Boson sampler timing control

Abstract

A system is provided. The system comprises a boson sampler and control logic. The boson sampler comprises a heralded single photon source (SPS) configured to probabilistically produce a photon in a time bin. The boson sampler further comprises an interferometer configured to interfere photons in different time bins. The boson sampler further comprises a detector arrangement comprising one or more photodetectors configured to detect photons output from the interferometer and produce corresponding detection event signals. The control logic is configured to control the boson sampler to produce a photon sequence over a time period comprising a plurality of time bins. The control logic is further configured to commence sampling an output distribution of the boson sampler by logging detection event signals received over the time period. The control logic is further configured to, prior to expiration of the time period and based on a received indication that production of the photon sequence has been unsuccessful: recommence sampling the output distribution of the boson sampler. Method and computer- readable media are also described.

Description

Boson Sampler Timing ControlTechnical Field

[0001] The present disclosure relates to hybrid quantum-classical systems that include a boson sampler. More particularly, the present disclosure relates to the control of hybrid quantum-classical systems that include a temporal boson sampler.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.

[0003] 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. Modern supercomputers fail to simulate boson sampler distributions generated from more than a few tens of modes.

[0004] Boson sampling has been shown to be useful in some optimization and machine learning tasks. Accordingly, improved boson sampling systems and methods are of interest.Summary

[0005] According to an aspect of the present disclosure, a system is provided. The system comprises a boson sampler and control logic. The boson sampler comprises a heralded single photon source (SPS) configured to probabilistically produce a photon in a time bin. The boson sampler further comprises an interferometer configured to interfere photons in different time bins . The boson sampler further comprises a detector arrangement comprising one or more photodetectors configured to detect photons output from the interferometer and produce corresponding detection event signals. The control logic is configured to control the boson sampler to produce a photon sequence over a time period comprising a plurality of time bins. The control logic is further configured to commence sampling an output distribution of the boson sampler by logging detection event signals received over the time period. The control logic is further configured to, prior to expiration of the time period and based on a received indication that production of the photon sequence has been unsuccessful: recommence sampling the output distribution of the boson sampler.

[0006] Advantageously, the time taken to sample a bosonic distribution is decreased. This improvement is more pronounced in systems in which the probability of producing a single photon is low, and so even with low- efficiency single-photon sources, difficult sampling problems can be handled in a more reasonable timeframe.

[0007] The control logic may be further configured to commence a control sequence configured to control the operation of one or more active photonic elements of the interferometer over the time period. The control logic may be further configured to, prior to expiration of the time period and based on the received indication that production of the sequence has been unsuccessful: recommence the control sequence.

[0008] The control logic may be further configured to, prior to expiration of the time period and based on the received indication that production of the photon sequence has been unsuccessful: recommence generating the photon sequence. In some examples, recommencing generating the photon sequence may comprise maintaining production of the photon sequence. For example, if the photon sequence is regular (e.g. a photon in every time bin, or a photon in alternating time bins), then the control logic may simply cause the boson sampler to continue producing that regular photon sequence.

[0009] The heralded SPS may be configured to produce a heralding signal when a single photon has been produced in a time bin. The received indication that production of the photon sequence has been unsuccessful may comprise a heralding signal not being received for a time bin. Advantageously, the indication that the production of the photon sequence has failed may be received and interpreted by the control logic at the earliest stage possible. The heralded SPS may comprise a non-linear photonic element that, when pumped, is configured to probabilistically produce a signal photon and an idler photon in a time bin. The detector arrangement may comprise a photodetector configured to detect the idler photon and produce a corresponding detection event signal to herald production of the signal photon. Controlling the boson sampler to produce a photon sequence may comprise controlling a pump laser to pump the non-linear photonic element.

[0010] The interferometer may comprise a reconfigurable beam splitter and a delay line, the delay line configured to connect one input port of the reconfigurable beam splitter with one output port of the reconfigurable beam splitter.

[0011] The interferometer may comprise a quantum memory, for example an atomic memory.

[0012] The control logic may be implemented in an application-specific integrated circuit (ASIC) or configured field programmable gate array (FPGA).

[0013] The photon sequence may comprise a train of single photons in consecutive time bins for the duration of the time period. The photon sequence may comprise a sequence of single photons in alternating time bins for the duration of the time period.

[0014] According to an aspect of the present disclosure, a computer-implemented method is provided. The method comprises controlling a boson sampler to produce a photon sequence over a time period comprising a plurality of time bins. The method further comprises commencing sampling an output distribution of the boson sampler by logging detection event signals received over the time period. The method further comprises, prior to expiration of the time period and based on a received indication that production of the photon sequence has been unsuccessful: recommencing sampling the output distribution of the boson sampler.

[0015] The method may further comprise, at the start of the time period, commencing a control sequence configured to control the operation of one or more active photonic elements of the interferometer over the timeperiod. The method may further comprise, prior to expiration of the time period and based on the received indication that production of the sequence has been unsuccessful: recommencing the control sequence.

[0016] The method may further comprise, prior to expiration of the time period and based on the received indication that production of the photon sequence has been unsuccessful: controlling the boson sampler to recommence production of the photon sequence.

[0017] According to an aspect of the present disclosure, a (non-transitory) computer -readable medium is provided, the computer -readable medium having instructions stored thereon which, when executed by a processor unit or controller or combination of processor unit and controller communicatively coupled to a boson sample, cause the processor unit and / or controller to perform a method as described herein.

[0018] 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

[0019] Illustrative embodiments of the present disclosure will now be described by way of example only, with reference to the accompanying figures.

[0020] Fig. 1 shows a block diagram of a system according to an example.

[0021] Fig. 2 shows an illustration of a heralded single photon source according to an example.

[0022] Fig. 3 shows an illustration of a boson sampler according to an example that may be used in the system of Fig. 1.

[0023] Fig. 4 shows an illustration of a boson sampler according to an example that may be used in the system of Fig. 1.

[0024] Fig. 5 shows a flowchart of a method suitable for performance by the system of Fig. 1 according to an example.

[0025] Fig. 6 shows a table comparing the expected time to produce the photon sequence of (EQ.2) using a traditional boson sampler with the expected time to produce the photon sequence of (EQ. 2) using a system according to the present disclosure.

[0026] Throughout the description and the drawings, like reference numerals refer to like parts.Detailed Description

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] The terms as used herein are provided merely to describe some embodiments thereof, but not to limit the scope of other embodiments of the disclosure. It is to be understood that the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. All terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of the disclosure belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealised or overly formal sense unless expressly so defined herein.

[0033] 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.

[0034] 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 communications fabric 102, which provides communications between a processor unit 104, memory unit 106, input / output unit 108, communications 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.

[0035] 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 communications module 110 may be provided in a first rack-mounted hardware module, the controller 122may be implemented in a second rack-mounted hardware module and electronically coupled to the first hardware module, the state generation unit 116 maybe 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.

[0036] The processor unit 104 is configured to execute instructions for software that may be loaded into the memory 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 graphics processing units (GPUs) or any combination thereof. If the processor unit 104 includes multiple processors and / or processing units, the multiple processors and / or processing units may operate individually or collectively.

[0037] The memory unit 106 may comprise any piece of hardware that is capable of storing information, such as, for example, data, program code in functional 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.

[0038] 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.

[0039] 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 maybe 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.

[0040] 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 functional 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.

[0041] 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.

[0042] 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.

[0043] 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 this document, a computer-readable storage medium may be any tangible medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device.

[0044] 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.

[0045] The boson sampler 114 comprises a state generation unit 116, a linear interferometer 118, a state detection unit 120 (also “detector arrangement”) and a dedicated (classical) control unit 122. More particularly, the boson sampler 114 is a temporal boson sampler. 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. Example boson sampler architectures are described further below in relation to Fig. 3 and Fig. 4.

[0046] The state generation unit 116 is configured to (probabilistically) generate a photon sequence over a time period comprising a plurality of time bins. More particularly, over a time period of duration T comprising a number M of time bins of duration t, the state generation module 116 is configured to probabilistically produce a single photon in each of N 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 t of each time bin. For example, the photon pulse duration may be approximately 500 picoseconds while the time between one photon and the next may be for example on the order of 100 nanoseconds or 1 microsecond.

[0047] For probabilistically producing the photon sequence, the state generation module 116 comprises one or more single photon sources (SPSs). Each single photon source is configured to probabilistically produce a single photon.

[0048] In the example of Fig. 1 the state generation module 116 comprises one or more heralded SPSs. A heralded SPS may emit photons in pairs, the detection of one photon heralding the generation of the other. An example implementation of a heralded SPS is illustrated in Fig. 2. A pump laser 202 is configured to generate a pump beam that is applied to a non-linear photonic element 206 that probabilistically generates a photon pair 208. A first photon 210 (which is referred to herein as an idler photon without loss of generality) is received by a detector 212, thereby heralding the production of the second photon 214 (which is referred to herein as a signal photon without loss of generality). The non-linear photonic element 206 in this example relies upon spontaneous parametric down-conversion (SPDC). More specifically, the non-linear photonic element 206 comprises a nonlinear photonic material such as periodically-poled lithium niobate (PPLN) or potassium titanyl phosphate (KTP) which, when pumped with a suitable pump beam 204, can probabilistically emit entangled photon pairs 208. As will be appreciated by the skilled person a heralded SPS may comprise further optical apparatus. For example, the light may be transferred via optical fibres. Furthermore, the skilled person would appreciate that other non-linear materials may be utilised to produce photon pairs, through SPDC or through other non-linear processes such as spontaneous four-wave mixing (SFWM).

[0049] With reference again to Fig. 1, the controller 122 may be configured to control the operation of a pump laser 202. The photodetector 212 may be built into the state generation unit 116 or the state detection unit 120. The photodetector 212 is configured to communicate a detection event signal, for example an electrical signal, to the controller 122 to indicate generation of the signal photon 214.

[0050] In some examples, the state generation module 116 may comprise a single heralded SPS 200 configured to probabilistically produce a single photon in each time bin of duration t over the time period T (in accordance with the desired photon sequence).

[0051] 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 SPSs 200 and a multiplexer. Each SPS may be configured to probabilistically produce a single photon in each time bin of duration t over the time period T. The multiplexer may be configured to receive successfully generated photons from the plurality of SPSs and couple them to an output port for onwards propagation to the interferometer.

[0052] 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 spectrally multiplexed sources. Furthermore, the state generation unit 116 may include further elements for enabling finer control over the photon sequence produced. For example, the state generation unit 116 may include one or more optical filters to controllably filter out photons that are produced in time bins in which, according to the prescribed sequence, no photon should be produced.

[0053] The interferometer 118 is configured to receive photons 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 modes of the input multimodal photonic state (photon sequence), thereby transforming the input multimodal photonic state to produce an output multimodal photonic state, and the interferometer 118 is arranged to output the output multimodal photonic state to the state detection unit 120.

[0054] Due to interference between photons in different temporal modes, in operation the boson sampler 114 transforms the input multimodal photonic state (photon sequence) into an output multimodal photonic state that may be expressed as a superposition of the different possible configurations of the photons in the output modes aswhere C is a configuration, rtj 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 {0}, 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 S of times, 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.

[0055] The interferometer 118 may be designed and manufactured in any suitable and desired way e.g. depending on the modes of electromagnetic radiation to be transformed by the interferometer 118. Thus, for example, when the electromagnetic radiation has an optical or infrared wavelength (e.g. between 400nm and 700nm or between 700nm and 1600nm), the optical paths through the interferometer 118 may be implemented at least partially using optical fibres. In some examples, the interferometer 118 may be implemented in bulk optics. However, in other examples, the interferometer 118 may comprise a photonic integrated circuit. In the photonic integrated circuit, the optical paths may be implemented with, for example, etched waveguides and coupling locations arranged in the integrated circuit. At each coupling location, tuneable elements may be arranged (e.g. EOM phase shifters) that are configured to control the coupling interaction between the waveguides. The integrated circuit may be implemented in silicon nitride (Si3N4) or any other suitable material such as thin-film lithium niobate.

[0056] The state detection unit 120 comprises an arrangement of one or more photodetectors configured to detect photons output from the interferometer 118 and produce corresponding detection event signals. In some examples, the photodetectors may comprise photon number resolving (PNR) detectors, capable of determining how many photons are received. For example, the detectors may comprise superconducting nanowire detectors that generate an output signal intensity proportional to the (discrete) number of photons that strike a detector. The PNR detectors may comprise transition edge sensors (TESs). In other examples, the photodetectors may comprise 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.

[0057] 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 maybe 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 comprisean application-specific integrated circuit (ASIC) or an application-specific standard product (ASSP) or another domain-specific architecture (DS A). Alternatively, the controller 122 maybe implemented in adaptive computing hardware (in other words, hardware comprising configurable hardware blocks / configurable logic blocks) that has been configured to perform functions, for example in a configured field programmable gate array (FPGA). The controller 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.

[0058] 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.

[0059] The controller 122 is configured to, in conjunction with the processor unit 104, establish a clock cycle for the boson sampler 114. In particular, the controller is capable of defining the duration t of time bins, for example by adjusting the pump rate of a heralded SPS 200 of the state generation unit 116.

[0060] The controller 122 is configured to control the boson sampler 114 to produce a prescribed photon sequence over a time period of duration T comprising a number M of time bins (of duration t). In particular, in operation the controller 122 may send one or more control signals to the state generation unit 116 to cause the state generation unit 116 to commence production of the photon sequence. For example, and with reference to Fig- 2, the controller 122 may control a pump laser 202 for pumping a non-linear photonic element 206 of the state generation unit 116. In some examples, the controller 122 may control one or more filters of the state generation unit 116 to filter out photons from time bins that should not contain photons according to the prescribed photon sequence. The prescribed photon sequence may be provided to the controller 122 from the one or more processor units 104, or may be implicit in the setup of the boson sampler 114 - for example, the boson sampler 114 may be configured to probabilistically produce a single photon in every time bin and so the controller 122 may, in some examples, merely start or stop the probabilistic production of single photons by the state generation unit 116.

[0061] The controller 122 is further configured to receive detection event signals from a photodetector 212 (of for example the state generation unit 116) that herald the production of signal photons. In particular, if a detection event signal is received from the photodetector 212 in a particular time bin / time slot, then the controller 122 may infer that a signal photon 214 has been produced by a heralded SPS 200 of the state generation unit 116 in a corresponding time bin / time slot. The controller 122 is configured to log detection event signals from the heralded SPSs. In this way, the controller 122 is able to determine whether the state generation unit 116 succeeds in producing the photon sequence.

[0062] The controller 122 is further configured to generate control signals to control the operation of one of more active optical elements of the interferometer 118. For example, the controller 122 may send control signals that tune the effective reflectivity / transmittance coefficient of a reconfigurable beam splitter or the phase imparted by a phase shifter of the interferometer 118. In particular, in operation, at the same time as commencing the photon sequence, the controller 122 is configured to commence a control sequence comprising a plurality oftimed control signals to control the operation of one or more active photonic elements of the interferometer over the time period.

[0063] For example, in operation the processor unit 104 may, according to an algorithm processed by the processor unit 104, define a set of parameter values {0} for a set of parameters 0 defining the operation of the interferometer 118. One or more of the parameters 0 may characterise a single mode operation. For example, a parameter may characterise the phase shift imparted by a phase shifter of the linear interferometer 118. One or more of the parameters 0 may characterise a multimodal operation. For example, a parameter may characterise a transmission (or equivalently, a reflection) coefficient of a reconfigurable beam splitter in a passive linear interferometer. The processor unit 104 may then communicate the parameter values to the controller 122. The controller 122 may receive the set of parameter values from the processor unit 104, translate those parameter values into a control sequence of timed control signals for controlling the operation of active optical elements of the interferometer 118, and then commence the control sequence when commencing the production of the photon sequence. The controller 122 accordingly can tune the set of parameter values {0} that govern the behaviour of the interferometer 118 and thereby controllably interfere photons in different time bins of the photon sequence. In this way, the controller 122 can control the transformation of the input photon sequence to the output superposition state that is implemented by the interferometer 118.

[0064] The controller 122 is further configured to receive detection event signals from the photodetector(s) of the state detection unit 120. For example, a detection event signal may comprise an electrical signal from a photodetector at which a detection event occurs. In examples wherein the photodetectors are PNR detectors, the electrical signals may further be indicative of the number of photons received in a particular time bin.

[0065] 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 over the time period. 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 120. In the event that the state generation unit successfully completes production of the photon sequence, then the controller 122 may infer that the correct output multimodal photonic state 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 that output state. 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 integer sequence to the processor unit 104. The controller 122 may then repeat commencing production of the photon sequence and sampling of the output distribution until a predetermined number S of samples have been collected and communicated to the processor unit 104.

[0066] For the empirical probability distribution of the bosonic configurations of the output state to be representative of the output distribution of the boson sampler, the number S of samples collected may be large which means that the length of time the boson sampler operates for may be long. Consider, as an example, a situation in which the prescribed photon sequence comprises a train of single photons in consecutive time bins for the duration of the time period (in other words, in which the photon sequence comprises a single photon in everytime bin). In this example, the input multimodal photonic state (photon sequence) generated by the state generation unit 116 may be denoted in the Fock basis as l^iv) = | l1, l2. 1M) (EQ. 2) where the subscript j refers to the jth temporal mode (time bin). Naively, one may in each consecutive time period (of duration T) attempt to generate the input state. If the probability of producing a photon in each time bin is given by p, then successfully generating the photon sequence can accordingly be thought of as performing a Bernoulli trial in each time period with the probability of successfully generating the input state given by the probability p to the power of the number of time bins M. In each time period in which the photon sequence is not produced, the output state (see (EQ. 1)) produced by the interferometer is not as expected and accordingly the corresponding measurement outcome may not be valid, which can in turn lead to errors in the performance of algorithms by the processor unit 104. The expected number of time bins that one would expect to pass before the input state of (EQ. 2) is successfully produced is given by the number of time bins M in the photon sequence divided by the probability p to the power of the number of time bins M. For example, if each time bin has a duration of 100ns (t = 100ns ), the probability of producing a photon in a given time bin is p = 0.75, and the photon sequence is M = 20 time bins long, then the expected length of time to produce a valid photon sequence is approximately 631 microseconds. Accordingly, to take for example 1000 samples would take approximately 631 milliseconds.

[0067] However, with reference again to Fig. 1, the system 100 is able to improve the time taken to produce a suitable empirical distribution. In particular, the controller 122 is configured to take corrective action in the event that an indication is received that the state generation unit 116 has failed to produce the photon sequence (said differently, production of the photon sequence was unsuccessful), the controller 122 may, after commencing production of the photon sequence but prior to expiration of the time period T, infer from an indication from a photodetector 212 of a heralded SPS 200 that production of the photon sequence has failed, and then take corrective action. The indication may be, for example, a lack of a detection event signal in a time bin in which a detection event signal is expected to be received. In other examples, the indication may comprise an active signal from photodetector 212 that a photon has not been produced.

[0068] The controller 122 is configured to, prior to expiration of the time period and based on the received indication, recommence production of the photon sequence. In some examples, recommencing generation of the photon sequence may comprise maintaining production of the photon sequence. For example, when the prescribed input state (photon sequence) is a train of single photons in consecutive time bins (e.g. (EQ. 2)), recommencing production of the photon sequence may comprise continuing to produce a train of single photons, and there may be no need to for the controller 122 to send further command signals to the state generation unit 116.

[0069] The controller 122 is configured to, prior to expiration of the time period and based on the received indication, recommence the control sequence for controlling the operation of one or more active optical elements of the interferometer 118.

[0070] The controller 122 is further configured to, prior to expiration of the time period and based on the received indication, recommence sampling the output distribution of the boson sampler 114. For example, the controller 122 may clear the record of all detection even signals received since the photon sequence was started, and then recommence logging of detection event signals received from the state detection unit 120.

[0071] Advantageously, as opposed to waiting until a time period occurs in which the desired photon sequence is successfully produced, the controller 122 of system 100 can quickly restart the operation of the boson sampler 114 to reattempt to produce the photon sequence. This can lead to a great speed up in the time taken to collect samples from the boson sampler 114. In particular, the expected number of time bins to produce the sequence is approximately given by:M(1 - pM)

[0072] For example, to produce the sequence of (EQ. 2) if each time bin has a duration of 100ns (t = 100ns), the probability of producing a photon in a given time bin is p = 0.75, and the photon sequence is 20 time bins long (M = 20), then the expected length of time to produce a valid photon sequence is approximately 126 microseconds. Accordingly, to take 1000 samples would take approximately 126ms. Fig. 6 shows a table comparing the expected length of time to produce the photon sequence of (EQ. 2) using the conventional approach with the expected length of time to produce the photon sequence using the system 100 for a variety of probabilities p and sequence lengths M and when the duration of a time bin is 100ns. Even assuming that the controller 122 is unable to immediately take corrective action in response to a received indication that production of the photon sequence has failed, there is still a speed up in the expected length of time to produce the sequence.

[0073] The skilled person would appreciate that the architecture described above in relation to Fig. 1 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 system 100 may include more or fewer components.

[0074] Fig. 3 shows a diagram of a boson sampler 114a that may be used with the system 100 of Fig. 1. The state generation unit 116a of Fig. 3 comprises a SPS 310 operable to produce a single photon in each time bin of a plurality of time bins, so that each photon enters the time-bin interferometer 118a separated from the next by a duration t. The SPS 310 comprises a heralded single photon source 200. The controller 122a controls the operation of a pump laser 202 that, when incident on a non-linear photonic element 206, causes the non-linear photonic element to probabilistically produce a pair 208 of entangled photons. Detection of an idler photon 210 of the photon pair heralds the generation of the signal photon 214 of the pair 208.

[0075] The interferometer 118a comprises a reconfigurable beam splitter 320 and a delay line 330. The delay line 330 is arranged to connect one input port of the reconfigurable beam splitter 320 with one output port of the reconfigurable beam splitter 320. The delay line may comprise, for example, optical fibre. The delay line 330 has a length ct 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 330 so as to interfere with the field of the photon(s) in the next temporal mode on the parametrised beam splitter 320. The time-bin interferometer may comprise further optical components including further optical switches.

[0076] A parametrised / reconfigurable beam splitter 320 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 (not indicated in Fig. 3). 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 may further 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 320 may comprise two 50 / 50 beamsplitters and a phase shifter element arranged therebetween.

[0077] The controller 122a is configured to tune the parameter value (e.g. transmittance) of the parametrised beam splitter 320 for each time interval. For example, the controller 122a may, as a first photon is emitted from the photon source 310, configure the reconfigurable beamsplitter 320 to loop the first photon into the delay line 330. The controller 122a 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 310), configure the reconfigurable beamsplitter 320 using parameter value 0, to cause interference between the first temporal mode and second temporal mode (e.g. the first and second photon) of the photon sequence. The controller 122a 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 310), configure the reconfigurable beamsplitter 320 using parameter value 92to cause interference between the second temporal mode and third temporal mode. The controller 122a 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 310, configure the reconfigurable beamsplitter 320 using parameter value 03to cause interference between the third temporal mode and fourth temporal mode. This may continue until a predetermined transformation has been performed on the input photon sequence of M time bins.

[0078] The state detection unit 120a comprises a photon number resolving (PNR) photodetector 340 configured to detect the number of photons in each temporal mode. By measuring the number of photons in each of M time bins output from the interferometer 118a, the boson sampler 114a takes a sample of the output distribution.

[0079] The skilled person would appreciate that the architecture of the temporal mode boson sampler 114a of Fig. 3 may be varied in several ways. For example, the boson sampler 114a may comprise further reconfigurable beamsplitters 320 and further delay lines 330 in order to generate more complicated interference between temporal modes. 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.

[0080] Fig. 4 shows a diagram of another example boson sampler 114b that may be used with the system 100 of Fig. 1. The interferometer 118b of the boson sampler 114b depicted in Fig. 4 comprises a quantum memory device 400. The quantum memory device 400 may be used to controllably place, for example, a photon in the first temporal mode into a superposition of being stored in the memory and being not stored in the memory, and then to controllably release any photon (or quantum state thereof) stored in the memory at the time the second temporal mode passes through, in order to interfere temporal modes in a similar way as a reconfigurable beam splitter can be used to interfere modes. An example of a suitable quantum memory device being used in such a way is described in international patent application number PC T / GB 2021 / 052447 filed on 21 September 2021 in the name of ORCA Computing Limited, the content of which is incorporated herein by reference.

[0081] According to an example, a quantum memory device 400 may comprise an atomic system having discrete energy states which may be termed ‘energy levels’. The atomic system may comprise a single type of neutral atoms or ions or a plurality of different types of neutral atoms and / or ions. The atomic system may be an atomic ensemble comprising a plurality of atoms, such as a gas of atoms, or ions, such as a rare earth ion locked in a hostmedium. Such an atomic system may use energy levels associated with electrons in inner or outer shells of atoms, ions, or crystal defects. Examples of suitable atomic ensembles include vapours containing Rubidium. Other energy levels of atomic system may be used including Rydberg - type atomic systems. Additionally, or alternatively the atomic system may comprise a single neutral atom or ion, for example single 87Rb atoms in a magneto-optical trap (MOT).

[0082] In an example, the quantum memory device 400 may comprise two dichroic mirrors with a vapour cell sandwiched therebetween, the vapour cell comprising a rubidium vapour. The rubidium vapour has a ladder-like energy level structure and the energy of the 5S energy level is lower than the energy of the 5P level which is lower than the energy of the 5D level. The quantum memory device 400 may be configured to receive a single photon from the SPS 310 (through a first of the dichroic mirrors), the single photon having a first frequency associated with a first energy. The quantum memory device 400 may be further configured to receive a counterpropagating control electromagnetic field from a control laser (not shown) via the second dichroic mirror, the control electromagnetic field having a second frequency associated with a second energy, such that the sum of the first energy and the second energy corresponds to a resonance between the 5S state and the 5D state. In this way, a single photon may be “stored” in the quantum memory. Upon receipt of a third electromagnetic field in a subsequent time bin, which may have a different frequency to the control field, the stored photon may be emitted. The quantum memory device 400 may therefore be used to store the single photon from the SPS 310 either in a deterministic manner or in a superposition state, the superposition state of a photon having components of both being stored and not stored. In this way, photons in different time bins may be interfered. The controller 122b may control a laser (not shown) to produce the control electromagnetic field and the third electromagnetic field to interfere photons in different time bins.

[0083] Fig. 5 shows a flowchart of a (e.g., computer-implemented) method 500 for performance by a system comprising one or more (classical) processor units and a boson sampler, such as system 100 of Fig. 1.

[0084] At 510, the method comprises controlling a boson sampler to produce a photon sequence over a time period comprising a plurality of time bins.

[0085] At 520, the method comprises commencing sampling of an output distribution of the boson sampler by logging detection event signals received over the time period.

[0086] At 530, the method comprises, prior to expiration of the time period, receiving an indication that production of the photon sequence has been unsuccessful. The indication may comprise, for example, a lack of receipt of a heralding signal at a time period that a heralding signal is expected. The indication may comprise, for example, a negative acknowledgement signal, for example an active electrical signal indicating that a photon has not been received.

[0087] At 530, the method comprises, prior to expiration of the time period and based on the received indication that production of the sequence has been successful: recommencing sampling of the output distribution of the boson sampler.

[0088] In some examples, the method 500 may further comprise, at the start of the time period (in other words, when commencing the photon sequence at 510), also commencing a control sequence configured to control the operation of one or more active photonic elements of an interferometer of the boson sampler over the time period. The method may further comprise, prior to expiration of the time period and based on the received indication: recommencing the control sequence.

[0089] In some examples, the method 500 may further comprise, prior to expiration of the time period and based on the received indication: recommencing the photon sequence.

[0090] Variations of the described embodiments are envisaged.

[0091] For example, 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 computation to be performed. Other photon sequences may be used based on for example the design or construction of the interferometer 118.

[0092] 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 1 0 may comprise pseudo-threshold detectors capable of detecting whether zero, one or more than one photon is received in a particular temporal mode.

[0093] The state generation unit 116 may be configured to be “always on” when boson sampler 114 of the system 100 is powered on. For example, turning on the system 100 may automatically turn on a pump beam configured to pump the non-linear photonic element of a heralded SPS. Accordingly, powering on the system 100 means there is a probability of producing a single photon in every time bin of duration t. In circumstances in which the desired photon sequence comprises a single photon in every time bin of the plurality M of time bins (for example to produce the input state of (EQ. 1)), then there is advantageously a probability of producing the desired photon sequence in every time period T without the need for further control signals to be produced by controller 122. For more complicated photon sequences, instead of controlling the SPSs themselves, the controller 122 may instead control filters at the output of the state generation unit 116 to filter out unwanted generated photons from particular time bins.

[0094] In other examples, the state generation unit 116 may controllably attempt to produce the desired photon sequence. For example, the controller 122 may be configured to control the SPSs to probabilistically produce single photons in appropriate time bins. For example, the controller 122 may actively generate control signals according to a prescribed timing sequence to cause a pump laser to pump a heralded SPS.

[0095] 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.

[0096] 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 or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functionsnoted 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.

[0097] 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: a boson sampler comprising: a heralded single photon source (SPS) configured to probabilistically produce a photon in a time bin; an interferometer configured to interfere photons in different time bins; and a detector arrangement comprising one or more photodetectors configured to detect photons output from the interferometer and produce corresponding detection event signals; and control logic configured to: control the boson sampler to produce a photon sequence over a time period comprising a plurality of time bins; commence sampling an output distribution of the boson sampler by logging detection event signals received over the time period; and prior to expiration of the time period and based on a received indication that production of the photon sequence has been unsuccessful: recommence sampling the output distribution of the boson sampler.

2. The system of claim 1, wherein the control logic is further configured to: commence a control sequence configured to control the operation of one or more active photonic elements of the interferometer over the time period; and prior to expiration of the time period and based on the received indication that production of the sequence has been unsuccessful: recommence the control sequence.

3. The system of claim 1, wherein the control logic is further configured to: prior to expiration of the time period and based on the received indication that production of the photon sequence has been unsuccessful: recommence production of the photon sequence.

4. The system of any preceding claim, wherein: the heralded SPS is configured to produce a heralding signal when a single photon has been produced in a time bin; and the received indication that production of the photon sequence has been unsuccessful comprises a heralding signal not being received for a time bin.

5. The system of any preceding claim, wherein: the heralded SPS comprises a non-linear photonic element that, when pumped, is configured to probabilistically produce a signal photon and an idler photon in a time bin; andthe detector arrangement comprises a photon detector configured to detect the idler photon and produce a corresponding detection event signal to herald production of the signal photon.

6. The system of claim 5, wherein controlling the boson sampler to produce a photon sequence comprises controlling a pump laser to pump the non-linear photonic element.

7. The system of any preceding claim, wherein the interferometer comprises a reconfigurable beam splitter and a delay line, the delay line configured to connect one input port of the reconfigurable beam splitter with one output port of the reconfigurable beam splitter.

8. The system of any preceding claim, wherein the interferometer comprises a quantum memory.

9. The system of any preceding claim, wherein the control logic is implemented in an application-specific integrated circuit (ASIC) or configured field programmable gate array (FPGA).

10. The system of any preceding claim, wherein the photon sequence comprises a train of single photons in consecutive time bins for the duration of the time period.

11. The system of any of claims 1 to 9, wherein the photon sequence comprises sequence of single photons in alternating time bins for the duration of the time period.

12. A computer-implemented method comprising: controlling a boson sampler to produce a photon sequence over a time period comprising a plurality of time bins, the boson sampler comprising a heralded single photon source (SPS) configured to probabilistically produce a photon in a time bin, an interferometer configured to interfere photons in different time bins, and a detector arrangement comprising one or more photodetectors configured to detect photons output from the interferometer and produce corresponding detection event signals; commencing sampling of an output distribution of the boson sampler by logging detection event signals received over the time period; and prior to expiration of the time period and based on a received indication that production of the photon sequence has been unsuccessful: recommence sampling the output distribution of the boson sampler.

13. The method of claim 12, further comprising: at the start of the time period, commencing a control sequence configured to control the operation of one or more active photonic elements of the interferometer over the time period; and prior to expiration of the time period and based on the received indication that production of the sequence has been unsuccessful: recommencing the control sequence.

14. The method of claim 12 or claim 13, further comprising: prior to expiration of the time period and based on the received indication that production of the photon sequence has been unsuccessful: controlling the boson sampler to recommence production of the photon sequence.

15. A computer-readable medium having instructions stored thereon which, when executed by one or more processors communicatively coupled to a boson sampler, cause a method according to any of claims 12 to 14 to be performed.

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