Phase adjustment system and method using photon statistics

Abstract

A system comprises an optical circuit and a controller. The optical circuit comprises (i) a photon source to produce a photon sequence, (ii) a configurable interferometer to interfere photons of the photon sequence to produce an interfered photon sequence, the interferometer comprising first and second temporal mode coupling devices, (iii) a phase shifter configured to adjust a relative phase between the first temporal mode coupling device and the second temporal mode coupling device of the interferometer; and (iv) a detector arrangement comprising one or more photodetectors and configured to measure the interfered photon sequence. The controller is configured to obtain measurements of the interfered photon sequence; determine, from the obtained measurements, a relative phase between the first temporal mode coupling device and the second temporal mode coupling device; and based on the determined relative phase, controlling the phase shifter to adjust the relative phase.

Description

Phase Adjustment System And Method Using Photon StatisticsTechnical Field

[0001] The present disclosure relates to hybrid quantum-classical systems. More particularly, the present disclosure relates to the control of hybrid quantum-classical systems that are able to interfere photons in different temporal modes.Background

[0002] A boson sampler is a non-universal quantum computer that relies on the interference of photons to generate its output. More particularly, a boson sampler comprises a network of optical components or elements (an interferometer) in which photons interfere with one another. As photons are quantum objects, the output of this network is described by a quantum superposition of all the possible outcomes. When a measurement is performed at the output of this network using one or more photodetectors, a single measurement outcome is realised from this superposition. For example, if photon number resolving (PNR) detectors are used, then each measurement outcome may be described by an array or string or sequence of integers indicating how many photons were found in each output mode of the output state; if threshold detectors are used, then each measurement outcome may be described by an array or string or sequence of integers, for example a binary sequence, indicating whether photons were present or absent in each output mode of the output state. By repeatedly sampling measurement outcomes, one can build up a picture of the probability distribution governing the quantum superposition. The photonic superposition states output from an interferometer of a boson sampler can be highly entangled. Accordingly, the output probability distributions generated by a boson sampler may have a complex structure and simulating this sampling task is understood to be intractable classically. Modem supercomputers fail to simulate boson sampler distributions generated from more than a few tens of modes.

[0003] If the interferometer is a temporal interferometer (e.g. it interferes photons in different temporal modes), then phase stabilisation is important for maintaining quantum coherence and enabling accurate interference effects. In the context of boson sampling, where the goal is to sample from the distribution of photons passing through a multi-mode interferometer, small phase fluctuations can drastically alter the output distribution. Furthermore, a loss of phase coherence can mean that the photons no longer interfere as expected, causing the output to deviate (or “wash out”) from the desired boson sampling distribution via phase averaging effects. Approaches to phase stabilisation may include Pound-Drever-Hall (PDH) mechanisms, but such mechanisms require additional optical components (e.g. an additional reference laser system dedicated to phase stabilisation, additional components to couple light from the reference laser into and out of the interferometer, and additional detectors to measure the reference light). These additional components may introduce further photon loss into the interferometer, and use of the reference laser may introduce backscattering noise in the signal modes of the boson sampler. Photon loss and backscattering noise may reduce the ability of the boson sampler to reliably generate samples of the desired output distribution.Summary

[0004] According to an aspect of the present disclosure, a system is provided. The system comprises an optical circuit and a controller. The optical circuit comprises a photon source operable to produce a photon sequencecomprising a number of time bins. The optical circuit further comprises a configurable interferometer operable to interfere photons of the photon sequence to produce an interfered photon sequence. The interferometer comprises a first temporal mode coupling device configurable to interfere photons in different time bins. The interferometer further comprises a second temporal mode coupling device configurable to interfere photons in different time bins. The optical circuit further comprises a phase shifter configured to adjust a relative phase between the first temporal mode coupling device and the second temporal mode coupling device of the interferometer. The optical circuit further comprises a detector arrangement comprising one or more photodetectors, the detector arrangement configured to measure the interfered photon sequence, the measurement indicative of a photon occupation of each time bin of the interfered photon sequence. The controller is configured to operate the optical circuit a number of times to obtain measurements of the interfered photon sequence. The controller is further configured to determine, from the obtained measurements of the interfered photon sequence, a relative phase between the first temporal mode coupling device and the second temporal mode coupling device. The controller is further configured to, based on the determined relative phase, control the phase shifter to adjust the relative phase between the first and second temporal mode coupling devices.

[0005] Advantageously, such a system is able to adjust a relative phase between two temporal mode coupling devices without additional cumbersome equipment that may be required for a PDH scheme. Furthermore, such a system can be more robust to environmental perturbations, for example jolts or vibrations when the system is being transported.

[0006] Determining, from the obtained measurements of the interfered photon sequence, a relative phase between the first temporal mode coupling device and the second temporal mode coupling device may comprise determining, from the obtained measurements of the interfered photon sequence, relative photon occupations of time bins of the interfered photon sequence, and determining, from the relative photon occupations of the time bins of the interfered photon sequence, the relative phase between the first temporal mode coupling device and the second temporal mode coupling device.

[0007] In some examples the photons sequence and the interfered photon sequence may each comprise three time bins. The first time bin of the photon sequence may be occupied by a photon, and a measurement of the interfered photon sequence is indicative of the second time bin or third time bin of the interfered photon sequence being occupied by the photon.

[0008] The first temporal mode coupling device may comprise a reconfigurable beamsplitter and a delay line, the delay line configured to connect output port of the reconfigurable beamsplitter with one input port of the reconfigurable beamsplitter.

[0009] In some examples, the photon source may comprise a quantum dot. In some examples, the photon source may comprise a heralded photon source. For example, the photon source may comprise a non-linear material inside a cavity.

[0010] In some examples, the one or more photons of the photon sequence have a first frequency. The photon source may be further operable to contemporaneously produce a second photon sequence comprising the number of time bins, at least one time bin occupied by a photon having a second frequency. The configurable interferometer may be further operable to interfere photons of the second photon sequence to produce a second interfered photon sequence. The detector arrangement may be further configured to measure the second interfered photon sequence. The controller may be further configured to determine the relative phase from the obtainedmeasurements of the interfered photon sequence and obtained measurements of the second interfered photon sequence. Advantageously, by evaluating photon statistics at a second frequency, a controller may be able to better determine what adjustment to make to the relative phase.

[0011] In some examples, controlling the phase shifter to adjust the relative phase may comprise controlling the phase shifter to eliminate or zero the relative phase.

[0012] According to an aspect of the present disclosure, a method is provided. The method comprises obtaining measurements of an interfered photon sequence, wherein the interfered photon sequence comprises a number of time bins, at least one time bin is occupied by a photon, and wherein each measurement is indicative of a photon occupation of each time bin of the interfered photon sequence. The method further comprises determining, from the obtained measurements of the interfered photon sequence, a relative phase between a first temporal mode coupling device and a second temporal mode coupling device of a configurable interferometer that produced the interfered photon sequence. The method further comprises controlling, based on the determined relative phase, a phase shifter to adjust the phase between the first temporal mode coupling device and the second temporal mode coupling device of the configurable interferometer.

[0013] According to an aspect of the present disclosure, a non-transitory-computer-readable medium is provided. The non-transitory computer-readable medium has instructions thereon which, when executed by one or more processors coupled to an optical circuit, cause the one or more processors to perform a method as described herein.

[0014] Many modifications and other embodiments set out herein will come to mind to a person skilled in the art in light of the teachings presented herein. Therefore, it will be understood that the disclosure herein is not to be limited to the specific embodiments disclosed herein. Moreover, although the description provided herein provides example embodiments in the context of certain example combinations of elements, steps and / or functions, it will be appreciated that different combinations of elements, steps and / or functions may be provided by alternative embodiments without departing from the spirit or scope of the disclosure.Brief Description of the Figures

[0015] Illustrative embodiments of the present disclosure will now be described by way of example only, with reference to the accompanying figures.

[0016] Fig. 1 shows a block diagram of a system according to an example.

[0017] Fig. 2 shows an illustration of a boson sampler according to an example.

[0018] Fig. 3 A shows a cartoon of an interferometer such as that depicted in Fig. 2;

[0019] Fig. 3B shows an illustration of a histogram indicating the expected result of measurements output from the interferometer when configured as in Fig. 3A.

[0020] Fig. 4A and Fig. 4B. show cartoons of the interferometer at two different time points.

[0021] Fig. 4C shows an illustration of a histogram indicating the expected result of measurements output from the interferometer when configured as in Figs. 4A and 4B.

[0022] Fig. 5A, Fig. 5B and Fig. 5C. show cartoons of the interferometer at three different time points.

[0023] Fig. 5D shows an illustration of a histogram indicating the expected result of measurements output from the interferometer when configured as in Figs. 5 A, 5B and 5C.

[0024] Fig. 6A, Fig. 6B and Fig. 6C. show cartoons of the interferometer at three different time points.

[0025] Fig. 6D shows an illustration of a histogram indicating the expected result of measurements output from the interferometer when configured as in Figs. 6 A, 6B and 6C.

[0026] Fig. 7 illustrates probabilities of measuring a photon in one of two different time bins as a function of the relative phase between two temporal mode coupling devices of an interferometer.

[0027] Fig. 8 illustrates probabilities of measuring a photon in one of two different time bins as a function of the relative phase between two temporal mode coupling devices of an interferometer when samples are taken for different frequencies.

[0028] Fig. 9 shows a flowchart of a method for adjusting a phase according to an example.

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

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

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

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

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

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

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

[0036] The terms “time bin” and “temporal mode” have been used interchangeably throughout this description.

[0037] The term “photon sequence” may be understood to mean a multimodal quantum state of light comprising multiple time bins. For example, a photon sequence may comprise a sequence of single photons, or a sequence in which a first time bin contains a first number of photons (or zero), a second time bin contains a second number of photons (or the first number of photons, or zero) and so on., or a sequence comprising other quantum states of light in which the number of photons is not precisely known such as a non-Gaussian light pulse formed from subtracting one or more photons from a Gaussian pulse. The term “interfered photon sequence” may be understood to mean a photon sequence output from an interferometer in which temporal modes may have been interfered.

[0038] Fig. 1 depicts a block diagram of a heterogeneous system 100 in which illustrative embodiments may be implemented. The heterogeneous system 100 comprises both classical processing apparatus and quantum processing apparatus. Other architectures to that shown in Fig. 1 may be used as will be appreciated by the skilled person. For example, system 100 may be distributed across multiple interconnected devices.

[0039] System 100 is an example of a specialised computing apparatus, in which computer usable program code or instructions implementing the processes may be located. In this example, system 100 includes communication fabric 102, which provides communications between a processor unit 104, memory unit 106, input / output unit 108, communication module 110, display 112, and boson sampler 114, the boson sampler comprising a state generation unit 116, an interferometer 118, a state detection unit 120 and a dedicated controller unit 122.

[0040] The system 100 may be implemented in any of a number of ways. For example, the system 100 may be provided as a number of hardware modules suitable for installation in a server / computer rack (for example a conventional 19-inch server rack). For example, the processor unit 104, memory unit 106, input / output unit 108, and communication module 110 may be provided in a first rack-mounted hardware module, the controller 122 may be implemented in a second rack-mounted hardware module and electronically coupled to the first hardware module, the state generation unit 116 may be implemented in a third rack-mounted hardware module electronically coupled to the controller 122, the interferometer 118 may be implemented in a fourth rack-mounted hardware module electronically coupled to the controller 122 and optical fibre -connected to the state generation unit 116, and the photodetectors of the state detection unit 120 may be provided in another hardware module electronically coupled to the controller 122 and optical fibre-connected to the interferometer module and, optionally, to the state generation unit 116. In other examples, the system 100 may be implemented using one or more separate devices communicatively coupled (at least in part) over a network such as the internet.

[0041] The processor unit 104 is configured to execute instructions for software that may be loaded into the memory unit 106. Processor unit 104 may be a set of one or more processors or may be a multi-processor core, depending on the particular implementation. Furthermore, processor unit 104 may be implemented using one or more heterogeneous processor systems in which a main processor is present with secondary processors on a single chip. The processor unit 104 may comprise one or more central processing units (CPUs), one or more graphicsprocessing units (GPUs) or any combination thereof. If the processor unit 104 comprises multiple processors, the multiple processors may operate individually or collectively.

[0042] The memory unit 106 may comprise any piece of hardware that is capable of storing information, such as, for example, data, program code in lunctional form, and / or other suitable information on a temporary basis and / or a permanent basis. The memory unit 106 may include, for example, a random-access memory or any other suitable volatile or non-volatile storage device. The memory unit 106 may include a form of persistent storage, for example a hard drive, a flash memory, a rewritable optical disk, a rewritable magnetic tape, or some combination thereof. The media used for persistent storage may also be removable. For example, the memory unit 106 may include a removable hard drive.

[0043] Input / Output unit 108 enables the input and output of data with other devices that may be in communication with the system 100. For example, input / output unit 108 may provide a connection for user input through a keyboard, a mouse, and / or other suitable devices. The input / output unit 108 may provide outputs to, for example, a printer.

[0044] Communications module 110 enables communications with other data processing systems or devices. The communications module 110 may provide communications through the use of either or both physical and wireless communications links. For example, the communications module 110 may be configured to communicate with other data processing systems or devices via a wired local area network connection, via WiFi or over a wide area network such as the internet.

[0045] Instructions for the applications and / or programs may be located in the memory unit 106, which is in communication with the processor unit 104 through communications fabric 102. Computer-implementable instructions may be in a lunctional form on persistent storage in the memory unit 106 and may be performed by processor unit 104. These instructions may sometimes be referred to as program code, computer usable program code, or computer-readable program code that may be read and executed by a processor in processor unit 104. The program code in the different embodiments may be embodied on different physical or tangible computer- readable media.

[0046] The program code may contain instructions which, when processed by the processor unit 104, cause the processor unit 104 to communicate with the boson sampler to sample a bosonic probability distribution.

[0047] In Fig. 1, computer-readable instructions 126 are located in a functional form on computer-readable storage medium 124 that is selectively removable and may be loaded onto or transferred to system 100 for execution by processor unit 104. Alternatively, computer-readable instructions 126 may be transferred to system 100 from computer-readable storage medium 124 through a communications link to communications module 110 and / or through a connection to input / output unit 108. The communications link and / or the connection may be physical or wireless.

[0048] A computer-readable storage medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or any suitable combination thereof. More specific examples of the computer-readable medium include the following: a portable computer diskette, a hard disk, a random-access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a portable compact disc read-only memory (CDROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of thisdocument, a computer-readable storage medium may be any tangible medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device.

[0049] In some illustrative embodiments, computer-implementable instructions 126 may be downloaded over a network to the memory unit 106 from a remote device for use with system 100. For instance, computer- implementable instructions stored in a remote server may be downloaded over a network from the server to the system 100.

[0050] The boson sampler 114 comprises an optical circuit and a dedicated (classical) control unit 122. The optical circuit comprises a state generation unit 116, a linear interferometer 118, and a state detection unit 120 (also referred to herein as a “detector arrangement”). In this particular example, the boson sampler 114 is a temporal boson sampler, in the sense that the interferometer 118 is able to interfere photons in different time bins. Accordingly, the controller 122 may generate one or more control signals to cause the state generation unit 116 to generate an input multimodal photonic state, and contemporaneously generate one or more control signals to dynamically configure the interferometer 118 while the photonic state passes through. The controller 122 may then receive from a photodetector of the state detection unit 120 a measurement outcome indicating whether photons were measured in each temporal mode of the output multimodal photonic state. The controller 122 may then interpret the number of photons detected in each output mode of the output multimodal photonic state as a sequence / string of integers and provide this integer sequence to the one or more processors 104. For example, each integer of the integer sequence may correspond to a detected number of photons in each mode. By re-running the boson sampler 114 a number of times, the controller 122 may provide a set of integer sequences to the processor unit 104. A diagram of the boson sampler 114 is set out in Fig. 2.

[0051] Due to interference between photons in different temporal modes, in operation the boson sampler 114 transforms the input multimodal photonic state (a photon sequence) into an output multimodal photonic state (an interfered photon sequence) that may be expressed as a superposition of the different possible configurations of the photons in the output modes aswhere C is a configuration,is the number of bosons in the jth output mode in configuration C, and acis the probability amplitude associated with configuration C. The skilled person would appreciate that while the state of (EQ. 1) is expressed as a pure state, this is for illustrative purposes only - photon loss may, for example, mean that the output state can be expressed only as a mixed state. By tuning the parameter values {q} of the interferometer 118, the probability amplitudes associated with each configuration may be changed. Accordingly, a measurement of the number of photons in each output mode can yield a measurement outcome representable as a string of integers corresponding to a configuration C. By operating the boson sampler 114 a number of times to produce a batch of samples S. it is possible to establish an empirical probability distribution of the bosonic configurations of the output state. One can expect that with many samples, the probability pcof obtaining a measurement outcome corresponding to configuration C is approximately given by pc= | ac|2.

[0052] The state generation unit 116 is configured to generate an input multimodal photonic state \lPIN) comprising N photons distributed across a plurality of M input modes. In this example, each input mode is a temporal mode (also referred to as a time bin), and the input multimodal photonic state accordingly comprises aphoton sequence. For example, over a time period of duration T comprising a number M of time bins, the state generation module 116 may be configured to produce a single photon in each of M time bins according to a prescribed photon sequence, with N less than or equal to M. The photon duration may be smaller than the duration of each time bin. For example, the photon pulse duration may be approximately 500 picoseconds while the time At between one photon and the next may be for example on the order of 100 nanoseconds or 1 microsecond.

[0053] With reference to Fig. 2, the state generation module 116 comprises a photon source 210 operable to produce a photon sequence comprising a number of time bins, at least one time bin occupied by a photon. The photon source 210 may comprise a heralded single photon source. For example, the photon source 210 may comprise a non-linear photonic material (such as periodically -poled lithium niobate (PPLN) or potassium titanyl phosphate (KTP)) configured to receive a pump beam from a pump laser and to probabilistically generate pairs of entangled photons, and may further comprise a photodetector configured to detect a photon of the entangled pair, thereby heralding the presence of the other photon of the pair. The non-linear photonic element may rely upon spontaneous parametric down conversion (SPDC) or may produce photon pairs through another non-linear process such as spontaneous four-wave mixing. The controller 122 may be configured to control the operation of a pump laser. The photodetector used to detect a photon of the entangled pair may be built into the state generation unit 116 or the state detection unit 120, and may be configured to communicate a detection event signal, for example an electrical signal, to the controller 122 to indicate or herald generation of the other photon of the photon pair.

[0054] While only a single photon source 210 is illustrated in Fig. 2, in order to increase the probability of successfully producing a photon in any given time bin, in some examples the state generation module 116 may comprise a plurality of heralded single photon sources and a multiplexer. Each single photon source may be configured to probabilistically produce a single photon in each time bin. The multiplexer may be configured to receive successfully generated photons from the plurality of heralded single photon sources and couple them to an output port for onwards propagation to the interferometer 118.

[0055] The skilled person will appreciate that modifications to the state generation unit 116 may be made in order to boost the probability of successfully generating photons in the requisite time bins. For example, the state generation unit 116 may comprise one or more multi-spectral sources. An example of such a multi-spectral source is a non-linear photonic element suitable for e.g. spontaneous parametric down conversion provided inside a photonic cavity having optically reflecting elements. For example, the non-linear photonic element may be placed between mirrors, for example bulk optic mirrors, or Bragg gratings. As an alternative, the non-linear photonic element may comprise a non-linear crystal waveguide with end facets covered with a reflective coating. The optical cavity enables the confinement of light with frequencies such that the round-trip distance is equal to an integer number of wavelengths and accordingly there is a plurality of frequency modes which satisfy this constraint. This set of frequency modes can be grouped as modal pairs that are equidistant in frequency about some central frequency. Each of these modal pairs can support the generation of a photon pair with frequencies equal to those of the associated modes. Accordingly, the non-linear photonic element and cavity are together capable of producing a plurality of photon pairs across a range of frequencies.

[0056] In other examples, the photon source 210 may comprise a quantum dot that may be driven by a pump laser to emit single photons.

[0057] The skilled person would appreciate that the state generation unit 116 may comprise further elements for enabling finer control over the photon sequence produced. For example, the state generation unit may include one or more optical filters to controllably filter out photons that are produced in time bins which, according to the prescribed photon sequence, no photon should be produced.

[0058] While in this example, the state generation unit may produce a photon sequence comprising single photons, in other examples the state generation unit 116 may be configured to generate an input multimodal photonic state | WIN ) comprising a Gaussian photonic input in each of the N input modes (in which case the boson sampler may be referred to as a Gaussian boson sampler). For example, a single mode squeezed state (SMSS), also referred to as a squeezed coherent state, may be input into each input mode. In other examples, the state generation unit may be configured to generate an input multimodal photonic state \WIN) comprising a nonGaussian photonic input in at least one of the N input modes (in which case the boson sampler may be referred to as a non-Gaussian boson sampler). For example, a photon may be subtracted from a single mode squeezed state and detected, thereby heralding the generation of a non-Gaussian state for input into an input mode. In some examples, the state generation unit 116 may be configured to generate an input multimodal photonic state \WIN) that comprises single photons in some modes and squeezed coherent states in other modes.

[0059] The state generation unit 116 is optically coupled to the interferometer 118, for example via optical fibre, such that a photon sequence generated by the state generation unit 116 is passed to the interferometer 118.

[0060] The reconfigurable interferometer 118 is arranged to receive the input multimodal photonic state (that is, a photon sequence) from the state generation unit 116 and to interfere photons in different time bins. The interferometer 118 comprises a plurality of optical elements arranged to interfere the photons of the modes of the input multimodal photonic state (photon sequence), thereby transforming the input multimodal photonic state to produce an output multimodal photonic state, which may be referred to herein as an interfered photon sequence. Several optical elements of the interferometer 118 are configurable, and each configurable optical element has one or more configurable parameters that influence the operation of that element when the boson sampler is used. Accordingly, the transformation from input multimodal photonic state to output multimodal photonic state is dependent on a set of parameter values q] that define the function of one or more configurable optical elements. One or more of the configurable parameters may characterise a single mode operation. For example, a configurable optical element may comprise a phase shifter, and a parameter value qaof the set of parameter values {q} may characterise the phase shift imparted by the phase shifter. One or more of the configurable parameters may characterise a multimodal operation. For example, a configurable optical element may comprise a reconfigurable beamsplitter having a tuneable transmission (or equivalently, a reflection) coefficient, and a parameter value qbof the set of parameter values {q} may characterise the transmission coefficient of the reconfigurable beam splitter.

[0061] A parametrised / reconfigurable beam splitter 230 is understood to mean any tuneable element or device or tuneable collection of elements / devices capable of coupling two modes of electromagnetic radiation with each other with a reconfigurable reflection / transmission coefficient and optionally a reconfigurable phase shift coefficient. The parametrised beam splitters may be implemented in any suitable way - for example a parametrised beam splitter may comprise a Mach-Zehnder type interferometer containing a variable phase shifter in one internal path for controlling the effective beam splitter reflection coefficient of the Mach-Zehnder interferometer. The phase shifter may be implemented using an electro-optical modulator. The Mach-Zehnder interferometer mayfurther comprise an external phase shifter on one external path of the Mach-Zehnder interferometer to control the relative phases of the two modes acted upon. For example, when implemented in bulk optics, a reconfigurable beamsplitter 230-1 may comprise two 50 / 50 beamsplitters and a phase shifter element arranged therebetween.

[0062] With reference to Fig. 2, in this example the interferometer 118 comprises a first temporal mode coupling device 220-1 and a second temporal mode coupling device 220-2. The first temporal mode coupling device 220- 1 comprises a reconfigurable beam splitter 230-1 and a delay line 240-1. The delay line 240-1 is arranged to connect one output port of the reconfigurable beam splitter 230-1 with one input port of the reconfigurable beam splitter 230-1. The delay line may comprise, for example, optical fibre. The delay line 240-1 has a length cAt where c is the speed of light in the fibre. In this way, the field of the photon in one temporal mode may be coupled, at least partially, into the delay line 240-1 so as to interfere with the field of the photon(s) in the next temporal mode on the parametrised beam splitter 230-1. The second temporal mode coupling device 220-2 in this example also comprises a reconfigurable beam splitter 230-2 and a delay line 240-2. The delay line 240-2 is arranged to connect one output port of the reconfigurable beam splitter 230-2 with one input port of the reconfigurable beam splitter 230-2. In this example, the delay line 240-2 also has a length cAt, such that a photon in one temporal mode may be made to interfere with a photon in a (temporally) adjacent temporal mode, however the skilled person will appreciate that the second delay line 240-2 may be designed to have a different delay length to the first delay line 240-1.

[0063] Each of the temporal mode coupling devices 220-1, 220-2 further comprises a corresponding phase shifter (250-1 and 250-2 respectively). A phase shifter may comprise, for example, a piezoelectric or electro- optical modulator controllable to adjust the phase of light passing through the delay line 240. In some examples, a phase shifter may comprise a thermo-optic element.

[0064] The interferometer 118 is optically coupled to the state detection unit 120, for example via optical fibre, such that an interfered photon sequence produced by the interferometer 118 is passed to the state detection unit 120.

[0065] The state detection unit 120 comprises a photodetector 260 configured to detect photons output from the interferometer 118 and produce corresponding detection event signals. In other words, the photodetector 260 is configured to measure the interfered photon sequence, the measurement indicative of a photon occupation of each time bin of the interfered photon sequence. In this example, the photodetector 260 comprises a photon number resolving (PNR) photodetector capable of determining how many photons are received. For example, the photodetector 260 may comprise a superconducting nanowire detector that generates an output signal having an intensity proportional to the (discrete) number of photons that strike the detector. As an alternative, the photodetector 260 may comprise a transition edge sensor (TES).

[0066] While only one photodetector 260 is shown in Fig. 2, the skilled person will appreciate that the detector arrangement 120 may comprise additional detectors that together are capable of measuring the interfered photon sequence output from the interferometer 118. The detector arrangement 120 may further comprise additional equipment for routing photons having particular characteristics to a particular photodetector, for example to enable photons from a multi-spectral source to be measured.

[0067] In other examples, the state detection unit 120 may comprise one or more threshold detectors, also known as on / off detectors. Threshold detectors are not capable of determining how many photons are received but are capable of determining the presence / absence of photons in an output mode.

[0068] The controller 122 is communicatively coupled to the processor unit 104, the state generation unit 116, the interferometer 118 and the state detection unit 120. The controller 122 may be any suitable classical computing resource for controlling the operation of the boson sampler 114. In some examples, the controller 122 is implemented in a dedicated, application-specific processing unit. For example, the controller 122 may comprise an application-specific integrated circuit (ASIC) or an application-specific standard product (ASSP) or another domain-specific architecture (DSA). Alternatively, the controller 122 may be implemented in adaptive computing hardware (in other words, hardware comprising configurable hardware blocks / configurable logic blocks) that has been configured to perform the required functions, for example in a configured field programmable gate array (FPGA). The controller 122 may have a dedicated random-access memory or other memory element for temporarily logging data. In some examples, the functionality of the controller 122 may be incorporated into the functionality of the processor unit 104.

[0069] The controller 122 is configured to communicate with (that is, receive signals from and send signals to) the processor unit 104. For example, the controller 122 may receive a signal, for example an electrical signal, from the processor unit 104 to commence sampling from an output distribution of the boson sampler 114. The controller may further communicate measurement outcomes to the processor unit 104, for example via electrical signals.

[0070] The controller is configured to, in conjunction with the processor unit 104, establish a clock cycle for the boson sampler 114. In particular, the controller 122 is capable of defining the duration of time bins, for example by adjusting a pump rate of the photon source 210 of the state generation unit 116.

[0071] The controller 122 is configured to control the photon source 210 to produce a prescribed photon sequence over a time period T comprising a number M of time bins. In particular, in operation the controller 122 may send one or more control signals to the state generation unit 116 to cause the photon source 210 to commence production of the photon sequence.

[0072] The controller 122 is further configured to generate control signals to control the operation of one or more active optical elements (e.g. reconfigurable beamsplitters or phase shifters) of the interferometer 118. With reference to Fig. 2, the controller 122 is configured to tune the parameter value (e.g. transmittance) of the parametrised beam splitter 230-1 for each time interval. For example, the controller 122 may, as a first photon is emitted from the photon source 210, configure the reconfigurable beamsplitter 230-1 to loop the first photon into the delay line 240. The controller 122 may then, at a time corresponding to the second time bin of the photon sequence (e.g. when a second photon is emitted from the photon source 210), configure the reconfigurable beamsplitter 230-1 using parameter value q to cause interference between the first and second photon of the photon sequence. The controller 122 may then, at a time corresponding to the third time bin of the photon sequence (e.g. when a third photon is emitted from the photon source 210), configure the reconfigurable beamsplitter 230- 1 using parameter value q2to cause interference between the photons of the second temporal mode and the photon of the third temporal mode. The controller 122 may then, at a time corresponding to the fourth time bin of the photon sequence (e.g. when a fourth photon is emitted from the photon source 210, configure the reconfigurable beamsplitter 230-1 using parameter value q3to cause interference between the photons of the third temporal mode and fourth temporal mode. Likewise, the controller 122 may configure the reconfigurable beamsplitter 230-2 of the second temporal mode coupling device 220-2 to further interfere photons. The controller 122 is furtherconfigured to adjust the phase imparted by the phase shifters 250-1, 250-2. This may continue until a predetermined transformation has been performed on the input photon sequence of M time bins.

[0073] The controller 122 is further configured to receive a response from the photodetector 260 of the state detection unit 120 in order to sample from the output distribution of the boson sampler.

[0074] In operation, at the same time as commencing the photon sequence, the controller 122 is configured to commence sampling the output distribution of the boson sampler 114 by logging detection event signals received from the state detection unit 120. In other words, while the prescribed photon sequence is being produced by the state generation unit 116, the controller 122 logs detection event signals received from the state detection unit. In the event that the state generation unit 116 successfiilly completes production of the photon sequence, then the controller 122 may infer that the correct output multimodal photonic state (the correct interfered photon sequence) has been produced by the interferometer 118 and that the sequence of detection event signals from the state detection unit 120 logged during the time period represents a valid measurement of the interfered photon sequence. The controller 122 is configured to communicate the measurement outcome or sample (for example, the collated record of the received detection event signals over that particular time period) to the processor unit 104. For example, the controller 122 may interpret the detection event signals as the number of photons in each measured output mode of the output state (EQ. 1) as a sequence or string of integers and provide this sequence to the processor unit 104. The controller 122 may then repeat commencing production of the photon sequence and sampling of the interfered output distribution until a predetermined number of samples have been collected and communicated to the processor unit 104.

[0075] During downtime in sample collection, the boson sampler 114 may enter a calibration mode in which the optical circuit is calibrated to mitigate against environmental impacts. For example, in the calibration mode the controller 122 may attempt to stabilise any phase difference that may arise between the temporal mode coupling devices. For example, it may be desirable to stabilise the phase difference between the two temporal mode coupling devices to ensure that photons in the interferometer are made to interfere as expected, and to mitigate the effects of phase averaging effects in collecting samples from the boson sampler 114. The controller 122 is further (alone or in conjunction with the processor unit 104) configured to control a relative phase between the first temporal mode coupling device 220-1 and the second temporal mode coupling device 220-2 of the interferometer 118. In particular, the controller 122 is configured to operate the optical circuit (in this example the state generation unit 116, interferometer 118 and state detection unit 120) a number of times to produce an interfered photon sequence and to obtain measurements (e.g. a batch of samples) of the interfered photon sequence. The controller 122 is further configured to determine, from the obtained measurements of the interfered photon sequence, a relative phase between the first temporal mode coupling device 220-1 and the second temporal mode coupling device 220-2. The controller 122 is further configured to, based on the determined relative phase, control one or both of the phase shifters 250-1, 250-2 to adjust the relative phase between the first and second temporal mode coupling devices 220-1, 220-2. Ways in which this may be accomplished are described fiirther below in relation to Fig. 3 A to Fig. 9.

[0076] The skilled person would appreciate that the architecture described above in relation to Figs. 1 and 2 is not intended to provide limitations on the systems with which the methods described herein may be implemented. Instead, the skilled person would appreciate that other architectures may be applicable. For example, the boson sampler 114 may comprise further reconfigurable beamsplitters 230 and further delay lines 240, or may comprisefurther phase shifters 250, in order to generate more complicated interference patterns. The skilled person would further appreciate that delay lines of different lengths may be used to vary which temporal modes are interfered with one another. The time-bin interferometer 118a may comprise further optical components including further optical switches.

[0077] To better explain how the controller 122 may determine a relative phase between the first temporal mode coupling device 220-1 and the second temporal mode coupling device 220-2, reference is now made to Figs. 3A- 7. It is helpful to first revisit some of the basic functionality of a time-bin interferometer, and the types of measurements one can perform using such an interferometer and a photodetector. Examples of temporal mode manipulation using a fibre-loop time-bin interferometer (such as interferometer 118) are pictorially depicted in Figs. 3A-6D. Specifically, Figs. 3A, 4A-4B, 5A-5C, and 6A-6C show cartoons of a first temporal mode coupling device 310 comprising a reconfigurable beamsplitter and delay line, a second temporal mode coupling device 320 comprising a reconfigurable beamsplitter and delay line, and a PNR photodetector 330. Phase shifters (e.g. phase shifters 250) are not depicted in the cartoons. The cartoons further show the expected locations of a photon at given time points, assuming that the photon sequence provided to the interferometer comprises a single photon populating a single time bin e.g. the photon sequence is [1 ,0,0, .. ,0] . In these cartoons, the expected location of the photon just before and just after the time point highlighted are illustrated with solid and hollow circles respectively. In the case of the latter, the probability that the photon is where it is shown is also explicitly highlighted.

[0078] In Fig. 3A, both coupling devices 310, 320 are configured to bypass the delay line e.g. there is a 0% probability that the photon will couple into either loop. As such, one would expect that at the output of the interferometer there has been no change to the photon sequence. If the photon sequence is provided to the interferometer a number of times, then one would expect that all detection events signalled by the photodetector 330 occur in the first time bin, that is at time t0. This is illustrated in the sketched histogram of Fig. 3B, in which 100% of the counts (the detection events) occur at time t0.

[0079] In Fig. 4A, the first temporal mode coupling device 310 is configured to couple photons into the loop with a probability of 100% in the first time bin, denoted by t0. In Fig. 4B, the first temporal mode coupling device 310 is configured to couple the photon out of the loop with probability 100% in the second time bin at time= tQ+ At. The second temporal mode coupling device 320 is configured to cause the photon to bypass the second delay line at both time points. Accordingly, one expects a single peak in the measurement histogram (Fig. 4C) that is delayed by one time bin compared to the histogram retrieved from the experiment performed in Fig. 3A.

[0080] In Figs 5A-5D, the idea is extended fiirther. In this case both temporal mode coupling device 310, 320 are treated the same at times t0and tT, except that at the second device 320 is configured to couple the photon into the loop with 100% probability (Fig. 5A). At time points t2= tq + At and t3= t2+ At the second device 320 is configured to couple the photon out of the loop with probabilities of 50% (Fig. 5B). and 100% (Fig. 5C). respectively.

[0081] In this case, neglecting loss, one can expect the histogram (Fig. 5D) to exhibit two equally spaced peaks at times t2and t3. For a fixed integration time, the height of these two peaks may sum to the height of the single peak in the previous experiments.

[0082] With this context, Figs. 6A, 6B, 6C, 6D and 7 are now referenced to describe how the relative phase difference between the two temporal mode coupling devices 310 and 320 may be determined. Once again, aphoton sequence is provided to the interferometer in which only the first time bin is populated with a photon. This time, however, at time t0(the first-time bin), the first and second temporal mode coupling devices 310, 320 are set to couple the photon into their respective delay lines with probability 50% and 100% respectively (Fig. 6A). This results in an equal probability of the photon going into either the first or the second loop. At time tT, the first loop is read out with 100% probability, and the second loop is set to read-out with 50% probability (Fig. 6B). Owing to interference effects, at this point, the photon will either couple back into the second loop or exit the second loop towards the detector 330, with a probability that is determined by the relative phase between the two paths that the photon could have taken. Note that in Fig. 6B, this probability has been represented with two dots; this is not to indicate that there are two photons, but rather that the probability amplitudes from the two paths have summed. Finally, as shown in Fig. 6C, at time t2= t + At, the second loop is fully read out (whilst the first loop implements the identity operation on the vacuum state). In this case, if the photon entered the loop in the previous time bin at time point tT, it would emerge here, but, if instead the photon exited the second loop in the previous time bin, the vacuum state will emerge. The probabilities of detecting a photon at time point or time point t2are therefore intrinsically linked by the relative phase of the two loops. More specifically, and as illustrated in Fig. 6D, the histogram measured at the output of the interferometer will exhibit two peaks at time points and t2, the heights of which are denoted as c and c2respectively. The relative height of these two peaks may be expressed aswhere P&(t>is the probability of detecting a count at time point t2, which in turn depends on the relative phase between the two loops A< >. Therefore, by measuring the interfered photon sequence multiple times, a controller such as controller 122 may determine the relative phase between two temporal mode coupling devices. The probability P&(f, of photons being found at time points t2and the probability (1 - P^ of photons being found at time point are illustrated as a function of the relative phase A< > in Fig. 7. Advantageously, by determining the relative phase using photon statistics, PDH locking schemes may be avoided. This means that additional hardware may be avoided, reducing the loss experienced by the photons in the signal path. The controller 122 may accordingly control one or both of the phase shifters 250 to adjust the relative phase between the temporal mode coupling devices 220-1, 220-2 towards a target value (which may be, for example, zero).

[0083] The skilled person will appreciate that the method described in relation to Figs. 6A-7 may be adapted for different architectures, for example to take account of temporal mode coupling devices of different delays or to take account of additional temporal mode coupling devices.

[0084] As can be seen in Fig. 7, for any given value of P&(f, the relative phase may be determined as one of two values. Trial and error may be utilised to determine whether the relative phase shift needs to be adjusted one way or another to achieve a target value. However, in an alternative approach, a multi-spectral source or a plurality of photon sources configured to produce photons at particular frequencies may be utilised to contemporaneously produce photons at several frequencies, such that the input photon sequence is generated at several frequencies (e.g. the input multimodal photonic distribution comprises spectral-temporal modes). The detector arrangement may be configured to measure the photon sequence at each frequency, for example by using a prism or other dispersive element to steer a photon having a particular frequency towards a corresponding detector of the detectorarrangement. Accordingly, the interferometer may be configured to produce the interfered photon sequence at multiple frequencies. As different frequencies experience different phase shifts as the photons pass through the interferometer, the determined relative phases at each frequency may be different. For example, the relative phase determined at a first frequency may be different to the relative phase determined at a second frequency, and this information may be utilised to better determine the relative phase at the first frequency. This is illustrated in Fig.8.

[0085] Similar to Fig. 7, Fig. 8 illustrates the probability curve P&(f, of photons being found at time point t2as a function of the relative phase A< > . However, in this example the interfered photon sequence is measured at multiple frequencies to determine the probabilities. For a first frequency, the probability may be found to have a first value (labelled 910) indicating that the relative phase at the first frequency is one of two values (labelled 920 and 930). As the relative phase may be dependent on the frequency of the photons, the probabilities for second and third frequencies (labelled 940 and 950 respectively) may be compared with the probability for the first frequency 910 to better determine which phase value 920 or 930 is correct for the first frequency. For example, if the probability 940 for the second frequency is greater than the probability value 910 for the first frequency, and if the probability value 950 for the third frequency is less than the probability value 910 for the first frequency, then a controller may determine that the value for the relative phase for the first frequency is given by the value at 930, while if the probability 940 for the second frequency is less than the probability value 910 for the first frequency, and if the probability value 950 for the third frequency is greater than the probability value 910 for the first frequency, then a controller may determine that the value for the relative phase for the first frequency is given by the value at 920, as illustrated in the figure.

[0086] A system such as system 100 may be adapted to obtain measurements of the interfered photon sequence across multiple frequencies, determine, for each frequency, a relative phase between the first temporal mode coupling device and the second temporal mode coupling device, and based on the determined relative phases, control one or both of the phase shifters 250-1, 250-2 to adjust the relative phase between the first and second temporal mode coupling devices 220-1, 220-2.

[0087] Fig. 9 shows a flowchart of a method 900 for adjusting a relative phase between a first temporal mode coupling device, such as first temporal mode coupling device 220-1 of Fig. 2 and a second temporal mode coupling device, such as second temporal mode coupling device 220-2 of Fig. 2, of an optical circuit. The skilled person will appreciate that while the method 900 is discussed with reference to the system 100 specifically this is for explanatory purposes only and other systems utilising a temporal interferometer may also be configured to perform the method 900.

[0088] At 910, the method comprises operating an optical circuit a number of times to obtain measurements of an interfered photon sequence, the optical circuit comprising (i) a photon source 210 operable to produce a photon sequence comprising a number of time bins, at least one time bin occupied by a photon, (ii) a configurable interferometer 118 operable to interfere photons of the photon sequence to produce the interfered photon sequence, (iii) a phase shifter 250, and (iv) a detector arrangement 120 comprising one or more photodetectors, the detector arrangement configured to measure the interfered photon sequence, the measurement indicative of a photon occupation of each time bin of the interfered photon sequence.

[0089] At 920, the method comprises determining, from the obtained measurements of the interfered photon sequence, a relative phase between a first temporal mode coupling device and a second temporal mode couplingdevice of the configurable interferometer, each temporal mode coupling device configurable to interfere photons in different time bins from the obtained measurements of the interfered photon sequence, a relative phase between the first temporal mode coupling device and the second temporal mode coupling device.

[0090] At 930 the method comprises, based on the determined relative phase, controlling the phase shifter to adjust the relative phase between the first and second temporal mode coupling devices.

[0091] Variations of the described embodiments are envisaged. For example, the boson sampler 114 may be varied in any of a number of ways, in terms of the source used, the number and configuration of delay lines and reconfigurable beamsplitters, the number of photodetectors and so on. In some examples, the delay lines of different temporal mode coupling devices may be of different lengths for example. In some examples, the interferometer may comprise further temporal mode coupling devices, and the method for adjusting relative phases between the temporal mode coupling devices may be amended accordingly. In other examples, a temporal mode coupling device may not utilise a reconfigurable beamsplitter and delay line but may instead utilise e.g. an atomic memory.

[0092] While in the examples above, the interferometers described only interfere photons in different temporal modes, in some examples, an interferometer may interfere photons in both different temporal and different spatial modes.

[0093] The disclosures herein may be used in conjunction with different photon sequences to those described herein. Other photon sequences may be used based on for example the design or construction of the interferometer.

[0094] In some examples, the state detection unit 120 may comprise one or more threshold detectors that are not capable of photon number resolution. In such circumstances, the generated integer sequences may comprise binary strings, with each element of a binary string indicative of the presence or absence of a detected photon in an output mode of the output multimodal photonic state. For example, an element of a binary sequence may have a value of one if one or more photons are detected in the corresponding output mode, while an element of the binary sequence may have a value of zero if no photons are detected in the corresponding output mode. In other examples, the state detection unit 120 may comprise pseudo -threshold detectors capable of detecting whether zero, one or more than one photon is received in a particular temporal mode.

[0095] While the methods herein have been described in connection with systems having a boson sampler, the skilled person will appreciate that similar methods may be applied to other systems having a temporal mode interferometer, and that the invention is not limited only to systems having boson samplers.

[0096] As will be appreciated by one skilled in the art, the present disclosure may be embodied as a system, method, or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, microcode, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present disclosure may take the form of a computer program product embodied in any one or more computer-readable medium / media having computer usable program code embodied thereon.

[0097] The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present disclosure. In this regard, each block in the flowchart orblock diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementingthe specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and / or flowchart illustration, and combinations of blocks in the block diagrams and / or flowchart illustration, can be implemented by special purpose hardware -based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

[0098] Each feature disclosed in this specification (including any accompanying claims, abstract or drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The disclosure is not restricted to the details of any foregoing embodiments. The disclosure extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

CLAIMS1. A system comprising: an optical circuit comprising: a photon source operable to produce a photon sequence comprising a number of time bins; a configurable interferometer operable to interfere photons of the photon sequence to produce an interfered photon sequence, the interferometer comprising a first temporal mode coupling device configurable to interfere photons in different time bins; and a second temporal mode coupling device configurable to interfere photons in different time bins; a phase shifter configured to adjust a relative phase between the first temporal mode coupling device and the second temporal mode coupling device of the interferometer; and a detector arrangement comprising one or more photodetectors, the detector arrangement configured to measure the interfered photon sequence, the measurement indicative of a photon occupation of each time bin of the interfered photon sequence; and a controller configured to: operate the optical circuit a number of times to obtain measurements of the interfered photon sequence; determine, from the obtained measurements of the interfered photon sequence, a relative phase between the first temporal mode coupling device and the second temporal mode coupling device; and based on the determined relative phase, control the phase shifter to adjust the relative phase between the first and second temporal mode coupling devices.

2. A system according to claim 1, wherein determining, from the obtained measurements of the interfered photon sequence, a relative phase between the first temporal mode coupling device and the second temporal mode coupling device comprises: determining, from the obtained measurements of the interfered photon sequence, relative photon occupations of time bins of the interfered photon sequence; and determining, from the relative photon occupations of the time bins of the interfered photon sequence, the relative phase between the first temporal mode coupling device and the second temporal mode coupling device.

3. A system according to claim 1 or claim 2, wherein the photon sequence and the interfered photon sequence each comprises three time bins; wherein the first time bin of the photon sequence is occupied by a photon; and wherein a measurement of the interfered photon sequence is indicative of the second time bin or third time bin of the interfered photon sequence being occupied by the photon.

4. A system according to any preceding claim, wherein the first temporal mode coupling device comprises: a reconfigurable beamsplitter and a delay line, the delay line configured to connect an output port of the reconfigurable beamsplitter with an input port of the reconfigurable beamsplitter.

5. A system according to any preceding claim, wherein the photon source comprises a quantum dot.

6. A system according to any of claims 1 to 4, wherein the photon source comprises a heralded photon source.

7. A system according to claim 6, wherein the photon source comprises a non-linear material inside a cavity.

8. A system according to clams 1 to 7, wherein the one or more photons of the photon sequence have a first frequency; wherein the photon source is further operable to contemporaneously produce a second photon sequence comprising the number of time bins, at least one time bin occupied by a photon having a second frequency; wherein the configurable interferometer is further operable to interfere photons of the second photon sequence to produce a second interfered photon sequence; wherein the detector arrangement is further configured to measure the second interfered photon sequence; and wherein the controller is further configured to determine the relative phase from the obtained measurements of the interfered photon sequence and obtained measurements of the second interfered photon sequence.

9. A system according to any preceding claim, wherein the phase adjust comprises an electro -optical modulator or a piezo-electric element.

10. A system according to any preceding claim, wherein controlling the phase shifter to adjust the relative phase comprises controlling the phase shifter to eliminate the relative phase.

11. A method comprising: obtaining measurements of an interfered photon sequence, wherein the interfered photon sequence comprising a number of time bins, at least one time bin is occupied by a photon, and wherein each measurement is indicative of a photon occupation of each time bin of the interfered photon sequence; determining, from the obtained measurements of the interfered photon sequence, a relative phase between a first temporal mode coupling device and a second temporal mode coupling device of a configurable interferometer that produced the interfered photon sequence; and controlling, based on the determined relative phase, a phase shifter to adjust the phase between the first temporal mode coupling device and the second temporal mode coupling device of the configurable interferometer.

12. A non-transitory computer-readable medium having instructions thereon which, when executed by one or more processors coupled to an optical circuit, cause the one or more processors to perform a method according to claim 11.

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