comprising: a first waveguide, a second waveguide, a coupling arrangement and a photodetector. The first waveguide and the second waveguide are suitable for supporting propagation of photons in a plurality of spectral channels. The first waveguide has an input port for coupling photons into the first waveguide and the second waveguide has an output port for outputting photons from the second waveguide. The coupling arrangement is configured to couple photons from the first waveguide to the second waveguide. The coupling arrangement comprises a plurality of discrete optical components each configured to couple photons in a different spectral channel from the first waveguide to the second waveguide. The photodetector is arranged to receive photons output from the output port of the second waveguide and configured to detect a time of arrival of photons at the photodetector. The photodetection apparatus is arranged such that an optical pathlength between the input port of the first waveguide and the output port of the second waveguide and coupled by a discrete optical component of the coupling arrangement is different for each of the plurality of discrete optical components. Each spectral channel has an associated optical pathlength between the input port of the first waveguide and the output port of the second waveguide.

[0008] Photons coupled from the first optical waveguide to the second optical waveguide by different discrete optical elements have different pathlengths between the input port of the first optical waveguide and the output port of the second optical waveguide. Consequently photons in different spectral channels have different arrival times at the photodetector(relative to a time of photon emission or photon entry into the photodetection apparatus). Detection of a time of arrival of photon at the photodetector may therefore be used to determine a spectral channel of the detected photon.

[0009] The discrete optical components may be independent of each other. For example, a first discrete optical component of the plurality of discrete optical components may configured independently of any other discrete optical component of the plurality of discrete optical components. Each discrete optical component may be independently configured to bring about a desired and independent coupling of each spectral channel. Each discrete optical component may couple a discrete spectral channel. As such the plurality of discrete optical components may not operate on a continuous frequency domain. In at least some examples, there may portions of the spectrum which are not coupled between the first waveguide and the second waveguide and thus may not be detected by the photodetection apparatus. For example, frequencies between frequency adjacent spectral channels which have an associated discrete optical component may have no discrete optical component which couples those frequencies from the first waveguide to the second waveguide. For example, a first discrete optical component may be arranged to couple a first spectral channel from the first waveguide to the second waveguide. A second discrete optical component may be arranged to couple a second spectral channel from the first waveguide to the second waveguide. Frequencies between the first spectral channel and the second spectral channel may not be coupled between the first waveguide and the second waveguide. For example, the photodetection apparatus may not include any discrete optical components configured to couple frequencies between the first spectral channel and the second spectral channel.

[0010] The discrete nature of the discrete optical components may provide for an efficient coupling between the first waveguide and the second waveguide. For example, when compared to the use of a dispersive element, the use of discrete optical components may provide for decreased photon loss and / or a more space efficient arrangement. The use of discrete optical components may additionally or alternatively provide flexibility for targeting and / or prioritising detection of predetermined spectral channels. This may have particular use in applications in which photons are produced in discrete spectral channels (for example, when detecting photons emitted from a cavity photon source) and / or one or more spectral channels may be associated with preferred operation modes of a system (such as a photon emission system).

[0011] The first waveguide and the second waveguide may comprise any suitable photonic waveguide. For example, the first waveguide and / or the second waveguide may comprise optical fibre. The first waveguide and / or the second waveguide may be formed on a substrate.For example, all or part of the photodetection apparatus may form part of a photonic integrated circuit. In such examples, the first waveguide and / or the second waveguide may be fabricated on a photonic chip.

[0012] The plurality of discrete optical components may be arranged such that an order of optical pathlengths associated with the spectral channels is different to an order of frequencies of the spectral channels.

[0013] The plurality of discrete optical components may be arranged such that a first order of the spectral channels ordered by the frequencies of the spectral channels is different to a second order of the spectral channels ordered by the optical pathlengths associated with the spectral channels.

[0014] Each spectral channel may have an associated frequency which may, for example, comprise a central frequency of the spectral channel. The spectral channels can be ordered consecutively by frequency, for example from a lowest frequency spectral channel to a highest frequency spectral channel (or equivalently from a highest frequency spectral channel to a lowest frequency spectral channel). An order of the spectral channels ordered by an order of their frequencies may be referred to as a first order.

[0015] Each spectral channel also has an associated optical pathlength between the input port of the first waveguide and the output port of the second waveguide. The optical pathlength associated with each spectral channel represents the optical pathlength which is traversed by photons in that spectral channel which enter the input port of the first waveguide, are coupled into the second waveguide by a respective discrete optical component and exit the output port of the second waveguide. The spectral channels can be ordered by their associated optical pathlength. For example, the spectral channels can be ordered consecutively from a spectral channel having a shortest optical pathlength to a spectral channel having a longest optical pathlength (or equivalently from a spectral channel having a longest optical pathlength to a spectral channel having a shortest optical pathlength). An order of the spectral channels ordered by an order of their associated optical pathlengths may be referred to as a second order.

[0016] The second order may be alternatively thought of as an order of time of arrival of different spectral channels. For example, photons in each spectral channel will take a different time to propagate from the input port of the first waveguide to the output port of the second waveguide (due to the different optical pathlengths associated with each spectral channel). Each spectral channel will therefore have a different associated time of arrival (relative to a time of a photon entering the input port of the first waveguide) at the output portof the second waveguide. The second order may be thought of as an order in which photons in each spectral channel arrive at the output port of the second waveguide if photons in all spectral channels were to enter the input port of the first waveguide at the same time. Alternatively, the second order may be thought of as a reverse of the order in which photons in each spectral channel arrive at the output port of the second waveguide if photons in all spectral channels were to enter the input port of the first waveguide at the same time. That is, the second order may correspond to an order from first to last arrival time or an order from last to first arrival time.

[0017] The first order may be different to the second order. That is, an order of the optical pathlengths of the spectral channels and the frequencies of the optical pathlengths may be different from another. For example, the optical pathlengths associated with the spectral channels may not monotonically increase or decrease with increasing or decreasing frequency of the spectral channels. The discrete optical components may instead be arranged to provide flexibility as to which spectral channels are associated with shorter optical pathlengths and which spectral channels are associated with longer optical pathlengths. For example, spectral channels may be prioritised such that relatively high priority spectral channels have a relatively short optical pathlength and relatively low priority spectral channels have a relatively long optical pathlength. The spectral channels may be prioritised in any order and need not be limited to an order of frequencies of the spectral channels.

[0018] The photodetection apparatus may further comprise a tuning apparatus configured to vary a spectral channel which is coupled from the first waveguide to the second waveguide by at least one of the discrete optical components.

[0019] The photodetection apparatus may comprise a plurality of tuning apparatus. Each tuning apparatus may be associated with a discrete optical component and may each be configured to vary the spectral channel which is coupled by the discrete optical component with which it is associated. A tuning apparatus may comprise any suitable components for modifying an optical property of a discrete optical component so as to vary the spectral channel which is coupled by a discrete optical component. A tuning apparatus may, for example, comprise a heating element configured to controllable heat a discrete optical component. A tuning apparatus may comprise an electro-optic modulator.

[0020] The tuning apparatus may allow for precise control of the coupling of spectral channels between the first waveguide and the second waveguide. The tuning apparatus may, for example, be controlled by control logic.

[0021] The plurality of discrete optical components may comprise a plurality of reflective components each configured to reflect photons in a different spectral channel.

[0022] The plurality of reflective components may each be configured to substantially transmit photons in spectral channels other than a spectral channel which the reflective component is configured to reflect. The plurality of reflective components may be arranged at different distances from the input port of the first waveguide and / or the output port of the second waveguide such that photons reflected by each reflective component traverse different optical pathlengths between the input port of the first waveguide and the output port of the second waveguide.

[0023] The plurality of reflective components may comprise a plurality of fibre Bragg gratings.

[0024] Each fibre Bragg grating may be arranged with a different grating pitch such that it is reflective to photons in different spectral channels. One or more fibre Bragg gratings may be associated with a tuning apparatus in the form of a heating element thermally coupled to the fibre Bragg grating. A heating element may be controlled in order to heat a fibre Bragg grating so as to vary and / or control a spectral channel which is reflected by the fibre Bragg grating.

[0025] The coupling arrangement may comprise a third waveguide. The plurality of reflective components may be situated in the third waveguide and arranged to reflect photons in a spectral channel propagating in the third waveguide.

[0026] The third waveguide may, for example, comprise an optical fibre. Additionally or alternatively, the third waveguide may comprise an on-chip waveguide.

[0027] The coupling arrangement may comprise an optical circulator comprising a first port, a second port and a third port. The plurality of reflective components may each be arranged to receive photons output from the second port of the optical circulator and reflect photons in a respective spectral channel back to the second port of the optical circulator. The optical circulator may be arranged to: receive photons from the first waveguide at the first port and output the photons received at the first port from the second port; and receive photons reflected by at least one of the plurality of reflective components at the second port and output the photons, received at the second port, from the third port. The second waveguide may be arranged to receive photons output from the third port of the optical circulator.

[0028] A photon entering the first port of the optical circulator may be coupled into the third waveguide and reflected back to the optical circulator for coupling into the second waveguide. A distance from the optical circulator at which the photon is reflected back to the opticalcirculator (and therefore an optical pathlength traversed by the photon) depends on which of the reflective components reflects the photon, which in turn depends on the spectral channel in which the photon lies. In this way, the optical pathlength traversed by a photon before reaching the photodetector depends on its spectral channel.

[0029] The plurality of discrete optical components may comprise a plurality of microresonators arranged between the first waveguide and the second waveguide. Each microresonator may be resonant to a different spectral channel such that each microresonator couples a different spectral channel from the first waveguide to the second waveguide.

[0030] Each microresonator couples photons corresponding with a resonant frequency of the microresonator from the first waveguide to the second waveguide. The microresonators may each have different resonant frequencies such that they couple photons in different spectral channels. The microresonators may arranged at different positions along the length of the first waveguide and the second waveguide. In this way photons which are coupled by different microresonators traverse different optical pathlengths between the input port of the first waveguide and the output port of the second waveguide. The microresonators, the first waveguide and / or second waveguide may be fabricated on a photonic chip.

[0031] One or more tuning apparatus may be associated with one or more of the microresonators and may be operable to vary a spectral channel which is coupled by at least one microresonator. A suitable tuning apparatus may, for example, comprise an electro-optic modulator. An electro-optic modulator may be operable to vary a refractive index of at least part of a microresonator so as to vary a resonant frequency of the microresonator.

[0032] The coupling arrangement may comprise: a plurality of optical transmission lines of different optical pathlength; a photonic demultiplexer configured to spatially separate photons in different spectral channels into one of the plurality of optical transmission lines; and a photonic multiplexer arranged to receive photons from any of the plurality of optical transmission lines and couple the photons into the second waveguide.

[0033] The optical transmission lines may, for example, comprise optical fibres of different lengths. One or more of the optical transmission lines may an optical delay line. An optical delay line may, for example, be realised as loops of optical fibre. The different optical pathlengths of the optical transmission lines result in different spectral channels experiencing different optical pathlengths in the photodetection apparatus.

[0034] The system may be arranged such that each spectral channel has a respective arrival time at the photodetector and wherein a smallest time difference between arrival times of different spectral channels is less than a dead time of the photodetector.

[0035] In such examples, in the event that multiple photons enter the photodetection apparatus (e.g., the input port of the first waveguide) substantially simultaneously (e.g., multiple photons emitted as part of the same photon emission event) then the photodetection apparatus may not detect all photons. For example, if two photons in different spectral channels enter the photodetection apparatus substantially simultaneously then they will reach the photon detector with a time separation between them. If the time separation is less than the dead time of the photodetector then the second photon to arrive at the photodetector will go undetected by the photodetector (by virtue of arriving during the detector deadtime triggered by detection of the first photon to arrive at the photodetector). In at least some examples, the photodetection apparatus may be arranged to prioritise detection of one or more preferred spectral channels (which may, for example, correspond with preferred and / or more efficient modes of operation). One or more preferred spectral channels may be prioritised by arrangement of the discrete optical components such that preferred spectral channels are associated with shorter optical pathlengths. Photons in one or more prioritised spectral channels will therefore arrive at the photodetector earlier and their detection is thus prioritised over spectral channels with longer associated optical pathlengths and later arrival times at the photodetector.

[0036] The system may be arranged such that each spectral channel has a respective arrival time at the photodetector and wherein a smallest time difference between arrival times of different spectral channels is greater than a dead time of the photodetector.

[0037] In such examples, the photodetection apparatus may be capable of detecting multiple photons in different spectral channels which enter the photodetection apparatus substantially simultaneously. Photons in different spectral channels will arrive at the photodetector at different times and will be separated by a time separation. If the dead time of the photodetector is less than the time separation then the photodetector may detect the first photon to arrive and any subsequent photons to arrive at the photodetector. In such examples, it may still be advantageous to prioritise one or more spectral channels for detection (by virtue of prioritised spectral channels being associated with shorter optical pathlengths) in order to detect photons in one or more prioritised spectral channels earlier.

[0038] The photodetector may comprise a single photon detector.

[0039] The photodetector may, for example, comprise a superconducting nanowire single photon detector. The photodetector may comprise parallel single photon detectors and / or multi-pixel single photon detectors. The photodetector may a comprise photon number resolving detector.

[0040] The photodetection apparatus may be configured to receive a single photon in one of the spectral channels of the plurality of spectral channels at the input port of the first waveguide and detect a time of arrival of the single photon at the photodetector.

[0041] The photodetection apparatus may be further suitable for receiving a plurality of photons substantially simultaneously and detecting a time of arrival of at least one of the photons at the photodetector. The photodetection apparatus may be configured to resolve the arrival of a single photon at the photodetector.

[0042] The photodetection apparatus may further comprise control logic configured to determine the spectral channel of a photon detected at the photodetector based on the time of arrival of the detected photon at the photodetector.

[0043] The control logic may, for example, comprise a field programmable gate array. The control logic may, for example, determine a time of arrival of the detected photon relative to a time indicative of emission of the photon and / or a time indicative of arrival of the photon at the photodetection apparatus. The control logic may determine a time bin of a plurality of time bins in which the photon is detected and may determine the spectral channel of the photon based on the determined time bin.

[0044] According to a second aspect of the present disclosure there is provided a photon emission system comprising: a photon pair source arranged to generate a frequency-entangled photon pair; a photodetection apparatus according to the first aspect and including control logic configured to determine the spectral channel of a photon detected at the photodetector based on the time of arrival of the detected photon at the photodetector. The photodetection apparatus is arranged to receive a first photon of a generated frequency-entangled photon pair. The photon emission system comprises control logic configured to determine a spectral channel of the second photon of the generated frequency-entangled photon pair based on the determined spectral channel of the first photon of the generated frequency-entangled photon pair.

[0045] The control logic which determines the spectral channel of the first photon may be the same or different to the control logic which determines the spectral channel of the second photon. The frequency of the first photon is correlated with the frequency of the second photon by virtue of their entanglement. Detection of the first photon by the photodetection apparatus may therefore herald the generation and spectral channel of the second photon.

[0046] The photon pair source may comprise a cavity photon source. The cavity photon source may comprise a non-linear photonic structure arranged inside a photonic cavity. The non-linear photonic structure may be subjected to a pump field such as a pump laser pulse inorder to promote emission of a frequency-entangled photon pair from the non-linear photonic structure.

[0047] According to a third aspect of the present disclosure there is provided a tuneable photon emission system comprising: a photon emission system according to the third aspect, wherein the control logic is further configured to determine a complementary frequency of a pump field based on the determined spectral channel of the second photon of the generated frequency-entangled photon pair; and a non-linear photonic element arranged to receive the second photon of the generated frequency-entangled photon pair and a pump field having the determined complementary frequency and to emit a photon having a predetermined frequency.

[0048] When the frequency of the photon emitted from the non-linear photonic element depends on the frequency of the second photon and the frequency of the pump field. The determined frequency of the second photon (determined by virtue of the determined spectral channel of the first photon) may be used to determine the frequency of pump field which generates an emitted photon of the predetermined (desired) frequency. Determining the complementary frequency of the pump field may comprise selected a pump field from a plurality of available pump fields. For example, a pump field generator may generate a plurality of pump fields (e.g., pump laser beams) having different frequencies. A pump field selector may select one or more of the plurality of pump fields to be incident on the non-linear photonic element.

[0049] The plurality of discrete optical components of the photodetection apparatus may be arranged such that the spectral channel having the shortest optical pathlength between the input port of the first waveguide and the output port of the second waveguide corresponds to a spectral channel of the first photon which is associated with a conversion mode in the nonlinear photonic element having a highest conversion efficiency.

[0050] There may be a plurality of different possible combinations of the frequency of the second photon and the frequency of the pump field which result in emission of a photon having the pre-determined frequency. Such different frequency combinations may be referred to as different conversion modes for producing a photon having the pre-determined frequency. Different conversion modes may be associated with different conversion efficiencies with which a photon having the pre-determined frequency is produced. For example, the possible different conversion modes may be ordered consecutively from a highest efficiency conversion mode to a lowest efficiency conversion mode. Each conversion mode is associated with a first photon frequency and a corresponding second photon frequency. Accordingly, one or more first photon frequencies may be associated with conversion modes having higherconversion efficiencies than other first photon frequencies. A prioritised first photon frequency may be associated with a conversion mode having a highest conversion efficiency (of all possible conversion modes which produce the pre-determined frequency). The photodetection apparatus may be arranged to prioritise detection of first photons in spectral channels associated with higher conversion efficiency conversion modes. For example, the photodetection apparatus may be arranged such that first photon spectral channels associated with higher conversion efficiency conversion modes experience shorter optical pathlengths in the photodetection apparatus. Consequently, if a plurality of first photons are emitted substantially simultaneously and in different spectral channels, a first photon in a prioritised spectral channel may be detected first (and / or instead of detecting a first photon in another spectral channel). Consequently, a pump field frequency may be selected based on detection of a first photon in a prioritised spectral channel and the tuneable photon emission system may be operated using a conversion mode having a higher conversion efficiency (when compared to other conversion modes which may be used based on the other emitted first photons).

[0051] According to a fourth aspect of the present disclosure there is provide a method of determining a spectral channel of a photon, the method comprising: detecting a time of arrival of a photon output from a photodetection apparatus, the photodetection apparatus comprising: a first waveguide and a second waveguide for supporting propagation of photons in a plurality of spectral channels, the first waveguide having an input port for coupling photons into the first waveguide and the second waveguide having an output port for outputting photons from the second waveguide; and a coupling arrangement configured to couple photons from the first waveguide to the second waveguide, the coupling arrangement comprising a plurality of discrete optical components each configured to couple photons in a different spectral channel from the first waveguide to the second waveguide; and determining a spectral channel of the detected photon, wherein the photodetection apparatus is arranged such that an optical pathlength between the input port of the first waveguide and the output port of the second waveguide and coupled by a discrete optical component of the coupling arrangement is different for each of the plurality of discrete optical components and each spectral channel has an associated optical pathlength between the input port of the first waveguide and the output port of the second waveguide, and wherein the spectral channel of the detected photon is determined in dependence on the detected time of arrival at which the photon is detected.

[0052] According to a fifth aspect of the present disclosure there is provided a photodetection apparatus comprising: a photon source operable to emit at least one photon; a photodetector arranged to receive photons originating from the photon source and configuredto detect a time of arrival of photons at the photodetector; and a coupling arrangement configured to couple photons emitted from the photon source to the photodetector, the coupling arrangement comprising a plurality of discrete optical components each configured to couple photons in a different spectral channel from the photon source to the photodetector. The photodetection apparatus is arranged such that an optical pathlength between the photon source and the photodetector is different for each of the plurality of discrete optical components and each spectral channel has a different associated optical pathlength between the photon source and the photodetector.

[0053] It will be appreciated from the foregoing disclosure and the following detailed description of the examples that certain features and implementations described as being optional in relation to any given aspect of the disclosure set out above should be understood by the reader as being disclosed also in combination with the other aspects of the present disclosure, where applicable. Similarly, it will be appreciated that any attendant advantages described in relation to any given aspect of the disclosure set out above should be understood by the reader as being disclosed as advantages of the other aspects of the present disclosure, where applicable. That is, the description of optional features and advantages in relation to a specific aspect of the disclosure above is not limiting, and it should be understood that the disclosures of these optional features and advantages are intended to relate to all aspects of the disclosure in combination, where such combination is applicable.BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Certain examples of the present disclosure will now be described, with reference to the accompanying drawings, in which:- FIG. 1 is a schematic illustration of an example photodetection apparatus;- FIG. 2 is a schematic illustration of a further example of a photodetection apparatus;- FIG. 3 is a schematic illustration of a still further example of a photodetection apparatus;- FIG. 4 is a schematic illustration of a still further example of a photodetection apparatus;- FIG. 5 is a schematic illustration of a photodetection arrangement including a plurality of photodetection apparatus;- FIG. 6 is a schematic illustration of a photon emission system including a photodetection apparatus; and- FIG. 7 is a flowchart of a method of determining a spectral channel of a photon.DETAILED DESCRIPTION

[0055] Hereinafter, examples of the disclosure are described with reference to the accompanying drawings. However, it should be appreciated that the disclosure is not limited to the described examples, 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.

[0056] 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.

[0057] 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 and B,” “at least one of A or 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.

[0058] 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.

[0059] 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 or connected 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 connectedwith / to” another element (e.g., a second element), no other element (e.g., a third element) intervenes between the element and the other element.

[0060] 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 idealized or overly formal sense unless expressly so defined herein.

[0061] FIG. 1 is a schematic illustration of an example photodetection apparatus 102. The photodetection apparatus 102 is operable to receive at least one photon and to determine a spectral channel of the photon, from a plurality of possible spectral channels. The photodetection apparatus 102 comprises a first waveguide 104, a second waveguide 106, a coupling arrangement 108 and a photodetector 110.

[0062] The first waveguide 104 and the second waveguide 106 support propagation of photons in a plurality of spectral channels. That is, the first waveguide 104 and the second waveguide 106 are configured to support propagation of photons of a plurality of different frequencies and wavelengths. The first waveguide 104 and / or the second waveguide 106 may, for example, comprise sections of optical fibre. Additionally or alternatively, the first waveguide 104 and / or the second waveguide 106 may comprise a waveguide forming part of and / or, which is suitable for inclusion in, a photonic integrated circuit. For example, the first waveguide 104 and / or the second waveguide 106 may be formed on one or more substrates and may have any suitable waveguide shape and configuration.

[0063] The first waveguide 104 includes an input port 112 for coupling photons into the first waveguide 104. The input port 112 may comprise an end of the first waveguide 104. As shown in FIG. 1, the input port 112 may be arranged to receive one or more photons from a photon source 120. The one or more photons may have a frequency in a spectral channel of a plurality of different spectral channels which the photodetection apparatus 102 is operable to detect. The photon source 120 may be any suitable source of photons but may in particular, comprise a discrete photon source 120. A discrete photon source 120 may be operable to emit a small countable number of photons and / or may allow for control of the timing of photon emission and / or the number of photons emitted. A discrete photon source 120 may, forexample, comprise a single photon source. A discrete photon source 120 may comprise a photon source 120 operable to emit a pair of photons, for example, an entangled photon pair.

[0064] The second waveguide 106 includes an output port 114 for outputting photons from the second waveguide 106. The output port 114 may comprise an end of the second waveguide 106. The photodetector 110 is arranged to receive photons output from the output port 114 of the second waveguide 106.

[0065] The coupling arrangement 108 is configured to couple photons from the first waveguide 104 to the second waveguide 106. The photodetection apparatus 102 is arranged such that a photon which is coupled into the input port 112 of the first waveguide 104 propagates along the first waveguide 104, is coupled from the first waveguide 104 to the second waveguide 106 by the coupling arrangement 108, propagates along the second waveguide 106 and is output from the output port 114 of the second waveguide 106. It will be understood that in at least some instances, photon loss may occur such that each and every photon entering the input port 112 of the first waveguide 104 may not be output from the output port 114 of the second waveguide 106. The first waveguide 104, the coupling arrangement 108 and the second waveguide 106 are however arranged to generally support the passage of photons in a plurality of spectral channels from the input port 112 of the first waveguide 104 to the output port 114 of the second waveguide 106 (via the coupling arrangement 108).

[0066] The coupling arrangement 108 comprises a plurality of discrete optical components 116a - 116d. In the example shown in FIG. 1 four discrete optical components 116a - 116d are shown. In other examples, a different number of discrete optical components may be included in the coupling arrangement 108. Each discrete optical component 116a - 116d is configured to couple photons in a different spectral channel from the first waveguide 104 to the second waveguide 106. For example, a first discrete optical component 116a may be configured to couple photons in a first spectral channel from the first waveguide 104 to the second waveguide 106. A second discrete optical component 116b may be configured to couple photons in a second spectral channel (different to the first) from the first waveguide 104 to the second waveguide 106. A third discrete optical component 116c may be configured to couple photons in a third spectral channel (different to the first and second) from the first waveguide 104 to the second waveguide 106. A fourth discrete optical component 116d may be configured to couple photons in a fourth spectral channel (different to the first, second and third spectral channels) from the first waveguide 104 to the second waveguide 106. Each spectral channel includes different photon frequencies. Each spectral channel may have at least a different central frequency. A spectral channel may comprise substantially a singlefrequency (corresponding to a central frequency) or a range of frequencies (centred on a central frequency and having a spectral width.) Example implementations of the coupling arrangement 108 will be described in more detail below with reference to FIG. 1, FIG. 2, FIG.3 and FIG. 4.

[0067] The photodetector 110 is configured to detect a time of arrival of photons at the photodetector 110. For example, the photodetector 110 may be configured to generate an electronic signal at a time indicative of a time of arrival of a photon at the photodetector 110. The photodetector 110 may comprise a single photon detector capable of detecting the time of arrival of a single photon. The photodetector 110 may, for example, comprise a superconducting nanowire single photon detector (SNSPD). SNSPDs are well suited to performing precise time measurements of an optical field. For example, SNSPDs may provide photon detection with relatively low jitter (e.g., less than about 3 picoseconds (ps)), high efficiency (e.g., about 98%) and / or a relatively small recovery time (e.g., on the order of 10 nanoseconds (ns)).

[0068] The photodetection apparatus 102 is arranged such an optical pathlength between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 is different for each of the plurality of discrete optical components 116a - 116d, which couple a spectral channel from the first waveguide 104 to the second waveguide 106. That is, each spectral channel has an associated optical pathlength between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106. The optical pathlength associated with each spectral channel travels via a discrete optical component 116a - 116d, which is configured to couple that spectral channel from the first waveguide 104 to the second waveguide 106. Put another way, photons in different spectral channels and which are coupled from the first waveguide 104 to the second waveguide 106 by different discrete optical components 116a - 116d, experience different optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106. Consequently, an arrival time of photons at the output port 114 and the photodetector 110 (relative to a time of a photon entering the input port 112 of the first waveguide 104) is different for photons in different spectral channels. That is, a time taken for a photon to traverse a respective optical pathlength between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 will be different for photons in different spectral channels. Put another way if photons in different spectral channels were to enter the input port 112 of the first waveguide 104 at the same time then they would arrive at the output port 114 of the second waveguide 106 and the photodetector 110 at different times, due to their different associated optical pathlengths via different discrete optical components 116a -116d. The discrete optical components 116a - 116d therefore serve to convert different spectral modes into different temporal modes and different spectral channels arrive at the photodetector 110 in different time bins.

[0069] The spectral channel of the photon can be determined by detecting the time of arrival of the photon at the photodetector 110. For example, the photon source 120 may comprise a discrete photon source 120 which is operable to emit at least one photon at a controllable photon emission time. The photodetector 110 is configured to detect a time of arrival of a photon at the photodetector 110. The detected time of arrival of a photon at the photodetector 110 can therefore be used, relative to the photon emission time, to determine a spectral channel of a detected photon. A time of arrival of a photon at the photodetector 110 may comprise a detection of a time bin (from a plurality of time bins) in which the photon is detected.

[0070] As shown in FIG. 1 the photodetector 110 may be coupled with control logic 118. For example, the photodetector 110 may generate and emit an electronic signal at a time indicative of a time of arrival of a photon at the photodetector 110. The control logic 118 may be electronically connected to the photodetector 110 and may receive an electronic signal generated by the photodetector 110, the electronic signal (e.g., the timing of the signal) being indicative of a time of arrival of a photon at the photodetector 110. The control logic 118 may be configured to determine the spectral channel of a photon detected at the photodetector based on the time of arrival of the detected photon at the photodetector 110. For example, the control logic 118 may determine, based on the time of arrival of a photon at the photodetector 110, the discrete optical component 116a - 116d responsible for coupling the photon from the first waveguide 104 to the second waveguide 106 and thus the spectral channel of the photon.

[0071] The control logic 118 may additionally control, or otherwise be in communication with, the photon source 120. For example, the control logic 118 may generate a control signal for initiating emission of a photon from the photon source 120. Additionally or alternatively, the control logic 118 may receive a signal indicative of a time at which photon emission is initiated at the photon source 120 (e.g., a trigger signal). The control logic 118 may therefore compare a time indicative of photon emission at the photon source 120 with a determined time of arrival of a photon at the photodetector 110 to determine a propagation time and / or an optical path length of the photon and thus its spectral channel. Additionally or alternatively, the control logic 118 may determine a time of arrival of a photon at the photodetector 110 relative to a clock signal. A time indicative of a time of generation oremission of a detected photon may then be compared to the time of arrival to determine a spectral channel of the photon.

[0072] The control logic 608 may take any suitable form. For example, the control logic 608 may comprise an integrated electronic circuit such as a field programmable gate array (FPGA). The integrated circuit (e.g., FPGA) or an application specific integrated circuit (ASIC) may, for example, implement a time-to-digital converter to generate and / or store a digital signal indicative of the time of arrival of a photon at the photodetector 110.

[0073] As was explained above, the photodetection apparatus 102 is arranged to convert different spectral channels into different temporal modes such that a spectral channel of a detected photon may be determined in dependence on a time of arrival of the photon at the photodetector 110. Such an arrangement allows for a determination of a spectral channel of a photon, from a plurality of different spectral channels, with a single photodetector 110.

[0074] In some examples, the photodetection apparatus 102 may detect and determine a spectral channel of a single photon per photon generation event at the photon source 120. In some examples, the photodetection apparatus 102 may be capable of detecting and determining a spectral channel of a plurality of photons per photon generation event at the photon source 120.

[0075] As was described above, the photon source 120 may comprise a discrete photon source 120 operable to emit a small countable number of photons. The photon source 120 may, for example, comprise an emitting element (such as a non-linear photonic element), which may be subjected to an excitation or pump field to cause a transition in the emitting element to an excited energy state. The emitting element may then relax to a lower energy state and emit at least one photon. Subjecting an emitting element to an excitation or pump field is an example of a photon generation event at the photon source 120. In some situations, a photon source 120 may emit a single photon as a result of a photon generation event. In other situations a photon source 120 may emit a plurality of photons as a result of a photon generation event. A plurality of photons emitted as a result of the same photon generation event may comprise photons in the same spectral channel and / or photons in different spectral channels.

[0076] In the event that the photon source 120 emits a plurality of photons in different spectral channels as a result of the same photon generation event, the photons may propagate through the photodetection apparatus 102 and arrive at the photodetector 110. The photons may enter the input port 112 of the first waveguide 104 at substantially the same time. The photons may arrive at the photodetector 110 at different times by virtue of being coupled bydifferent discrete optical components 116a - 116d and thus traversing different optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106.

[0077] In some examples, when a plurality of photons are emitted in different spectral channels (and as a result of the same photon generation event), the photodetector 110 may only detect a first photon arriving at the photodetector 110 (of a plurality of photons emitted as a result of the same photon generation event). That is, the photodetector 110 may only detect a photon in a spectral channel having the shortest optical pathlength (of the spectral channels of all of the photons emitted as a result of the same photon generation event) between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 (and correspondingly the earliest arrival time at the photodetector 110). This may be a result of a dead time of the photodetector 110 being greater than a time difference between arrival times of photons in different spectral channel. In such an example, the photodetector 110 may be triggered by the arrival of a first photon in a first spectral channel at the photodetector 110. Triggering of the photodetector 110 by the first photon starts a dead time during which the photodetector 110 is unable to detect another photon. A second photon in a second spectral channel arriving at the photodetector 110 during the dead time of the photodetector 110 will therefore go undetected.

[0078] In other examples, when a plurality of photons are emitted in different spectral channels (and as a result of the same photon generation event), the photodetector 110 may detect a plurality of photons in a plurality of spectral channels. The photodetector 110 may be triggered by the arrival of a first photon in a first spectral channel at the photodetector 110. Triggering of the photodetector 110 by the first photon starts a dead time during which the photodetector 110 is unable to detect another photon. If the dead time of the photodetector 110 is less than a time difference between a time of arrival of the first photon in the first spectral channel and a time of arrival of a second photon in a second spectral channel, then the second photon 618 may also be detected by the photodetector 110.

[0079] In some examples, the dead time of the photodetector 110 may be greater than a smallest time difference between arrival times of photons in different spectral channels. Consequently, the photodetector 110 may not be able to detect all spectral channels for each photon generation event. For example, the photodetector 110 may only detect the spectral channel of the first photon to arrive at the photodetector 110. In some examples, a time difference between a spectral channel (of all of the spectral channels detectable by the photodetection apparatus 102) having an earliest time of arrival (shortest optical pathlength) and a spectral channel having a latest time of arrival (longest optical pathlength) may be lessthan the dead time of the photodetector 110. In such examples, the photodetector 110 will detect the first photon to arrive for each photon generation event and may not detect any subsequent photons resulting from the same photon generation event. As will be described in further detail below, the photodetection apparatus 102 may be arranged to prioritise detection of photons in spectral channels associated with shorter optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 (and thus having an earlier arrival time at the photodetector 110).

[0080] The dead time of a typical SNSPD may be of the order of around 30-50 nanoseconds (ns). However, some photodetectors 110 may have shorter detector dead times. For example, the photodetector 110 may comprise parallel SNSPDs and / or multi-pixel SNSPDs which may enable a shorter detector dead time for the photodetector 110. The use of parallel SNSPDs and / or multi-pixel SNSPDs may enable photon number resolution (PNR), for example, by detecting an amplitude of a signal pulse resulting from photon detection (e.g., using parallel SNSPDs) and / or through the use of photonic logic gates (such as XOR and / or AND logic), for example using multi -pixel SNSPDs. In such examples, a plurality of photons in the same spectral channel may be detected and counted.

[0081] In some examples, a dead time of the photodetector 110 may be less than a smallest time difference between arrival times of photons in different spectral channels. In such examples a plurality of photons in different spectral channels and resulting from the same photon generation event may be detected. Such an implementation may be achieved by one or both of a photodetector 110 having a relatively short dead time and / or by arrangement of the discrete optical components 116a - 116d such that a time difference between arrival times of photons in different spectral channels is relatively large.

[0082] In general, a maximum number N of spectral channels which can be distinguished depends on a temporal length T of emitted photons and a repetition rate R with which photons are emitted by the photon source 120. The maximum number A of spectral channels may be approximated according to equation (1) below.

[0083] As was explained above, in at least some examples, the photodetection apparatus 102 may not be able to detect photons in each spectral channel emitted as part of the same photon generation event. For example, the dead time of the photodetector 110 and / or a time separation of different spectral channels arriving at the photodetector 110 may mean that foreach photon generation event, the photodetector 110 only detects a photon in a spectral channel associated with an earliest arrival time at the photodetector 110. Detection in spectral channels associated with shorter optical pathlengths in the photodetection apparatus 102 and earlier arrival times at the photodetector 110 may therefore be prioritised over detection in spectral channels associated with longer optical pathlengths in the photodetection apparatus 102 and later arrival times at the photodetector 110.

[0084] Each spectral channel has an associated frequency which may, for example, comprise a central frequency of the spectral channel. The spectral channels can be ordered consecutively by frequency, for example from a lowest frequency spectral channel to a highest frequency spectral channel (or equivalently from a highest frequency spectral channel to a lowest frequency spectral channel). An order of the spectral channels ordered by an order of their frequencies may be referred to as a first order.

[0085] As will be explained in further detail below with reference to the examples of FIG. 2 - FIG. 4, the discrete optical components 116a - 116d may be arranged to establish an order of spectral channels ordered by the optical pathlengths (from the input port 112 of the first waveguide 104 to the output port 114 of the second waveguide 106) associated with the spectral channels. Such an order may be referred to as a second order. The second order may, for example, comprise the spectral channels ordered consecutively from a spectral channel having a shortest optical pathlength to a spectral channel having a longest optical pathlength (or equivalently from a spectral channel having a longest optical pathlength to a spectral channel having a shortest optical pathlength).

[0086] The second order may alternatively be thought of as an order of a time of arrival of different spectral channels. Each spectral channel has a different associated time of arrival (relative to a time of a photon entering the input port 112 of the first waveguide 104) at the output port 114 of the second waveguide 106. The second order may be thought of as an order in which photons in each spectral channel arrive at the output port 114 of the second waveguide 106, if photons in all spectral channels were to enter the input port 112 of the first waveguide 104 at the same time. Alternatively, the second order may be thought of as a reverse of the order in which photons in each spectral channel arrive at the output port 114 of the second waveguide 106, if photons in all spectral channels were to enter the input port 112 of the first waveguide 104 at the same time. That is, the second order may correspond to an order from first to last arrival time or an order from last to first arrival time.

[0087] The use of discrete optical components 116a - 116b to establish different optical pathlengths and arrival times of photons in different spectral channels may provide flexibilityas to an order in which different spectral channels arrive at the photodetector 110. Put another way, the discrete optical component 116a - 116d may provide flexibility as to which spectral channels are associated with relatively shorter optical pathlengths and which spectral channels are associated with relatively longer optical pathlengths. Furthermore, an order of optical pathlengths associated with the spectral channels need not be limited with respect to frequencies (e.g., central frequencies) associated with the spectral channels. For example, the discrete optical component 116a - 116d may be arranged such that the first order of the spectral channels (ordered by their frequencies) need not correspond to the second order of the spectral channels (ordered by their optical pathlength). Furthermore, the second order need not correspond to a reverse of the first order and / or may not be constrained by the first order at all. This may allow prioritisation of any chosen spectral channels to have a relatively short associated optical pathlength and relatively early arrival time at the photodetector 110.

[0088] In at least some examples, the photon source 120 may be configured to emit photons in a finite number of spectral channels. For example, the photon source 120 may only emit photons at specific discrete frequencies. The photon source 120 may, for example, comprise a cavity photon source. For example, the photon source 120 may comprise an emitting element (such as a non-linear photonic element) arranged in an optical cavity. The optical cavity may be arranged to confine light at discrete resonant frequencies and may support photon emission at those resonant frequencies. Consequently, a cavity photon source may emit photons at specific discrete frequencies and in a finite number of pre-determined spectral channels.

[0089] The use of discrete optical components 116a -116d allows for configuration of the photodetection apparatus 102 to detect photons in a finite number of pre-determined spectral channels. For example, in examples in which the photon source 120 comprises a cavity photon source (or other form of photon source) which emits photons in a limited number of discrete emission spectral channels, the discrete optical components 116a -116d may each be configured to couple photons in a spectral channel corresponding to an emission spectral channel of the photon source 120. This may, for example, limit the number of discrete optical components 116a - 116d which are included in the photodetection apparatus 102 to the number of discrete spectral channels, which the photon source 120 emits in. The photodetection apparatus 102 may therefore be configured to provide an efficient detection arrangement for a given photon source 120.

[0090] FIG. 2 is a schematic illustration of an example photodetection apparatus 202. The photodetection apparatus 202 of FIG. 2 includes corresponding components to the photodetection apparatus 102 described above with reference to FIG. 1. Correspondingreference numerals are used in FIG. 1 and FIG. 2 to indicate corresponding components. The photodetection apparatus 202 of FIG. 2 may be considered to be an example of the type of photodetection apparatus 102 described above with reference to FIG. 1. Any of the features and / or components described above with reference to FIG. 1 may also apply to the example of FIG. 2.

[0091] In the example of FIG. 2 the coupling arrangement 108 comprises an optical circulator 204 and a third waveguide 206. The plurality of discrete optical components 116a - 116d comprise a plurality of reflective components arranged in the third waveguide 206. The reflective components 116a - 116d are each configured to reflect photons in different spectral channels. Each reflective component 116a - 116d may be configured to reflect photons in an associated spectral channel and to transmit photons outside of the associated spectral channel. The reflective components 116a - 116d may, for example, comprise fibre Bragg gratings, each configured to reflect photons in different spectral channels. For example, the fibre Bragg gratings may each have different grating periods such that they reflect photons in different spectral channels. The refractive index change and / or grating period in each fibre Bragg grating is configured to reflect photons in a given spectral channel.

[0092] The optical circulator 204 comprises a first port 214, a second port 216 and a third port 218. The optical circulator 204 is arranged to receive photons from the first waveguide 104 at the first port 214 and output photons received at the first port 214 from the second port 216. The third waveguide 206 is arranged to receive photons emitted from the second port 216 of the optical circulator 204. The plurality of reflective components 116a - 116d are each arranged to receive photons output from the second port 216 of the optical circulator 204 and reflect photons in a respective spectral channel back to the second port 216 of the optical circulator 204. The optical circulator 204 is arranged to receive photons reflected by at least one of the plurality of reflective components 116a - 116d at the second port 216 and output the photons, received at the second port 216 from the third port 218. The second waveguide 106 is arranged to receive photons output from the third port 218 of the optical circulator 204. Photons entering the second waveguide 106 from the third port 218 of the optical circulator 204 propagate along the second waveguide 106 to the output port 114 and the photodetector 110. The plurality of reflective components 116a - 116d are arranged at different positions along the length of the third waveguide 206 such that photons reflected by different reflective components 116a - 116d traverse different optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106. Correspondingly, photons in different spectral channels traverse different optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the secondwaveguide 106. As was described above with reference to FIG. 1, this allows the spectral channel of a photon to be determined in dependence on a time of arrival of a photon detected at the photodetector 110.

[0093] As also shown in FIG. 2, the photodetection apparatus 202 may further comprise a tuning apparatus 208. In the example shown in FIG. 2 the photodetection apparatus 202 comprises a plurality of tuning apparatus 208, where each tuning apparatus 208 is associated with a corresponding reflective component 116a - 116d. For example, each tuning apparatus 208 may be arranged to be in physical proximity to or otherwise coupled to a corresponding reflective component 116a - 116d. Each tuning apparatus 208 is arranged to vary a spectral channel which is coupled from the first waveguide 104 to the second waveguide 106 by at least one of the reflective components 116a - 116d. For example, each tuning apparatus 208 may be arranged to vary a spectral channel which is reflected by a reflective component which the tuning apparatus 208 is situated in close proximity to. The tuning apparatus 208 may, for example, be configured to heat one or more of the reflective components 116a - 116d. For example, the tuning apparatus 208 may comprise one or more controllable heating elements thermally coupled to the reflective components 116a - 116d. The spectral channel which is reflected by at least some reflective components may be controlled through controllable heating of the reflective components. For example, changing a temperature of a fibre Bragg grating may serve to change one or both of the refractive index of components of the fibre Bragg grating and a grating period of the fibre Bragg grating. Changing the temperature of a fibre Bragg grating may therefore serve to change the spectral channel which is reflected by the fibre Bragg grating. One or more controllable heating elements thermally coupled to one or more fibre Bragg gratings may therefore be controlled to vary a spectral channel which is coupled from the first waveguide 104 to the second waveguide 106 by at least one of the reflective components 116a - 116d

[0094] In the example of FIG. 2, the reflective components 116a - 116d may be arranged in the third waveguide 206 in any suitable order. This is enabled by the independent and discrete nature of the reflective components. Reflective components 116a - 116d situated closer to the optical circulator 204 (and their associated spectral channels) are associated with shorter optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 (and correspondingly earlier arrival times at the photodetector 110 for photons reflected by the reflective components). Reflective components 116a - 116d situated further away from the optical circulator 204 (and their associated spectral channels) are associated with longer optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 (andcorrespondingly earlier arrival times at the photodetector 110 for photons reflected by the reflective components). The use of discrete optical components 116a - 116d in the form of reflective components such as a fibre Bragg gratings enables flexibility as to which spectral channels are associated with shorter optical pathlengths and earlier arrival times. For example, one or more spectral channels may be prioritised over other spectral channels by arranging reflective components configured to reflect a prioritised spectral channel closer to the optical circulator 204. Consequently, photons in the one or more prioritised spectral channels will have an earlier time of arrival at the photodetector 110 (relative to other spectral channels which are reflected by reflective components arranged further from the optical circulator 204).

[0095] Whilst in the the example of FIG. 2, four reflective components 116a - 116d are shown, it will be appreciated that any suitable number of reflective components 116a - 116d may be used to couple any suitable number of spectral channels.

[0096] FIG. 3 is a schematic illustration of a further example of a photodetection apparatus 302. The photodetection apparatus 302 of FIG. 3 includes corresponding components to the photodetection apparatus 102 described above with reference to FIG. 1. Corresponding reference numerals are used in FIG. 1 and FIG. 3 to indicate corresponding components. The photodetection apparatus 302 of FIG. 3 may be considered to be an example of the type of photodetection apparatus 102 described above with reference to FIG. 1. Any of the features and / or components described above with reference to FIG. 1 may also apply to the example of FIG. 3.

[0097] In the example of FIG. 3 the coupling arrangement 108 comprises a plurality of discrete optical components 116a - 116d each comprising an optical microresonator arranged between the first waveguide 104 and the second waveguide 106. A microresonator is a resonator with sub-micron features and that supports optical resonance. Light that enters the closed circuit of the microresonator is amplified at at least one resonant frequency due to constructive interference and total internal reflection in the microresonator. Example materials for microresonators include silicon, silica, silicon nitride, lithium niobate and crystalline fluorides. Example diameters of microresonators range from microns to hundreds of microns.

[0098] In the example shown in FIG. 3 the microresonators 116a - 116d comprise ring resonators arranged to form travelling wave microcavities. Each microresonator 116a - 116d have one or more resonant wavelengths and frequencies, dependent on their physical dimensions and refractive index.

[0099] Photons propagating in the first waveguide 104 in a spectral channel corresponding to a resonant frequency of a microresonator are coupled into the microresonator. Photons propagating in a microresonator can then be coupled from the mi crore senator and into the second waveguide 106. The plurality of microresonators 116a - 116d may each be configured to have different resonant wavelengths and frequencies such that different microresonators 116a - 116d are resonant to different spectral channels and couple different spectral channels from the first waveguide 104 to the second waveguide 106. For example, different microresonators 116a - 116d may be fabricated with different diameters such that the they have different resonant frequencies. Since the microresonators 116a -116d are each arranged at different positions along the lengths of the first waveguide 104 and the second waveguide 106, each spectral channel (which is coupled by a different microresonator 116a - 116d) traverses a different optical pathlength between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106. Different spectral channels will therefore have different respective arrival times at the photodetector 110 and the spectral channel of a detected photon can be determined in dependence on the time of detection of the photon at the photodetector 110 as described above.

[0100] In the example shown in FIG. 3, the photodetection apparatus 302 further includes tuning apparatus 304. Each microresonator 116a - 116d has a corresponding tuning apparatus 304, each tuning apparatus 304 being operable to vary a resonant frequency of the corresponding microresonator 116a - 116d. The tuning apparatus 304 may therefore be controlled in order to control the spectral channels which are coupled by each of the microresonators 116a - 116d. The tuning apparatus 304 may take any suitable form which is operable to adjust the physical or refractive properties of a respective microresonator 116a -116d so as to adjust a resonant frequency of the respective microresonator 116a - 116d. In at least some examples, a tuning apparatus 304 may comprise an electro-optic modulator (EOM). An EOM may be operable to vary a refractive index of the microresonator 116a -116d so as to vary the resonant frequency of the microresonator 116a - 116d. In this way, one of more EOMs may be controlled to provide precise control over the spectral channels which are coupled from the first waveguide 104 to the second waveguide 106 by each microresonator 116a - 116d.

[0101] Similarly to the reflective components of the example of FIG. 2, in the example of FIG. 3, the microresonators 116a - 116d may be arranged in in any suitable order. This is enabled by the independent and discrete nature of the microresonators. Microresonators 116a - 116d situated closer to the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 (and their associated spectral channels) are associated withshorter optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 (and correspondingly earlier arrival times at the photodetector 110 for photons coupled by the microresonators). Microresonators 116a - 116d situated further away from the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 (and their associated spectral channels) are associated with longer optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 (and correspondingly earlier arrival times at the photodetector 110 for photons reflected by the reflective components). The use of discrete optical components 116a - 116d in the form of microresonators enables flexibility as to which spectral channels are associated with shorter optical pathlengths and earlier arrival times. For example, one or more spectral channels may be prioritised over other spectral channels by arranging microresonators configured to couple a prioritised spectral channel closer to the input port 112 of the first waveguide 104. Consequently, photons in the one or more prioritised spectral channels will have an earlier time of arrival at the photodetector 110 (relative to other spectral channels which are reflected by microresonators arranged further from the input port 112 of the first waveguide 104).

[0102] The example, of FIG. 3 may be realised as and / or as part of an photonic integrated circuit. For example, one of more of the first waveguide 104, second waveguide 106 and the microresonators 116a - 116d may be fabricated on a substrate and / or a chip. The first waveguide 104, second waveguide 106 and / or the microresonators 116a - 116d may be realised using one or more of silicon photonics, silicon nitride, indium phosphide, lithium niobate, silica and gallium arsenide. The first waveguide 104, second waveguide 106 and / or the microresonators 116a - 116d may be fabricated together as part of the same or corresponding fabrication process.

[0103] Whilst in the the example of FIG. 3, four microresonators 116a - 116d are shown, it will be appreciated that any suitable number of microresonators 116a - 116d may be used to couple any suitable number of spectral channels.

[0104] FIG. 4 is a schematic illustration of a further example of a photodetection apparatus 402. The photodetection apparatus 402 of FIG. 4 includes corresponding components to the photodetection apparatus 102 described above with reference to FIG. 1. The photodetection apparatus 402 of FIG. 4 may be considered to be an example of the type of photodetection apparatus 102 described above with reference to FIG. 1. Corresponding reference numerals are used in FIG. 1 and FIG. 4 to indicate corresponding components. Any of the features and / or components described above with reference to FIG. 1 may also apply to the example of FIG. 4.

[0105] In the example of FIG. 4 the coupling arrangement 108 comprises a photonic demultiplexer 404, a photonic multiplexer 406 and discrete optical components 116a - 116d each comprising an optical transmission line extending between the demultiplexer 404 and the multiplexer 406. The demultiplexer 404 is configured to spatially separate different spectral channels propagating in the first waveguide 104 and couple the different spectral channels into different optical transmission lines 116a - 116d. The multiplexer 406 is configured to spatially combine each of the spectral channels propagating in the optical transmission lines 116a - 116d and couple the spectral channels into the second waveguide 106. The optical transmission lines 116a - 116d extending between the demultiplexer 404 and the multiplexer 406 each have different optical lengths. The optical transmission lines may comprise waveguides. For example, the optical transmission lines 116a - 116d may comprise different lengths of optical fibre. As depicted in FIG. 4, at least some of the optical transmission lines 116a - 116d may include optical delay lines 408a - 408d, which provide for different optical pathlengths between the demultiplexer 404 and the multiplexer 406 for different spectral channels. The optical delay lines 408a - 408d may, for example, comprise loops of optical fibre having different optical pathlengths. In some examples, one or more of the optical delay lines 408a - 408d may comprise fixed delay lines. Additionally or alternatively one or more of the optical delay lines 408a - 408d may comprise variable optical delay lines operable to vary an optical pathlength between the demultiplexer 404 and the multiplexer 406.

[0106] As was described above, different optical transmission lines 116a - 116d extending between the multiplexer 406 and the demultiplexer 404 have different optical pathlengths and each optical transmission lines 116a - 116d carries a different spectral channel (by virtue of the spatial separation of spectral channels introduced by the demultiplexer 404). Consequently, different spectral channels have different optical pathlengths between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106. Different spectral channels will therefore have different respective arrival times at the photodetector 110 and the spectral channel of a detected photon can be determined in dependence on the time of detection of the photon at the photodetector 110 as described above.

[0107] Similarly to the reflective components of the example of FIG. 2 and the microresonators of the example of FIG. 3, in the example of FIG. 4 the optical pathlengths of the optical transmission lines 116a - 116d extending between the demultiplexer 404 and the multiplexer 406 may be arranged in any suitable order. This is enabled by the independent and discrete nature of the optical transmission lines. The optical pathlength of each optical transmission lines 116a - 116d may be set at any suitable length such that the order of time ofarrival of different spectral channels at the photodetector 110 is any suitable order. The use of discrete optical components 116a - 116d in the form of optical transmission lines with different optical pathlengths enables flexibility as to which spectral channels are associated with shorter optical pathlengths and earlier arrival times at the photodetector 110. For example, one or more spectral channels may be prioritised over other spectral channels by configuring optical transmission lines 116a - 116d which carry the one or more prioritised spectral channels to have shorter optical pathlengths than other spectral channels. Consequently, photons in the one or more prioritised spectral channels will have an earlier time of arrival at the photodetector 110 (relative to other spectral channels which are associated with optical transmission lines 116a - 116d having longer optical pathlengths).

[0108] Whilst in the the example of FIG. 4, four optical transmission lines 116a - 116d are shown, it will be appreciated that any suitable number of optical transmission lines 116a -116d may be used to couple any suitable number of spectral channels.

[0109] FIG. 5 is a schematic illustration of an example photodetection arrangement 502. The photodetection arrangement 502 comprises a plurality of photodetection apparatus 506 each arranged to detect photons emitted from a common photon source 120. Each of the photodetection apparatus 506 may be arranged to detect photons in different spectral bands. The photodetection arrangement 502 comprises a photonic demultiplexer 504 arranged to receive photons from a photon source 120. The demultiplexer 504 is configured to spatially separate photons in different spectral bands (e.g., frequency bands) and direct different spectral bands to different photodetection apparatus 506. Each spectral band may include a plurality of spectral channels.

[0110] Each photodetection apparatus 506 may be a photodetection apparatus having any of the features and / or components described above with reference to FIG. 1 - FIG. 4. Each photodetection apparatus 506 may be configured to resolve a spectral channel of a photon from a plurality of spectral channels which lie within the spectral band which is received from the demultiplexer 504. The use of a plurality of photodetection apparatus 506, each photodetection apparatus 506 arranged to receive and detect in a different spectral band, may allow the total number of resolvable spectral channels to be increased when compared to the use of a single photodetection apparatus 506. The photodetection arrangement 502 may include as few as a single photodetector 110 per photodetection apparatus 506. The total number of photodetectors 110 included in the photodetection arrangement 502 may therefore be as few as the number of spectral bands in to which photons are spatially separated by the demultiplexer 504 (each spectral band including a plurality of spectral channels which can be detected by teach photodetection apparatus 506).

[0111] Various examples of photodetection apparatus 102, 202, 302, 402 and a photodetection arrangement 502 have been described above with reference to FIG. 1 - FIG.5. Each photodetection apparatus 102, 202, 302, 402 and photodetection arrangement 502 is arranged such that photons in different spectral channels traverse different optical pathlengths to a photodetector 110 and thus have different arrival times at the photodetector 110. The photodetection apparatus 102, 202, 302, 402 and photodetection arrangement 502 therefore enable determination of a spectral channel of one or more photons in dependence on a time at which a photon is detected at a photodetector 110. Furthermore such a determination of a spectral channel of one or more photons may be performed with as few as one photodetector 110. In at least some examples, the use of discrete and independent optical components 116a - 116d allows for flexibility as to which spectral channels are associated with shorter optical pathlengths and earlier arrival times at the photodetector 110. As was explained above, this may allow prioritisation of the detection of one or more spectral channels over one or more other spectral channels. A photodetection apparatus as described herein may find a variety of different uses and may be suitable for detection of photons in originating from a variety of different types of photon source 120. A particular example use of a photodetection apparatus 102, 202, 302, 402 will be described below with reference to FIG. 6.

[0112] FIG. 6 is a schematic illustration of a photon emission system 602. The photon emission system 602 comprises a photon pair source 604, a photodetection apparatus 606, control logic 608, a pump field generator 610, a pump field selector 612 and a non-linear photonic element 614. The photon emission system 602 is operable to produce an output photon 620 having a desired pre-defined frequency.

[0113] The photon pair source 604 is arranged to generate a frequency-entangled photon pair comprising a first photon 616 and a second photon 618. The photon pair source 604 may be configured to probabilistically generate pairs of frequency-correlated photons across a range of spectral channels. For example, a first photon 616 and a second photon 618 may be emitted as a photon pair and may each have a frequency from among a plurality of spectral channels. The frequency of the first photon 616 is correlated with the frequency of the second photon 618 such that their frequency states are entangled.

[0114] The photon pair source 604 may comprise a non-linear media having a second-order or third-order non-linearity. For example, the photon pair source 604 may generate photon pairs based on spontaneous four wave mixing (SFWM) or spontaneous parametric down conversion (SPDC). Any suitable photon pair source 604 capable of producing photon pairs across a range of spectral channels may be used. For example, the photon pair source 604 maycomprise a plurality of non-linear elements, each pumped by slightly different pump fields (e.g., pump lasers).

[0115] In at least some examples, the photon pair source 604 may comprise a cavity source. For example, the photon pair source 604 may comprise a non-linear photonic structure arranged inside a photonic cavity. The photonic cavity may enable the confinement of light with frequencies such that the round-trip distance is equal to an integer number of wavelengths. There may be a plurality of different frequency modes which satisfy this constraint and as such the cavity may have a plurality of resonant frequencies. 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 source may be capable of producing a plurality of photon pairs across a range of frequencies. The non-linear photonic structure may, for example, use spontaneous four wave mixing (SFWM) or spontaneous parametric down conversion (SPDC) to convert a pump field (e.g., pump light) into photon pairs.

[0116] The first photon 616 of a generated photon pair emitted by the photon pair source 604 is measured by the photodetection apparatus 606. For example, whilst not shown in FIG.6, the photon emission system 602 may comprise an element (e.g., a beam splitter) arranged to direct the first photon 616 to the photodetection apparatus 606 and to direct the second photon 618 to the non-linear photonic element 614. The photodetection apparatus 606 may comprise a photodetection apparatus described herein. For example, the photodetection apparatus 606 may comprise a photodetection apparatus 102 as described above with reference to FIG. 1, a photodetection apparatus 202 as described above with reference to FIG.2, a photodetection apparatus 302 as described above with reference to FIG. 3, a photodetection apparatus 402 as described above with reference to FIG. 4 and / or a photodetection arrangement 502 as described above with reference to FIG. 5. The photodetection apparatus 606 outputs a signal indicative of a spectral channel of the first photon 616. For example, the photodetection apparatus 606 may include control logic 118 which determines a spectral channel of the first photon 616 in dependence on a signal generated by a photodetector 110 with a timing which is indicative of a time of arrival of the first photon 616 at the photodetector 110 (where the time of arrival is indicative of a spectral channel of the first photon 616).

[0117] The signal output by the photodetection apparatus 606 is received by control logic 608. Whilst control logic 608 is illustrated in FIG. 6 as being separate to the photodetection apparatus 606, all or part of control logic 118 which forms part of the photodetectionapparatus 606 (and which determines a spectral channel of the first photon 616) may be shared with all or part of the control logic 608. For example, a single or integrated control logic may be configured to perform part of the functionality of the photodetection apparatus 606 and / or all or part of the functionality of control logic 608.

[0118] The control logic 608 is configured to determine a spectral channel of the second photon 618 based on the determined spectral channel of the first photon 616. As was explained above, the photon pair comprising the first photon 616 and the second photon 618 are frequency entangled such that the frequency of the second photon 618 is correlated with the frequency of the first photon 616. The control logic 608 may therefore determine the frequency (or equivalently a spectral channel) of the second photon 618 in dependence on the determined spectral channel of the first photon 616. In at least some examples, the first photon 616 may be referred to as a herald photon and the second photon 618 may be referred to as a signal photon. Due to the entanglement between the first photon 616 and the second photon 618, the detection of the first photon 616 is indicative of the generation of the second photon 618. Detection of a spectral channel of the first photon 616 is further indicative of the frequency of the second photon 618. In this way detection of the first photon 616 acts as a herald for the presence and frequency of the second photon 618.

[0119] The second photon 618 is directed to be incident on a non-linear photonic element 614. The non-linear photonic element 614 is configured to receive the second photon 208 and a selected pump field 622 and produce an output photon 620 having a predefined frequency based on the frequency of the second photon 618 and the selected pump field 622. The control logic 608 may be configured to determine a complementary frequency of the selected pump field 622. In particular, the control logic 608 may determine a frequency of the selected pump field 622, in dependence on the determined frequency of the second photon 618, which will produce an output photon 620 having a predefined frequency. That is, the control logic 608 may use the determined frequency (or spectral channel) of the second photon 618 and a predefined desired frequency of an output photon 620 and determine a frequency of the selected pump field 622 which will produce an output photon 620 having the predefined desired frequency.

[0120] The control logic 608 may, for example, provide a signal indicative of the frequency of the selected pump field 622 to the pump field selector 612. In the example of FIG. 6, the pump field selector 612 receives a plurality of pump fields 624 of different frequencies. The pump field generator 610 is configured to generate the plurality of pump fields 624 at a plurality of pump frequencies. A pump field 624 may, for example, comprise a coherent laser field. Whilst four pump fields 624 are shown in FIG. 6, the pump field generator 610 mayproduce more or fewer pump fields 624. The pump field generator 610 may generate the pump fields in any suitable way. In at least some examples, the pump field generator 610 comprises a plurality of laser sources, each generating a pump field 624 at a different frequency.

[0121] The pump field selector 218 is configured to receive a signal from the control logic 608. As was described above, the control logic 608 may determine a complementary frequency for the selected pump field 622 and may provide a signal indicative of the determined frequency of the selected pump field 622. In some examples, some or all of the control logic may be incorporated into the pump field selector 612. For example, the control logic 608 may form part of the pump field selector 612. Additionally or alternatively, the control logic 608 may produce a signal indicative of the detected spectral channel of the first photon 616, the determined spectral channel of the second photon 618 and / or the predefined frequency for the output photon 620. In such examples, the pump field selector 612 may determine a frequency for the selected pump field 622 in dependence on the signal received from the control logic 608.

[0122] The pump field selector 612 is further configured to select a pump field from the plurality of pump fields 624. For example, the pump field selector 612 may select a pump field with a frequency closest to the determined frequency for the selected pump field 622. The selection may be performed in any suitable way, and any pump field selector 612 capable of selecting a single pump field or filtering out other pump fields may be utilised. In at least some examples, the pump field selector 612 may comprise controllable filters for filtering out any unselected pump fields 624. The pump field selector 612 may comprise a wavelength converter such as a wavelength division multiplexer which directs any unfiltered pump fields 624 to be incident on the non-linear photonic element 614 as the selected pump field 622. In at least some examples, the pump field generator 610 may be operable to tune the frequency of one or more of the pump fields 624 (e.g., under control of the pump field selector 612). In this way a selected pump field 622 may be produced which is as close as possible in frequency to a determined selected pump field 622 frequency for producing the output photon 620 of the desired predetermined frequency.

[0123] The non-linear photonic element 614 is configured to receive the second photon 618 and the selected pump field 622 and produce an output photon 620 having the predefined frequency based on the frequency of the second photon 618 and the complementary selected pump field 622. In some examples, the non-linear photonic element 614 may comprise a beam combiner with non-linear material. The non-linear photonic element 614 may, for example, comprise a material having a second order non- linearity. For example, a suitable material may be periodically-poled lithium niobate (PPLN) or potassium titanyl phosphate (KTP). Thenon-linear photonic element 614 may use sum-frequency generation (SFG), in which the received second photon 618 and a photon of the selected pump field 622 are annihilated and an output output photon 620 is generated having a frequency that is the sum of the frequencies of the annihilated second photon 618 and the annihilated photon of the selected pump field 622. The second order selected pump field 622 may use difference-frequency generation (DFG) in which the received second photon 618 and a photon of the selected pump field 622 are annihilated and an output photon 620 is generated having a frequency that is the difference of the frequencies of the annihilated second photon 618 and the annihilated photon of the selected pump field 622.

[0124] In some examples, the non-linear photonic element 614 may comprise a material having a third order non-linearity, for example a material that produces an output photon 620 based on Bragg-scattering four-wave mixing. In such examples, the pump field selector 612 may select two pump fields as two pump fields may be required for the non-linear effect to be seen.

[0125] Further details and embodiments of various components of a photon emission system of the type described above with reference to FIG. 6 are provided in US patent application having publication no. US 2024 / 0011829 Al, which is incorporated by reference herein in its entirety.

[0126] As was described above, in the photon emission system 602 of FIG. 6, a photodetection apparatus 606 is used to determine the spectral channel of a first photon 616 of a frequency-entangled photon pair. The determination of the spectral channel of the first photon 616 is then used to determine the spectral channel of the second photon 618, which is then used to determine a frequency of a selected pump field 622 in order to generate an output photon 620 having a desired pre-determined frequency. In the context of a photon emission system 602 of the type described above with reference to FIG. 6, a photodetection apparatus as described herein may have several advantages.

[0127] For example, the photon pair source 604, in at least some examples, may emit photons at discrete frequencies and in a finite number of spectral channels. For example, if the photon pair source 604 comprises a cavity photon source then photons may be emitted at discrete frequencies corresponding with resonant frequencies of the cavity. In this context, the use of discrete optical components 116a - 116d in a photodetection apparatus 102 is especially efficient since each of the discrete optical components 116a - 116d can be configured to couple a spectral channel corresponding with a spectral channel which the photon pair source 604 emits in. Other portions of the spectrum in which the photon pair source 604 does notemit may be discarded and no component need be provided for coupling other portions of the spectrum from the first waveguide 104 to the second waveguide 106.

[0128] Furthermore, as was described above, the discrete and independent nature of discrete optical components 116a - 116d in a photodetection apparatus allows flexible prioritisation of one or more spectral channels for detection over other spectral channels. In the photon emission system 602 of FIG. 6, since the photon pair source 604 is configured to generate frequency-entangled photon pairs across a range of frequencies, there is a non-zero probability that two or more photon pairs are generated contemporaneously. For example, two or more photon pairs may be generated substantially simultaneously and as a result of the same photon generation event (e.g., as a result of the same pump pulse in the the photon pair source 604). In such circumstances two or more first photons 616 may enter the photodetection apparatus 606 at substantially the same time. The two or more first photons 616 may have different frequencies.

[0129] As was explained above, when two or more photons enter a photodetection apparatus 606 at substantially the same time then, depending on the specific configuration of the photodetection apparatus 606, only one of the photons may be detected by a photodetector 110 forming part of the photodetection apparatus 606. For example, if a dead time of the photodetector 110 is greater than a time difference between arrival times of two photons (in different spectral channels) at the photodetector 110 then only the first photon arriving at the photodetector 110 will be detected.

[0130] In the context of a photon emission system 602 of the type described with reference to FIG. 6 some photon pair frequencies may be more suitable than others for causing the photon emission system 602 to produce a good quality output photon 620 reliably. For example, dependent on the materials used in the photon emission system 602, the photon emission system 602 may be more susceptible to photon loss at one frequency than another. As another example, different pump fields may have different intensities which can influence the conversion rate for the output photon 620. As another example, depending on mismatches between the available pump fields to select from, and the frequencies of the second photons 618 produced by the photon pair source 604, some second photons 618 may be used to generate output photons 620 having a higher purity than those generated by other second photons 618. Additionally or alternatively, the non-linear photonic element 614 may have different conversion efficiencies for different frequencies of heralded second photons 618. As a consequence of one or more (or additional or alternative) of these factors, one or more possible first photon spectral channels may be preferred over other possible first photonspectral channels as corresponding to an improved operation mode of the photon emission system 602.

[0131] The photodetection apparatus 606 may be arranged to prioritise detection of first photons 616 in one or more preferred first photon spectral channels. For example, the discrete optical components 116a - 116d may be arranged in the photodetection apparatus 606 such that one or more preferred first photon spectral channels are associated with relatively shorter optical pathlengths in the photodetection apparatus 606. Consequently, in the event that two first photons 616 enter the photodetection apparatus 606 substantially simultaneously and one first photon 616 is in a preferred spectral channel and the other first photon 616 is not in a preferred spectral channel, the first photon 616 in the preferred spectral channel will arrive at the photodetector 110 earlier than the first photon 616 not in a preferred spectral channel. The first photon 616 in the preferred spectral channel will therefore be detected and the first photon 616 not in a preferred spectral channel will either go undetected or will be detected at a later time (depending on the configuration of the photodetection apparatus 606).

[0132] In the event that only the first photon 616 in the preferred spectral channel is detected (e.g., due to the dead time of the photodetector 110 being greater than a time of arrival difference between the two photons) the pump field selector 612 will select a pump field 622 corresponding to a preferred mode of operation of the photon emission system 602 and will produce a good quality output photon 620 more reliably (when compared to selecting a pump field 622 based on detection of a first photon 616 not in a preferred spectral channel). In the event that the photodetection apparatus 606 detects both the first photon 616 in the preferred spectral channel and the first photon 616 not in a preferred spectral channel (e.g., because the dead time of the photodetector 110 is less than a time of arrival difference between the two photons) then it may still be advantageous to detect the first photon 616 in the preferred spectral channel earlier than detecting the first photon 616 not in the preferred spectral channel. For example, the pump field selector 612 may select a pump field 622 corresponding to detection of the first photon 616 in the preferred spectral channel earlier, thereby reducing any delays in the system. For example, the pump field selector 612 need not wait for detection of the first photon 616 not in a preferred spectral channel before selecting the selected pump field 622 corresponding to detection of the first photon 616 in the preferred spectral channel.

[0133] In at least some examples, the plurality of discrete optical components 116a - 116d of the photodetection apparatus 606 may be arranged such that a spectral channel corresponding with a highest efficiency conversion mode in the non-linear photonic element 614 has a shortest optical pathlength between the input port 112 of the first waveguide 104 and the output port 114 of the second waveguide 106 in the photodetection apparatus 606. Aspectral channel corresponding with a highest efficiency conversion mode in the non-linear photonic element 614 may comprise a spectral channel of a first photon 616 which corresponds to a spectral channel of the second photon 618 which, together with a selected pump field 622, can produce an output photon 620 of a desired predetermined frequency with a highest conversion efficiency. For example, for a given photon pair source 604, there may be a plurality of different frequency modes which can be used to produce an output photon 620 having a desired predetermined frequency. That is, there may be a plurality of different combinations of a second photon 618 spectral channel and selected pump field 622 frequency which results in an output photon 620 having the desired predetermined frequency. Each such possible combination of a second photon 618 spectral channel and selected pump field 622 frequency may be referred to as a different conversion mode.

[0134] As was explained above, different conversion modes may be associated with different conversion efficiencies with which an output photon 620 is produced. For example, the possible different conversion modes may be ordered consecutively from a highest efficiency conversion mode to a lowest efficiency conversion mode. The discrete optical components 116a - 116d of the photodetection apparatus 606 may be arranged such detection of a first photon 616 spectral channel associated with a highest efficiency conversion mode is prioritised. For example, the photodetection apparatus 606 may be arranged that a first photon 616 spectral channel associated with a highest efficiency conversion mode has a shortest optical pathlength in the photodetection apparatus 606. In at least some examples, the discrete optical components 116a - 116d may be arranged such that an order of spectral channels in terms of their associated optical pathlength in the photodetection apparatus 606 (the second order described above) corresponds with an order of the conversion efficiency of the conversion modes. That is, the discrete optical components 116a - 116d may be arranged such that the optical pathlengths of spectral channels are ordered in terms of the conversion efficiency of a corresponding conversion mode in the non-linear photonic element 614 (i.e., a conversion mode using a second photon 618 having a frequency corresponding with the first photon 616 spectral channel which is coupled by each discrete optical component 116a -116d).

[0135] The discrete and independent nature of the discrete optical components 116a - 116d allows for the flexible prioritisation one or more spectral channels over other spectral channels. In particular, the order of prioritisation of the spectral channels is not constrained by the frequencies of the spectral channels (e.g., the prioritisation is not limited by a frequency (first) order of the spectral channels).

[0136] Whilst various advantages of a photodetection apparatus of the type described herein has been described with reference to a photon emission system 602 of the type shown in FIG.6, the utility of a photodetection apparatus of the type described herein is not limited to such applications. For example, a photodetection apparatus of the type described herein may find utility in a variety of applications in which a spectral channel of one or more photons is to be determined.

[0137] FIG. 7 is a flowchart of a method 700 for determining a spectral channel of a photon. The method 700 may be performed using all or part of a photodetection apparatus 102, 202, 302, 402 and / or a photodetection arrangement 502 as described herein. At step 702, a time of arrival of a photon output from a photodetection apparatus is detected. The photodetection apparatus may include any of the components described above with reference to the photodetection apparatus 102, 202, 302, 402 of FIG. 1 - FIG. 4 and / or a photodetection arrangement 502 of FIG. 5. In particular, an optical pathlength to a photodetector used to detect the photon is dependent on a discrete optical component (of a plurality of discrete optical components) which couples the photon to the photodetector. Each discrete optical component is arranged to couple photons in different spectral channels and the photodetection apparatus is arranged such that an optical pathlength to the photodetector is different for each of the plurality of discrete optical components. Each spectral channel therefore has a different associated optical pathlength between to the photodetector.

[0138] At step 704 a spectral channel of the detected photon is determined in dependence on the detected time of arrival at which the photon is detected. Since an optical pathlength to the photodetector is different for different spectral channels, a time of arrival of a photon at the photodetector (and hence the detected time of arrival) is dependent on the spectral channel of the photon. The spectral channel of the detected photon may be determined by control logic as described herein.

[0139] Features, integers, characteristics or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of 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 such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention 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 orprocess so disclosed. In particular, any dependent claims may be combined with any of the independent claims and any of the other dependent claims.

[0140] Each feature disclosed in this specification (including any accompanying claims, abstract and 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. The invention is not restricted to the details of any foregoing embodiments. The invention 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 construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

CLAIMS1. A photodetection apparatus comprising:a first waveguide and a second waveguide for supporting propagation of photons in a plurality of spectral channels, the first waveguide having an input port for coupling photons into the first waveguide and the second waveguide having an output port for outputting photons from the second waveguide;a coupling arrangement configured to couple photons from the first waveguide to the second waveguide, the coupling arrangement comprising a plurality of discrete optical components each configured to couple photons in a different spectral channel from the first waveguide to the second waveguide; anda photodetector arranged to receive photons output from the output port of the second waveguide and configured to detect a time of arrival of photons at the photodetector, wherein the photodetection apparatus is arranged such that an optical pathlength between the input port of the first waveguide and the output port of the second waveguide and coupled by a discrete optical component of the coupling arrangement is different for each of the plurality of discrete optical components and each spectral channel has an associated optical pathlength between the input port of the first waveguide and the output port of the second waveguide.

2. The photodetection apparatus of claim 1, wherein the plurality of discrete optical components are arranged such that an order of optical pathlengths associated with the spectral channels is different to an order of frequencies of the spectral channels.

3. The photodetection apparatus of claim 1 or 2, further comprising a tuning apparatus configured to vary a spectral channel which is coupled from the first waveguide to the second waveguide by at least one of the discrete optical components.

4. The photodetection apparatus of any one of claims 1 to 3, wherein the plurality of discrete optical components comprise a plurality of reflective components each configured to reflect photons in a different spectral channel.

5. The photodetection apparatus of claim 4, wherein the plurality of reflective components comprise a plurality of fibre Bragg gratings.

6. The photodetection apparatus of claim 4 or 5, wherein the coupling arrangement comprises a third waveguide and the plurality of reflective components are situated in the thirdwaveguide and arranged to reflect photons in a spectral channel propagating in the third waveguide.

7. The photodetection apparatus of any one of claims 4 to 6, wherein the coupling arrangement comprises an optical circulator comprising a first port, a second port and a third port, wherein the plurality of reflective components are each arranged to receive photons output from the second port of the optical circulator and reflect photons in a respective spectral channel back to the second port of the optical circulator, andwherein the optical circulator is arranged to:receive photons from the first waveguide at the first port and output the photons received at the first port from the second port; andreceive photons reflected by at least one of the plurality of reflective components at the second port and output the photons, received at the second port, from the third port, andwherein the second waveguide is arranged to receive photons output from the third port of the optical circulator.

8. The photodetection apparatus of any one of claims 1 to 7, wherein the plurality of discrete optical components comprise a plurality of microresonators arranged between the first waveguide and the second waveguide, wherein each microresonator is resonant to a different spectral channel such that each microresonator couples a different spectral channel from the first waveguide to the second waveguide.

9. The photodetection apparatus of any one of claims 1 to 8, wherein the coupling arrangement comprises:a plurality of optical transmission lines of different optical pathlength;a photonic demultiplexer configured to spatially separate photons in different spectral channels into one of the plurality of optical transmission lines; anda photonic multiplexer arranged to receive photons from any of the plurality of optical transmission lines and couple the photons into the second waveguide.

10. The photodetection apparatus of any one of claims 1 to 9, wherein the system is arranged such that each spectral channel has a respective arrival time at the photodetector and wherein a smallest time difference between arrival times of different spectral channels is less than a dead time of the photodetector.

11. The photodetection apparatus of any one of claims 1 to 10, wherein the system is arranged such that each spectral channel has a respective arrival time at the photodetector and wherein a smallest time difference between arrival times of different spectral channels is greater than a dead time of the photodetector.

12. The photodetection apparatus of any one of claims 1 to 11, wherein the photodetector is a single photon detector.

13. The photodetection apparatus of any one of claims 1 to 12, wherein the photodetection apparatus is configured to receive a single photon in one of the spectral channels of the plurality of spectral channels at the input port of the first waveguide and detect a time of arrival of the single photon at the photodetector.

14. The photodetection apparatus of any one of claims 1 to 13, further comprising control logic configured to determine the spectral channel of a photon detected at the photodetector based on the time of arrival of the detected photon at the photodetector.

15. A photon emission system comprising:a photon pair source arranged to generate a frequency-entangled photon pair;a photodetection apparatus according to claim 14 and arranged to receive a first photon of a generated frequency-entangled photon pair; andcontrol logic configured to determine a spectral channel of the second photon of the generated frequency-entangled photon pair based on the determined spectral channel of the first photon of the generated frequency-entangled photon pair.

16. A tuneable photon emission system comprising:a photon emission system according to claim 15, wherein the control logic is further configured to determine a complementary frequency of a pump field based on the determined spectral channel of the second photon of the generated frequency-entangled photon pair; and a non-linear photonic element arranged to receive the second photon of the generated frequency-entangled photon pair and a pump field having the determined complementary frequency and to emit a photon having a predetermined frequency.

17. The tuneable photon emission system of claim 16, wherein the plurality of discrete optical components of the photodetection apparatus are arranged such that the spectral channel having the shortest optical pathlength between the input port of the first waveguide and the output port of the second waveguide corresponds to a spectral channel of the first photonwhich is associated with a conversion mode in the non-linear photonic element having a highest conversion efficiency.

18. A method of determining a spectral channel of a photon, the method comprising:detecting a time of arrival of a photon output from a photodetection apparatus, the photodetection apparatus comprising:a first waveguide and a second waveguide for supporting propagation of photons in a plurality of spectral channels, the first waveguide having an input port for coupling photons into the first waveguide and the second waveguide having an output port for outputting photons from the second waveguide; anda coupling arrangement configured to couple photons from the first waveguide to the second waveguide, the coupling arrangement comprising a plurality of discrete optical components each configured to couple photons in a different spectral channel from the first waveguide to the second waveguide; anddetermining a spectral channel of the detected photon, wherein the photodetection apparatus is arranged such that an optical pathlength between the input port of the first waveguide and the output port of the second waveguide and coupled by a discrete optical component of the coupling arrangement is different for each of the plurality of discrete optical components and each spectral channel has an associated optical pathlength between the input port of the first waveguide and the output port of the second waveguide, and wherein the spectral channel of the detected photon is determined in dependence on the detected time of arrival at which the photon is detected.

19. A photodetection apparatus comprising:a photon source operable to emit at least one photon;a photodetector arranged to receive photons originating from the photon source and configured to detect a time of arrival of photons at the photodetector; anda coupling arrangement configured to couple photons emitted from the photon source to the photodetector, the coupling arrangement comprising a plurality of discrete optical components each configured to couple photons in a different spectral channel from the photon source to the photodetector;wherein the photodetection apparatus is arranged such that an optical pathlength between the photon source and the photodetector is different for each of the plurality of discrete optical components and each spectral channel has a different associated optical pathlength between the photon source and the photodetector.

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