Apparatuses and methods for detecting quantum entanglement and managing generation of quantum entanglement

Abstract

Methods and apparatus for generating quantum entanglement detect success of entanglement attempts by monitoring output signals from one or more detectors for heralding patterns indicating that a pair of quantum systems is in an entangled quantum state. In response to detecting a heralding pattern that indicates a successful entanglement attempt a low latency signal is output on one or more signal lines; and a higher latency message is transmitted by a messaging interface. The higher latency message contains information that identifies the pair of quantum systems entangled by the successful entanglement attempt. The pair of quantum systems may be inhibited from participating in further entanglement attempts while further entanglement attempts are performed on other pairs of quantum systems.

Description

APPARATUSES AND METHODS FOR DETECTING QUANTUM ENTANGLEMENT AND MANAGING GENERATION OF QUANTUM ENTANGLEMENTCross-Reference to Related Application

[0001] This application claims priority from US application No 63 / 610976 filed 15 December 2023 and entitled APPARATUSES AND METHODS FOR DETECTING QUANTUM ENTANGLEMENT AND MANAGING GENERATION OF QUANTUM ENTANGLEMENT which is hereby incorporated herein by reference for all purposes. For purposes of the United States of America, this application claims the benefit under 35 U.S.C. §119 of US application No. 63 / 610976 filed 15 December 2023 and entitled APPARATUSES AND METHODS FOR DETECTING QUANTUM ENTANGLEMENT AND MANAGING GENERATION OF QUANTUM ENTANGLEMENT which is hereby incorporated herein by reference for all purposes.Field

[0002] The present invention relates to the fields of quantum communication and quantum informatics. Embodiments of the invention provide methods and apparatus for detecting quantum entanglement and managing generation of quantum entanglement in distributed quantum systems.Background

[0003] Two or more quantum systems are said to be “entangled” when the quantum state of any one of the entangled quantum systems cannot be described independently of the state of the other one(s) of the entangled quantum systems. Quantum entanglement has applications that include transferring a quantum state from one quantum system to another over an arbitrary distance using a quantum teleportation protocol, applying two qubit quantum gates to quantum systems that are separated by arbitrary distances using a protocol for teleporting controlled gates, quantum cryptography and quantum key distribution, and other applications.Entangled quantum systems may, for example, be applied to transfer and manipulate quantum information in quantum informatics networks.

[0004] Various protocols may be used to generate quantum entanglement. Where quantum systems to be entangled are spatially separated, photons may be used to mediate entanglement of the quantum systems. For example, an entanglement protocol using photons is described in Sean D. Barrett, Pieter Kok Efficient high-fidelity quantum computation using matter qubits and linear optics, Phys. Rev. A 71 , 060310(R) (2005) which is hereby incorporated herein by reference for all purposes. This protocol is referred to herein as the “BK protocol”. The BK protocol is applicable to a wide range of quantum systems. For example, the BK protocol may be used to generate entanglement of matter-based quantum systems that have energy levels in a Lambda configuration (i.e. a ground state having two energy levels and one excited state).

[0005] Many entanglement protocols are probabilistic. A probabilistic entanglement protocol has a probability p of successfully entangling two quantum systems that is less than 100% but more than 0%, even in the case where the probabilistic entanglement protocol is executed perfectly, all apparatus functions perfectly, and, where the protocol is photon mediated, there is no loss of photons. Departure from these ideal conditions (e.g. caused by loss of photons, deviation from the protocol, or failure of apparatus) will effectively reduce the probability p of successfully entangling the two quantum systems when the protocol is executed.

[0006] Where a probabilistic entanglement protocol is used to entangle two quantum systems, it is likely that the entanglement protocol will need to be executed on average more than once to achieve entanglement of the quantum systems. The expected number of times the protocol will need to be executed to achieve entanglement of the quantum systems increases as the probability p decreases.

[0007] An entanglement protocol is said to be “heralded” if success of an attempt to entangle two quantum systems using the protocol is indicated by a result (e.g. detection of a photon, a pattern of photon detections, a measurement result or a pattern of measurement results). The BK protocol is an example of a heralded entanglement protocol.

[0008] The inventors have recognized that there are applications in the fields of quantum communication and quantum informatics where it is necessary or desirable to generate entanglement of quantum systems rapidly and / or to generate entanglement of a significant number of pairs of entangled quantum systems.Summary

[0009] The present technology has several aspects. These include, without limitation: methods for generating entanglement of quantum systems; apparatus for generating entanglement of quantum systems; control systems for apparatus for generating quantum entanglement and instructions for configuring a control system to performmethods according to the invention.

[0010] Further aspects and example embodiments are illustrated in the accompanying drawings and / or described in the following description.

[0011] It is emphasized that the invention relates to all combinations of the above features, even if these are recited in different claims.

[0012] One aspect of the present disclosure provides a control apparatus for use in a system for generating quantum entanglement. The control apparatus comprises entanglement success detection logic configured to: monitor output signals from one or more detectors for a heralding pattern indicating a successful entanglement attempt, wherein a successful entanglement attempt is an entanglement attempt that places a pair of quantum systems in an entangled quantum state; and in response to detecting a heralding pattern that indicates a successful entanglement attempt: cause a low latency signal to be output on one or more signal lines; and cause a higher latency message to be transmitted by a messaging interface, the higher latency message containing information that directly or indirectly identifies the pair of quantum systems entangled by the successful entanglement attempt.

[0013] In some embodiments, the control apparatus is configured to simultaneously monitor a plurality of concurrent entanglement attempts for one or more successful entanglement attempts.

[0014] In some embodiments, the control apparatus is configured to monitor a series of the plurality of concurrent entanglement attempts for the one or more successful entanglement attempts.

[0015] In some embodiments, the signal line is connected to deliver the low latency signal to trigger at least one timer, an output of the at least one timer being connected to inhibit logic operative to deselect one or more quantum systems associated with the timer from participating in further entanglement attempts when the timer is active.

[0016] In some embodiments, the timer is associated with a plurality of quantum systems and the inhibit logic is configured to cause each of the plurality of quantum systems to be inhibited from participating in further entanglement attempts while the timer is active.

[0017] In some embodiments, the at least one timer comprises a plurality of timers, each of the plurality of timers having a respective output connected to a respective one of a plurality of inhibit logic, the respective inhibit logic operative to deselect a respective plurality of the quantum systems associated with the respective timer fromparticipating in further entanglement attempts while the respective timer is active.

[0018] In some embodiments, each of the plurality of inhibit logic is configured to cancel the respective timer if the information in the higher latency message that identifies the pair of quantum systems entangled by the successful entanglement attempt does not include any of the quantum systems associated with the respective timer.

[0019] In some embodiments, the low latency signal comprises a hardware trigger.

[0020] In some embodiments, the low latency signal is generated within 100 ns of the successful entanglement attempt.

[0021] In some embodiments, the control apparatus is configured to coordinate concurrently, for each of a plurality of pairs of quantum systems, serially executing entanglement attempts on each of the pairs of quantum systems using successive iterations of a heralded entanglement protocol.

[0022] In some embodiments, the heralded entanglement protocol comprises initializing a state of each of the quantum systems in each iteration of the heralded entanglement protocol.

[0023] In some embodiments, the heralded entanglement protocol is the BK protocol.

[0024] In some embodiments, the heralded entanglement protocol comprises delivering optical pulses and / or microwave pulses to the quantum systems and the control apparatus is operative to selectively engage and disengage the quantum systems from the optical pulses and / or the microwave pulses.

[0025] In some embodiments, the control apparatus comprises a plurality of analog interfaces connected to engage and disengage the quantum systems from the optical pulses and / or the microwave pulses by altering magnetic fields and / or electric fields at the quantum systems.

[0026] In some embodiments, the quantum systems are divided into plural groups and the control apparatus is operable to engage and disengage the quantum systems of any one of the groups independently of the quantum systems belonging to other ones of the groups.

[0027] In some embodiments, the control apparatus comprises a timer associated with each of the groups, the timer connected to be triggered to be active for a period of time by the low latency signal wherein, the quantum systems of the group are disengaged from the optical pulses and / or the microwave pulses while the timer associated with the group is active.

[0028] In some embodiments, the set period is in the range of about 150 ns to about 1.5 ps.

[0029] In some embodiments, the entanglement success detection logic is connected to receive signals output by a plurality of single photon detectors and the monitoring comprises the entanglement success detection logic monitoring the output signals from the single photon detectors for the heralding pattern that indicates success of the entanglement attempt.

[0030] In some embodiments, the single photon detectors are incorporated into a plurality of Bell state analyzers (BSAs) and the entanglement success detection logic is configured to monitor outputs of the single photon detectors of each of the BSAs for the heralding pattern.

[0031] In some embodiments, the entanglement success detection logic comprises a separate logic unit associated with each of the BSAs, the separate logic units each configured to detect, in the output signals of the single photon detectors of the corresponding BSA, the heralding pattern that indicates success.

[0032] In some embodiments, each of the logic units comprises a corresponding core configured in a configurable logic device.

[0033] One aspect provides a method for controlling a system for generating quantum entanglement. The method comprises: monitoring for successful entanglement attempts, wherein a successful entanglement attempt is an entanglement attempt that places a pair of quantum systems in an entangled quantum state; and in response to detecting a successful entanglement attempt: causing a low latency signal to be generated; and causing a higher latency message to be transmitted, the higher latency message containing information that directly or indirectly identifies the pair of quantum systems entangled by the successful entanglement attempt.

[0034] In some embodiments, the low latency signal comprises a hardware trigger.

[0035] In some embodiments, the low latency signal is generated within 100 ns of the successful entanglement attempt being heralded.

[0036] In some embodiments, the method comprises, concurrently, for each of a plurality of pairs of quantum systems, serially executing entanglement attempts on each of the pairs of quantum systems using successive iterations of a heralded entanglement protocol.

[0037] In some embodiments, executing the heralded entanglement protocol comprises initializing a state of each of the quantum systems in each iteration of theheralded entanglement protocol.

[0038] In some embodiments, the heralded entanglement protocol is the BK protocol.

[0039] In some embodiments, the successive entanglement attempts are separated by a time AT, wherein AT is 1 ps or less.

[0040] In some embodiments, the heralded entanglement protocol comprises delivering optical pulses and / or microwave pulses to the quantum systems and the method comprises selectively engaging and disengaging the quantum systems from the optical pulses and / or the microwave pulses.

[0041] In some embodiments, disengaging the quantum systems from the optical pulses and / or the microwave pulses comprises altering magnetic fields and / or electric fields at the quantum systems.

[0042] In some embodiments, the quantum systems are divided into plural groups and selectively engaging and disengaging the quantum systems from the optical pulses and / or microwave pulses comprises engaging and disengaging the quantum systems of one of the groups independently of the quantum systems belonging to other ones of the groups.

[0043] In some embodiments, the method comprises, in response to the low latency signal, triggering a plurality of timers to be active for a period of time, each of the groups being associated with a corresponding one of the timers, the method comprising, for each of the groups, disengaging the quantum systems of the group from the optical pulses and / or the microwave pulses while the corresponding timer associated with the group is active.

[0044] In some embodiments, the period of time is in the range of about 150 ns to about 1 .5 ps.

[0045] In some embodiments, the method comprises, in response to the low latency signal, disengaging the plurality of pairs of quantum systems from participating in further entanglement attempts for a time period.

[0046] In some embodiments, the method comprises within the time period, for one or more of the pairs of quantum systems for which the entanglement attempt succeeded, using the entanglement of the pair of quantum systems; transferring the entanglement of the pair of quantum systems; or protecting the entanglement of the pair of quantum systems.

[0047] In some embodiments, each of the entanglement attempts comprises a sequence of optical pulses being delivered to the quantum systems of one of thepairs of quantum systems while an optical transition of each of the quantum systems is resonant with the optical pulses.

[0048] In some embodiments, the optical pulses are delivered simultaneously to the quantum systems of the pairs of quantum systems.

[0049] In some embodiments, the optical pulses are delivered in a continuous stream in which the sequence of optical pulses repeats with a period that is not more than 15% longer than the sequence of optical pulses.

[0050] In some embodiments, disengaging the plurality of pairs of quantum systems from participating in further entanglement attempts for a time period comprises altering the optical transition of the quantum systems of the plurality of pairs of quantum systems so that the optical transition is not resonant with the optical pulses.

[0051] In some embodiments, the higher latency message is transmitted to one or more controllers and the method comprises, by the one or more controllers, processing the higher latency message, and based on the processing, selectively altering the transitions of some of the quantum systems, other than the quantum systems of the one or more pairs of quantum systems for which the entanglement attempt succeeded, to be resonant with the optical pulses.

[0052] In some embodiments, the latency between occurrence of a heralding pattern that indicates success and completion of reception of the higher latency message at the one or more controllers is longer than a time AT between successive entanglement attempts.

[0053] In some embodiments, the quantum systems each belong to one of a plurality of groups of the quantum systems, the one or more controllers comprises a plurality of controllers, each of the plurality of controllers being associated with one of the groups of quantum systems and operable to control the transitions of the quantum systems belonging to the associated group of quantum systems, and the method comprises processing the higher latency message at each of the plurality of controllers, based on the processing determining whether the associated group of quantum systems includes any of the quantum systems of the pairs of quantum systems for which the entanglement attempt succeeded, and if the associated group of quantum systems does not include any of the quantum systems of the pairs of quantum systems for which the entanglement attempt succeeded, engaging the associated group of quantum systems to participate in further entanglement attempts.

[0054] In some embodiments, the entanglement protocol includes delivering one ormore microwave pulses to the quantum systems while a microwave transition of each of the quantum systems is resonant with the one or more microwave pulses.

[0055] In some embodiments, inhibiting the plurality of pairs of quantum systems from participating in further entanglement attempts for a time period comprises altering the microwave transition of the quantum systems of the plurality of pairs of quantum systems so that the microwave transition is not resonant with the one or more microwave pulse.

[0056] In some embodiments, altering the optical transition of the quantum systems of the plurality of pairs of quantum systems comprises varying electric fields at locations of the quantum systems.

[0057] In some embodiments, altering the optical transition of the quantum systems of the plurality of pairs of quantum systems comprises varying strain in a crystalline lattice at locations of the quantum systems.

[0058] In some embodiments, the probability that any one of the entanglement attempts will result in success does not exceed 5%.

[0059] In some embodiments, the monitoring comprises monitoring the output signals from plural single photon detectors for a heralding pattern that indicates a successful entanglement attempt.

[0060] In some embodiments, the single photon detectors are incorporated into a plurality of Bell state analyzers (BSAs) and the method comprises making Bell state measurements (“BSMs”) using the BSAs.

[0061] In some embodiments, monitoring outputs of each of the BSAs for a successful entanglement attempt using a distinct logic unit associated with the BSA.

[0062] In some embodiments, the method comprises simultaneously monitoring a plurality of concurrent entanglement attempts for one or more successful entanglement attempts.

[0063] In some embodiments, the method comprises, in response to the low latency signal, deselecting one or more quantum systems from participating in further entanglement attempts for a period of time.

[0064] In some embodiments, the method comprises, in response to the low latency signal, deselecting a plurality of the quantum systems from participating in further entanglement attempts for a period of time.

[0065] In some embodiments, the method comprises engaging the plurality of the quantum systems prior to the end of the period of time in response to determining thatthe pair of quantum systems identified in the higher latency message as being entangled by the successful entanglement attempt does not include any of the plurality of the quantum systems.

[0066] One aspect provides apparatus for generating quantum entanglement. The apparatus comprises: a plurality of quantum systems; a plurality of detector units, each of the detector units comprising a Bell state analyzer having first and second optical inputs optically coupled to receive photons from a pair of the quantum systems that are respectively optically coupled to the first and second inputs; one or more control units operable to deliver control signals to the quantum systems according to an entanglement protocol; one or more inhibit mechanisms operable to engage or disengage the quantum systems from the control signals; and a control system. The control system is configured to: operate the control units to serially execute entanglement attempts on the pairs of quantum systems using successive iterations of a heralded entanglement protocol; monitor outputs of the Bell state analyzers for success of each of the entanglement attempts, wherein success comprises placing the corresponding pair of quantum systems in an entangled quantum state; in response to detecting success of any of the entanglement attempts, generate a low latency signal and a higher latency message; in response to the low latency signal, operate the one or more inhibit mechanisms to disengage the plurality of pairs of quantum systems from participating in further entanglement attempts for a time period; and during the time period receive and process the higher latency message.

[0067] In some embodiments, the apparatus comprises an optical switching network operable to optically couple to the first and second optical inputs of each of the Bell state analyzers a pair of the quantum systems.

[0068] Further aspects and example embodiments are illustrated in the accompanying drawings and / or described in the following description.

[0069] It is emphasized that the invention relates to all combinations of the above features, even if these are recited in different claims.Brief Description of the Drawings

[0070] The accompanying drawings illustrate non-limiting example embodiments of the invention.

[0071] Fig. 1 is a schematic drawing of an example apparatus for generating entanglement of two quantum systems.

[0072] Fig. 2 is a schematic illustration showing serial execution of an entanglement protocol.

[0073] Fig. 2A is a drawing showing an example pattern of optical and microwave pulses that may be applied to quantum systems to implement a quantum entanglement protocol.

[0074] Fig. 3 is a schematic diagram showing an example apparatus that may be controlled to generate entanglement of pairs of quantum systems according to an example embodiment.

[0075] Fig. 4 is a schematic block diagram showing an apparatus according to an example embodiment.

[0076] Fig. 5 is a schematic block diagram illustrating controller apparatus according to an example embodiment.

[0077] Fig. 5A is a functional block diagram of an example detector controller.

[0078] Fig. 5B is a functional block diagram of an example select controller.

[0079] Fig. 6 is a flowchart illustrating a method according to an example embodiment of the invention.

[0080] Fig. 6A is a flowchart illustrating a method according to an example embodiment of the invention.

[0081] Fig. 7 is a flowchart illustrating a method according to an example embodiment of the invention.

[0082] Figs. 8A and 8B are time charts showing a sequences of actions in example embodiments of the invention.Detailed Description

[0083] Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive sense.

[0084] Fig. 1 is a schematic illustration showing apparatus 10 for generating entanglement of two quantum systems, 11 A and 11 B. Apparatus 10 includes a detector unit 12 that has optical ports 12A, 12B that are optically coupled to quantum systems 11A and 11 B by optical paths 13. Detector unit 12 may, for example, comprise a Bell state analyzer (BSA). Optical paths 13 may comprise any or anysuitable combination of optical waveguides (e.g. optical fibers, hollow waveguides, rectangular waveguides, integrated waveguides etc.), free space optics, optical switches, optical switching networks, etc.

[0085] Apparatus 10 also includes one or more control units 14 that are operative to emit control signals 15 onto quantum systems 11A and 11 B. The control signals 15 may, for example comprise optical signals and / or RF (e.g. microwave) signals that affect the quantum state of the respective quantum system 11 A or 11 B according to a chosen entanglement protocol. Apparatus 10 may, for example, be controlled to generate entanglement of quantum systems 11 A and 11 B according to the BK protocol.

[0086] Each execution of an entanglement protocol may be called an entanglement attempt. As discussed above, it may be necessary to perform more than one entanglement attempt to successfully entangle quantum systems 11 A and 11 B. The probable number of entanglement attempts to achieve entanglement will depend on factors such as the choice of entanglement protocol, the lossiness of optical paths 13, the efficiency of detector 12, and the precision with which control units 14 are controlled to deliver control signals 15 according to the protocol. In some realistic cases, generating entanglement of quantum systems 11 A and 11 B may be expected to require on the order of dozens, hundreds, or thousands of entanglement attempts. It may take a significant amount of time to generate entanglement of quantum systems 11A and 11 B.

[0087] In many applications in the fields of quantum communications and quantum informatics it is undesirable to have to wait more than a short time for quantum entanglement to be available to perform an operation involving the quantum state of a quantum system (e.g. teleporting the quantum state to another quantum system, using the entanglement, or teleporting a quantum gate to act on the quantum state of the quantum system). This is because the fidelity of the entangled state of the quantum system degrades over time (the quantum state decoheres). It is also undesirable to generate quantum entanglement more than a short time before the quantum entanglement can be used since the fidelity of the entanglement degrades over time and it is often possible to use hardware resources (e.g. quantum systems, detectors, optical switches, etc.) more efficiently when quantum entanglement is generated “just-in-time”.

[0088] Fig. 2 is a schematic illustration showing a sequence of / V entanglementattempts 20 on a pair of quantum systems. Each entanglement attempt 20 requires a time to complete.is primarily determined by the choice of entanglement protocol. Each successive entanglement attempt 20 is separated from the immediately prior entanglement attempt 20 by a time period AT. It is generally desirable to minimize AT because, by doing so, the rate at which entanglement can be generated will be increased and the time required to generate an entangled state of the pair of quantum systems can be decreased.

[0089] In some embodiments, each entanglement attempt 20 is completed in a time that is about 15 ps or less. In some embodiments, the time for completion of each entanglement attempt 20 is in the range of about 200 ns to about 10 ps. In some embodiments, AT is about 1 ps or less or about 2 ps or less. For example, AT may be in the range of about 90 ns to about 1 ps or 2 ps.

[0090] If a particular entanglement attempt 20 succeeds (entanglement of the pair of quantum systems is generated), it is undesirable to perform another entanglement attempt on the pair of quantum systems because performing another entanglement attempt is certain or likely to destroy the entanglement. Where AT is small, it can be a problem to both provide a control system that allows entanglement attempts to be repeated with only a very small period between successive entanglement attempts and to include a mechanism operative to recognize that an entanglement attempt has succeeded and, if so, inhibit performance of another entanglement attempt 20 on the pair of quantum systems.

[0091] Fig. 3 is a block diagram illustrating apparatus 30 according to an example embodiment. Apparatus 30 includes a controller 31 which controls control unit(s) 14 to execute a desired entanglement protocol (e.g. by causing control units 14 to expose quantum systems 11 -quantum systems 11 A and 11 B are shown- to optical and / or microwave pulses having selected properties such as selected wavelengths, durations and waveforms).

[0092] Controller 31 includes a low level controller 31 A that directly controls control units 14 to execute entanglement attempts on quantum systems 11A and 11 B using a desired entanglement protocol. Low level controller 31 A may, for example comprise a fast electronic control system. A high level controller 31 B executes a program 31 C that controls overall operation of apparatus 30. For example, high level controller may comprise a classical computer system that configures apparatus 30 to perform quantum informatics processing, some steps of which may consume entanglement ofquantum systems 11 .

[0093] Fig. 2A (which is not to scale) schematically illustrates an example optical control signal 15A and microwave control signal 15B that may be applied to each of quantum systems 11A and 11 B to attempt entanglement of quantum systems 11 A and 11 B in accordance with the BK protocol. Control signal 15A may be generated, for example, by operating a light source (e.g. a laser) to emit light into an optical shutter and opening the optical shutter to illuminate one or more quantum systems 11 with pulses of light. The light has a wavelength (or equivalently frequency) selected to match (e.g. be resonant with) a chosen optical transition of the quantum system(s) 11 . The light pulses have durations selected to cause desired changes to the quantum states of quantum systems 11 A and 11 B.

[0094] Control signal 15B may be generated, for example, by controlling an RF signal generator to emit microwave pulses that have a frequency (or equivalently wavelength) chosen to have a desired effect on the quantum states of quantum system(s) 11 e.g. microwave pulses with a wavelength (or equivalently frequency) selected to match (e.g. be resonant with) a chosen microwave transition of the quantum system(s).

[0095] Each entanglement attempt 20 includes an initialization phase 23 which initializes quantum systems 11 A, 11 B to have desired initial quantum states. In the illustrated example, initialization phase 23 includes a first optical pulse 23A that optically pumps a quantum system 11 to have a known quantum state and a second optical pulse 23B that modifies the quantum state of the quantum systems 11 to be the desired initial quantum state. After initialization phase 23 is complete, an optical IT pulse 24A is applied to each of the quantum systems 11 A and 11 B. During a first detection window 25A following optical pulse 24A, a detector unit 12 is monitored to detect photon(s) emitted by quantum systems 11 A and / or 11 B.

[0096] After a sufficient time has passed for excitations of quantum systems 11 A, 11 B to relax, a microwave pulse 26 is applied to each of quantum systems 11 A and 11 B. The microwave pulse can be described as a IT pulse or as performing an X operation on quantum systems 11 A and 11 B. The microwave pulse has a frequency and duration selected to flip computational states of quantum systems 11 A and 11 B.

[0097] After application of microwave pulse 26, an optical IT pulse 24B is applied to each of the quantum systems 11 A and 11 B. During a second detection window 25B following optical pulse 24B, detector unit 12 is monitored to detect photon(s) emittedby quantum systems 11 A and / or 11 B. After a time 27, which corresponds to AT, a next entanglement attempt on quantum systems 11 A and 11 B begins.

[0098] In some embodiments, optical pulses 23B, 24A and 24B each have a duration that does not exceed about 100 ns.

[0099] Control signals 15 may repeat the pattern of optical and microwave pulses as illustrated in Fig. 2A continuously. In some embodiments, AT is short enough that control signals 15 comprise a continuous stream of optical pulses in which a sequence of optical pulses for each entanglement attempt 20 repeats with a period that is not more than 15% longer than the sequence of optical pulses.

[0100] Execution of the entanglement protocol results in patterns of detection (“heralding patterns”) by optical detectors of detection unit 12. For example, detection unit 12 may include photon detectors 12C and 12D which respectively output results 12E and 12F which each can have the value FALSE (no photons detected in the current detection window) or TRUE (one photon detected in the current detection window). In some examples, no result may be output when no photons are detected e.g. the absence of a result may indicate that no photons were detected. Certain heralding patterns correspond to successful generation of entanglement of quantum systems 11 A and 11 B. The heralding pattern(s) that correspond to successful generation of entanglement depends on the particular entanglement protocol that is being used. For example, in the BK protocol, successful entanglement is heralded in the detection of one photon in each of two detection windows (e.g. detection windows 25A and 25B).

[0101] The present technology is not limited to the case where the BK protocol is used to achieve entanglement of pairs of quantum systems 11 , nor to quantum systems 11 that can be controlled or are controlled using both optical and microwave pulses. Other entanglement protocols may be performed using other control units, by delivering other patterns of optical and / or microwave pulses to quantum systems 11 and / or monitoring for other heralding patterns. As such, it will be appreciated that the foregoing and following description may be adapted for other control units, heralded entanglement protocols and / or quantum systems 11. Information relating to example alternative entanglement protocols that may be used with the present technology is provided in “Entanglement of single atom quantum bits at a distance”, Moehring et al, Nature 449:68-71 (2007); “Creation of entangled states of distant atoms by interference”, Cabrillo et al, Physical Review A 59(2): 1025-1033 (1999); and“Deterministic delivery of remote entanglement on a quantum network”, Humphreys et al., Nature 558(7709):268-273 (2018).

[0102] The time required to complete each entanglement attempt and the probability that any individual entanglement attempt will succeed may vary among protocols and quantum systems. Some embodiments apply entanglement protocols that do not require initialization of quantum systems 11 to any particular quantum state in each entanglement attempt. For some such protocols, the time required to execute one entanglement attempt can be less than the time required to execute an entanglement attempt using the BK protocol. A tradeoff may be that the probability that an individual entanglement attempt will succeed could be lower than that of the BK protocol (and therefore may require, on average, a larger number of entanglement attempts to achieve entanglement of two quantum systems 11 than the expected number of entanglement attempts to achieve entanglement using the BK protocol).

[0103] Apparatus 30 includes a trigger 32. Trigger 32 may be referred to as a fast trigger 32. Fast trigger 32 comprises electronics that are configured to detect a heralding pattern that indicates that entanglement of quantum systems 11 A and 11 B has been achieved. Fast trigger 32 may, for example, comprise electronics that implements logic that identifies a heralding pattern that corresponds to success. For the BK protocol, the logic may, for example comprise: (12E XOR 12F )25A AND (12E XOR 12F )25B where the subscripts respectively indicate the first and second detection windows 25A, 25B. In an example embodiment the logic is implemented in whole or in part by hard wired logic circuits and / or configuration of one or more configurable logic devices such as a field programmable gate array (FPGA).

[0104] Fast trigger 32 outputs a signal 33 that triggers a timer 34. Timer 34 may be called a “lockout timer”. Upon being triggered, timer 34 causes one or more inhibit mechanisms 35 to operate for a set period of time. In some embodiments, timer 34 invokes the one or more inhibit mechanisms at the hardware level (i.e. without needing any involvement from controller 31 or, more specifically, from a processing unit of controller 31). The inhibit mechanisms 35 prevent associated quantum systems 11 from engaging with optical control signals 15A (e.g. optical pulses as shown in Fig. 2A) until timer 34 expires (times out) or is cancelled.

[0105] The illustrated apparatus 30 includes inhibit mechanisms 35A and 35B. Inhibit mechanisms 35 are selectively operable to protect the entangled state of quantum systems 11 A and 11 B. Inhibit mechanisms 35 may, for example operate by one ormore of:• disabling control units 14 from emitting control signals 15;• changing wavelengths of control signals 15 so that control signals 15 are decoupled from quantum systems 11 A and 11 B;• changing properties of a path by which control signals 15 are normally delivered to quantum systems 11A and 11 B so that control signals 15 are attenuated, blocked or changed so as not to reach or not to affect the quantum states of quantum systems 11 A and 11 B; and / or• changing energy levels of quantum systems 11 A and 11 B so that control signals 15 are decoupled from quantum systems 11 A and 11 B.

[0106] In some embodiments, inhibit mechanisms 35 operate to change energy levels of quantum systems 11 A and 11 B by one or more of: changing the magnitude of an electric field at quantum system 11A, changing the magnitude of an electric field at quantum system 11 B, changing the magnitude of a magnetic field at quantum system 11 A, changing the magnitude of a magnetic field at quantum system 11 B, changing a strain in a substrate at the location of quantum system 11 A, changing a strain in a substrate at the location of quantum system 11 B, and any other suitable approach for changing an energy level of quantum systems 11 A and 11 B. In some embodiments inhibit mechanisms 35 may operate by not doing something or ceasing to do something. For example, in some embodiments, inhibit mechanisms 35 may operate by ceasing to cause quantum systems 11 A and 11 B to be coupled to one or more control signals 15 (e.g. by ceasing to cause electric fields, magnetic fields and / or substrate strain at locations of quantum systems 11 A and 11 B to have a value or values such that quantum systems 11A and 11 B are coupled to control signals 15).

[0107] In some embodiments, inhibit mechanisms 35 do not alter control signals 15. In such embodiments control signals 15 may be delivered continuously while inhibit mechanisms 35 are activated and deactivated. An inhibit mechanism 35 may operate to selectively uncouple (or disengage or deselect or inhibit) those one or more of quantum systems 11 that are affected by the inhibit mechanism 35 from optical control signals 15A and / or microwave control signals 15B. In some embodiments, it is sufficient to uncouple the quantum systems 11 from optical control signals 15A while the quantum systems 11 remain coupled to microwave control signals 15B.

[0108] In an example embodiment, an inhibit mechanism 35 is operable to change the wavelengths of optical and / or microwave transitions of one or more quantumsystems 11 using, for example, electric fields, magnetic fields and / or strain application. This may be referred to as selectively tuning individual quantum systems 11 into or out of resonance with optical control signals 15A and / or microwave control signals 15B.

[0109] In some embodiments, inhibit mechanisms 35 are operative to Stark shift energy levels of quantum systems 11 A and 11 B by electronically controlling electric fields at the locations of quantum systems 11 A and 11 B. For example, inhibit mechanisms 35 may change an electrical potential difference between electrodes located adjacent to quantum system 11 A and 11 B. This may, for example be done by charging or discharging a capacitor connected between the electrodes. During the set period of time after timer 34 has been triggered, inhibit mechanism 35 may cause Stark shifting of quantum systems 11A and 11 B so that optical control signals 15B are uncoupled from quantum systems 11 A and 11 B. Before and after the set period of time after timer 34 has been triggered, inhibit mechanisms 35 may set the potential difference between the electrodes such that optical control signals 15B are coupled to quantum systems 11 A and 11 B (i.e. a transition between selected quantum states of quantum systems 11 A and 11 B has an energy that corresponds to a wavelength of optical control signals 15B).

[0110] In some embodiments, inhibit mechanisms 35 are operative to couple or decouple quantum systems 11 A and 11 B from microwave control signals 15B by altering a magnetic field at the location of quantum systems 15A and 15B. For example, inhibit mechanisms may change an electrical current provided to an electromagnet that generates a magnetic field at the location of quantum system 11 A or 11 B, which has a strength that depends on the electrical current. In some embodiments the electromagnet comprises a conductive loop that extends around quantum systems 11 A and / or 11 B.

[0111] In some embodiments, inhibit mechanisms 35 associated with quantum systems 11 are controlled in groups such that a signal control input controls a group of inhibit mechanisms that are associated with a corresponding group of quantum systems 11 . Such embodiments permit an entire group of quantum systems 11 to be selectively coupled to or uncoupled from control signals 15 or a component thereof with a single control input. In some embodiments inhibit mechanisms 35 associated with individual quantum systems 11 are individually controllable.

[0112] Timer 34 causes inhibit mechanisms 35 to operate for a time that is longenough for controller 31 to receive notification of the generated entanglement and take action e.g. to continue to protect the generated entanglement of quantum systems 11 A and 11 B after timer 34 times out (e.g. using inhibit mechanisms 35). For example, in some embodiments timer 34 is pre-configured with a delay equal to the worst-case latency for messages (e.g. higher latency messages 81 - See Figs. 8A and 8B) indicating successful entanglement of quantum systems 11 A and 11 B plus the time required to process and act on such messages. The messages may, for example, originate from low level controller 31 A and be sent to high level controller 31 B, which may be configured to take action based on the messages. In some embodiments, the messages are transmitted by a fast messaging protocol, for example a SERDES messaging protocol.

[0113] With the generated entanglement protected, controller 31 may proceed to apply other actions that affect the entangled quantum systems 11 A and / or 11 B, for example, transfer the entanglement from one or both of quantum systems 11 A and 11 B to one or more other quantum systems, apply one or more quantum gates to one or more of the entangled quantum systems (for example using microwave or optical pulses), etc.

[0114] In some embodiments, timer 34 operates inhibit mechanisms 35 to decouple quantum systems 11A and 11 B from control signals 15 for a time on the order of a few hundred nanoseconds (e.g. a time in the range of about 150 ns to about 1.5 ps).

[0115] There are many potential applications that could advantageously use entanglement of quantum systems that is generated at a high rate. The entanglement of quantum systems may be used as a resource for moving quantum information (e.g. qubit states) within a quantum informatics processor by quantum teleportation, teleporting two qubit gates to be applied between distributed qubits, entangling specific qubits by entanglement swapping, and other applications of entanglement.

[0116] Fig. 3 shows only two quantum systems 11 and one detector 12-1 for clarity of explanation. More typically, apparatus 30 will include significantly more than two quantum systems 11 . The rate at which entanglement can be generated can be scaled up by making concurrent entanglement attempts on plural pairs of quantum systems.

[0117] Apparatus 30 may be configured to execute entanglement attempts on multiple pairs of quantum systems 11 concurrently. In some embodiments, apparatus as described herein is configurable to perform least 30, 50, 80, 100, a few hundred, ormore entanglement attempts concurrently.

[0118] For example, apparatus 30 may be scaled up to perform larger numbers of concurrent entanglement attempts by providing a desired number of quantum systems and an optical network that is configurable to selectively connect pairs of the quantum systems to respective detectors for entanglement attempts. In some embodiments the apparatus may be designed so that additional quantum systems and / or additional detectors are provided in modules which can be added to increase the number of entanglement attempts that the apparatus can perform concurrently. As the number of quantum systems and detectors in such apparatus is increased, it becomes a hard problem to identify successful entanglement attempts and to preserve the resulting entanglement before the entanglement is spoiled by execution of another entanglement attempt involving already entangled quantum systems.

[0119] In some embodiments, apparatus 30 includes plural sets 37 which each include a pair of quantum systems 11 connected to respective inputs of a corresponding detector unit 12. Each set 37 may be controlled to generate entanglement of the corresponding pair of quantum systems 11 . Different sets 37 may be operated synchronously or asynchronously. In some embodiments, a control unit or control units are common to plural sets 37. In some embodiments some or all quantum systems of a plurality of sets of quantum systems receive control signals 15 that are synchronized and delivered from the same or different optical and / or microwave sources. That is, the same control signals 15 may be used to perform an entanglement protocol on multiple pairs of quantum systems.

[0120] In some embodiments, apparatus 30 includes a switching network that is operable to optically connect plural pairs of quantum systems 11 to respective inputs of plural detector units 12. For example, apparatus 30 may include a network as shown in Fig. 4.

[0121] Fig. 4 is a schematic diagram showing an example apparatus 40 that includes plural quantum systems 11-1 to 11-N (generally and collectively quantum systems 11) where N is a positive integer. In some embodiments, N is larger than 30 or larger than 90 or larger than 250 or larger than 800.

[0122] Quantum systems 11 are each optically connected to an optical switching network 41. Plural detector units 12-1 to 12-M (generally and collectively detector units 12) have ports connected to optical switching network 41 . Optical switching network 41 includes one or more optical switches that are configurable to provideoptical paths that optically connect each optical port of each detector unit 12 to a respective one of quantum systems 11 . The optical paths may comprise any suitable optical waveguides such as optical fibers, integrated waveguides, as well as free space optics or combinations of two or more of these.

[0123] With M detector units 12 an entanglement protocol may be performed concurrently on M pairs of quantum systems 11 . Optical switching network 41 may also be configurable to optically connect selected quantum systems 11 .

[0124] Quantum systems 11 A and 11 B in apparatus 30 may, for example be provided by any two quantum systems 11 of apparatus 40 that can be optically coupled to respective optical ports of the same one of detector units 12 by optical switching network 41 . The construction of optical switching network 41 determines which pairs of quantum systems 11 can be simultaneously optically connected to respective optical ports of any one of detector units 12. In some embodiments optical switching network 41 has all-to-all connectivity such that any two quantum systems chosen from quantum systems 11 may be optically connected to respective ports of at least one of detector units 12. In some embodiments optical switching network 41 has a topology that does not permit some pairs of quantum system 11 to be simultaneously connected to respective optical ports of any of detector units 12.

[0125] There are various options for arranging control units 14 to deliver control signals 15 to quantum systems 11 of apparatus 40. Some embodiments include a dedicated control unit 14 for each quantum system 11. Some embodiments divide quantum systems 11 into groups and include a control unit 14 associated with each group. The control units 14 may each be operative to deliver the same control signals 15 to each member of the associated group of quantum systems 11 . Some embodiments include a global control unit 14 that is operative to simultaneously deliver control signals 15 to all quantum systems 11. Control signals 15 may be carried to quantum systems 11 through free space, suitable waveguides, or in any other suitable manner,

[0126] Embodiments that include a larger number of independently control units 14 may provide more flexibility to provide different control signals 15 to different quantum systems 15 at the same or different times.

[0127] There are several options for arranging inhibit mechanisms 35. In some embodiments, one or more inhibit mechanisms 35 are configured to facilitate independent control over whether each quantum system 11 is or is not inhibited frombeing affected by control signals 15. In some embodiments, one or more inhibit mechanisms are configured to facilitate independent control over whether the quantum systems 11 belonging to each group are either all inhibited from being affected by control signals 15 or all not inhibited from being affected by control signals 15. At any given time, some groups of quantum systems 11 may be inhibited while one or more other groups of the quantum systems 11 are not inhibited. In some embodiments one or more global inhibition mechanisms are configured to control whether all quantum systems 11 are inhibited or all quantum systems 11 are not inhibited from being affected by control signals 15.

[0128] Depending upon the nature of quantum systems 11 and the entanglement protocol being used, an inhibit mechanism 35 may inhibit the effects of all control signals 15 or a selected component of signals 15 (e.g. optical control signals 15A or some part thereof or microwave control signals 15B or some part thereof). The term “selected” when applied to a quantum system 11 or a group of quantum systems 11 indicates that the action of control signals 15 on the quantum system 11 or group of quantum systems 11 is not inhibited (e.g. the quantum system 11 or group of quantum systems 11 is coupled to the control signals 15 to participate in entanglement attempts). The term “deselected” means the opposite of selected (i.e. the quantum system 11 or group of quantum systems 11 is uncoupled from control signals 15 or some component thereof such that entanglement of the quantum system 11 or any one or more quantum systems 11 of a group of quantum systems 11 is not destroyed by the application of control signals 15).

[0129] In some embodiments, it is advantageous to provide a control system 31 that is modular and / or decentralized, at least in part. Such control systems may advantageously be scalable to control larger numbers of quantum systems 11 and / or provide lower latency in control functionality.

[0130] Fig. 5 is a functional block diagram of an example control system 50. Control system 50 may, for example, be used as controller of apparatus as described herein (e.g. as controller 31 of apparatus 30). Control system 50 comprises a circuit controller 50A, a switch controller 50B, a detector controller 50C, a global microwave controller 50D, a global optical controller 50E, one or more optical select controllers 50F, and one or more microwave select controllers 50G. Any one or combination of the controllers 50A, 50B, 50C, 50D, 50E, 50F may be referred to as a control apparatus.

[0131] For example, low level controller 31A of apparatus 30 (see Fig. 3) may comprise a switch controller 50B, a detector controller 50C, a global microwave controller 50D, a global optical controller 50E, one or more optical select controllers 50F, and one or more microwave select controllers 50G. A high level controller 31 B of apparatus 30 may comprise a circuit controller 50A.

[0132] In some embodiments circuit controller 50A comprises a computer comprising a data processor that executes computer software that facilitates configuring controllers 50B to 50G for performing sequences of operations on quantum systems 11 (e.g. to implement a quantum circuit). Controllers 50B to 50G may be implemented by electronic circuits and / or configurable logic operable, when configured by circuit controller 50A, to execute sequences of actions involving quantum systems 11 . Controllers 50B to 50G may perform such actions, according to their respective configurations, in real time on nanosecond time scales. In some embodiments configuring controllers 50B to 50G comprises one or more of setting up one or more configurable logic devices to implement logic for performing or coordinating the actions and / or storing data relevant to performing the actions in data stores accessible to one or more of controllers 50B to 50G.

[0133] Optical select controllers 50F and microwave select controllers 50G are respectively operable to control inhibit mechanisms 35OP and 35MW to selectively couple and uncouple one or more quantum systems 11 from optical control signals 15A and microwave control signals 15B. Each of optical select controllers 50F and microwave select controllers 50G is operable to control coupling and uncoupling of a corresponding group of quantum systems 11 from optical control signals 15A and microwave control signals 15B respectively (i.e. the optical select controllers and / or microwave select controllers serve to control inhibit mechanisms 35 as described herein). Optical selects 35OP and microwave selects 35MW may respectively operate to inhibit one or more quantum systems 11 from being affected by optical control signals 15A and 15B by any suitable inhibit mechanism including the example inhibit mechanisms described herein.

[0134] In some embodiments, each of optical select controllers 50F and microwave select controllers 50G functions to couple or uncouple all quantum systems 11 belonging to a corresponding group of quantum systems from optical control signals 15A or microwave control signals 15B respectively. For example, one optical select controller 50F and a corresponding microwave select controllers 50G may togethercontrol coupling or uncoupling of a group of quantum systems 11 from both optical control signals 15A and microwave control signals 15B. The group may include any suitable number of quantum systems 11 (e.g. two or more quantum systems 11). For example, the group may include 10 to 100 quantum systems 11.

[0135] Apparatus as described herein may include any suitable number of groups of quantum systems. For example, apparatus according to some embodiments includes 3 to 100 groups of quantum systems where each group of quantum systems is associated with a corresponding optical select controller 50F and a corresponding microwave select controller 50G.

[0136] Global optical controller 50E and global microwave controller 50D are respectively operative to control generation of optical pulses from one or more optical sources (e.g. optical pulse generators 51 OP) and to control generation of microwave pulses from one or more microwave sources (e.g. microwave pulsers 51 MW) as required by an entanglement protocol (e.g. the optical pulses and microwave pulses may be as illustrated in Fig. 2A or as required to implement some other heralded entanglement protocol).

[0137] The architecture of controller 50, which includes plural optical select controllers 50F and plural microwave select controllers 50G, allows some groups of quantum systems 11 to be coupled to optical control signals 15A and microwave control signals 15B respectively provided by global microwave controller 50D and global optical controller 50E, while some groups of quantum systems 11 are not coupled to optical control signals 15A and microwave control signals15B.

[0138] In some embodiments, microwave control signals 15B are not damaging to entangled states and methods according to certain embodiments do not involve microwave select controllers 50G. In some embodiments, microwave select controllers 50G are not present and / or are not connected to receive low latency signals 33 and / or higher latency messages 81 .

[0139] Detector controller 50C controls operation of detectors 12 and may include functionality described above in relation to fast trigger 32. In some embodiments, detector controller 50C both generates fast trigger 32 and transmits higher latency messages 81 . For example, detector controller 50C may incorporate one or more logic circuits that process output signals from detectors 12 to identify heralding patterns that indicate that the quantum systems 11 that are coupled to any of the detectors 12 have been successfully entangled. In response to detecting suchentanglement, detector controller 50C may generate a signal 33 that is distributed to each of optical select controllers 50F (or each of optical select controllers 50F and each of microwave select controllers 50G). Signal 33 may be generated with low latency (e.g. within 100 ns of detecting a heralding pattern that corresponds to successful entanglement).

[0140] Detector controller 50C may commence sending a higher latency message 81 concurrently with generating signal 33. The time required to complete sending higher latency messages 81 can depend on the length of the messages 81 , the data transmission protocol used to transmit higher latency messages 81 and the distances over which higher latency messages 81 are transmitted. In some embodiments, the time required to complete transmission of higher latency messages 81 is a few ps or less or about 1 ps or less for each higher latency message 81 .

[0141] In some embodiments, it is known in advance which pairs of quantum systems are subject to entanglement attempts at any time. In some embodiments, detector controller 50C, or another controller configured for transmitting higher latency messages 81 , may prepare the data for possible higher latency messages 81 in advance. For example, a controller (e.g. detector controller 80C) may assemble data for potential higher latency messages 81 indicating success of each of the concurrent entanglement attempts being made. If one of these entanglement attempts is determined to have succeeded then the corresponding pre-assembled data may be send as a higher latency message 81 with no time required to assemble the data for the higher latency message 81 .

[0142] In some embodiments, control system 50 includes multiple independent signal paths (e.g. multiple hardware triggers) that may be used to distribute low latency signal 33. Each signal path may connect to a subset made up of one or more of the optical select controllers 50F and microwave select controllers 50G. In such cases, low latency signals 33 may be selectively delivered only to certain ones of optical select controllers 50F and microwave select controllers 50G. With this construction, in the event that a heralding pattern that identifies a successful entanglement attempt is detected, low latency signal 33 may be distributed only to the one or two subsets of the optical select controllers 50F (and microwave select controllers 50G if applicable) which contain optical select controllers 50F that control the quantum systems 11 involved in the entanglement attempt. Timers 34 of optical select controllers 50C that do not receive low latency signal 33 can continue to select entanglement attempts.For a system 50 that includes Q optical select controllers 50F and can support R independent trigger paths for distributing low latency signals 33, for each successful entanglement attempt, either (Q / R) or (2*Q / R) optical select controllers 50F will receive low latency signals 33.

[0143] In some embodiments, detector controller 50C is connected to receive output signals from detectors 12A, 12B and send data characterizing the signals to other controllers of control system 50. The output signals may be analog signals and the detector controller 50C may digitize those signals before sending the data. The data may, for example be transmitted in a multicast message to other controllers of control system 50.

[0144] Signal 33 may start timers 34 in each of optical select controllers 50F and microwave select controllers 50G. Timers 34 each count down a wait period. Each of optical select controllers 50F and microwave select controllers 50G may cause those of quantum systems 11 that they are associated with to be inhibited from being affected by the optical pulses or microwave pulses delivered under the control of global optical controller 50E and global microwave controller 50D respectively for as long as the corresponding timer 34 remains active (i.e. until the corresponding timer times out or is cancelled).

[0145] In some embodiments, control system 50 does not disengage quantum systems 11 from microwave control signals (e.g. microwave pulses), in response to signal 33. This may be appropriate, for example, where the microwave control signals are not damaging to entangled states of the quantum systems 11 . In such embodiments signal 33 may not be distributed to microwave select controllers 50G and / or microwave select controllers 50G may be configured to not disengage quantum systems 11 from the microwave control signals in response to signal 33.

[0146] Switch controller 50B is operative to control an optical switching network (e.g. optical switching network 41) to couple selected quantum systems 11 to ports of corresponding detectors 12.

[0147] A higher latency message that identifies, directly or indirectly, the particular quantum systems 11 that were involved in the successful entanglement attempt(s) or identifies, directly or indirectly, the group(s) of quantum systems 11 to which those quantum systems 11 belong is generated, directly or indirectly, in response to the detection of the heralding pattern(s) indicating successful entanglement attempt(s). In some embodiments, higher latency messages 81 additionally include results ofmeasurements and / or a pattern of measurement results that heralded the entanglement. Higher latency messages 81 may, for example, be sent via a high speed data transmission protocol (e.g. a SERDES protocol such as Aurora ™). The higher latency messages may, for example, be sent on a point to multi point serial link.

[0148] In some embodiments, the higher latency messages are generated and sent by detector controller 50C or switch controller 50B or circuit controller 50A. For example, in one example embodiment detector controller 50C identifies which quantum systems 11 are associated with successful entanglement attempt(s) and generates and sends higher latency messages that directly or indirectly identify those quantum systems 11 that were entangled by the successful entanglement attempts.. For example, prior to commencing the entanglement attempts, detector controller 50C may be configured to hold or have access to data that comprises identifiers for entanglement attempts and / or identifiers for quantum systems 11 that will take part in entanglement attempts. The data may, for example, be provided by circuit controller 50A while control system 50 is being configured to implement a quantum circuit. In such embodiments, optical select controllers 50F and microwave select controllers 50G may be configured to hold or access information as to which entanglement attempts they are currently processing and / or which quantum systems 11 they control. This information allows each optical select controller to determine from the contents of a higher latency message 81 whether any quantum systems 11 that are controlled by the optical select controller were involved in a successful entanglement attempt.

[0149] In another example embodiment, circuit controller 50A is connected to receive signals 33 which indicate that at least one detector unit 12 has detected heralding patterns that indicate that a pair of quantum systems 11 that are connected to the detector unit 12 have just been successfully entangled. In some embodiments, signals 33 indicate which specific detector unit 12 detected the successful entanglement. In some embodiments, circuit controller 50A is configured to access a memory (e.g. a register) of detector controller 50C to identify which detector unit(s) 12 have detected heralding patterns indicating success of the most recent entanglement attempt 20.

[0150] Circuit controller 50A may keep track of which pairs of quantum systems 11 are currently connected to which detector unit 12 so that entanglement attempts maybe executed on those pairs of quantum systems 11 . Given the detector unit 12 which detected the successful entanglement, circuit controller 50A may identify which of quantum systems 11 are currently connected to the detector unit 12 and are now entangled. Circuit controller 50A may then generate and transmit the higher latency message to optical select controllers 50F and / or microwave select controllers 50G, as applicable.

[0151] In some embodiments, higher latency messages 81 may be received after the next entanglement attempt 20 would normally begin. For example, the latency between occurrence of the heralding pattern that indicates success and completion of reception of higher latency message 81 at its destination (e.g. an optical select controller 50F) may be longer than AT.

[0152] Upon receiving the higher latency message 81 , those of microwave select controllers 50G and / or optical select controllers 50F, as applicable which are not associated with any of the successfully-entangled quantum systems 11 may either cancel (e.g. reset) their timer 34 and resume entanglement attempts or wait for their timer 34 to time out and then resume entanglement attempts .

[0153] It can be appreciated that the provision of low latency signal 33 and timer(s) 34 facilitates short times AT between successive entanglement attempts while allowing entanglement attempts following a successful entanglement attempt to be paused in response to low latency signal 33, thereby avoiding or reducing the risk of destruction of generated entanglement. Reduction of AT allows more entanglement attempts 20 to be performed in a given time. The increase in the rate at which entanglement attempts can be made is particularly advantageous in cases where the likelihood of success of any particular entanglement attempt is lower. In some embodiments the probability of success of an individual entanglement attempt 20 is less than 1 in 4, or less than 1 in 10, or less than 1 in 20 (5%) or less than 1 in 50 or less than 1 in 100 or less than 1 in 250.

[0154] In the example apparatus 50, circuit controller 50A exerts overall control of the ways that quantum states of quantum systems 11 are manipulated to execute a desired result. For example, circuit controller 50A may configure switch controller 50B to optically connect a pair of quantum systems 11 to input ports of a selected detector unit 12 so that entanglement attempts may be performed on that pair of quantum systems 11 . Circuit controller 50A may coordinate steps to consume created entanglement for purposes such as quantum teleportation of quantum states and / orquantum gates among quantum systems 11 , performing entanglement swapping to create entanglement of other pairs of quantum systems, distilling entanglement of plural entangled pairs of quantum systems 11 to create purer entangled states, and / or constructing desired entangled states of quantum systems 11 . Circuit controller 50A may additionally execute a program to coordinate performance of quantum informatics processing (e.g. quantum computing, quantum key distribution, etc.) using quantum systems 11 .

[0155] In some embodiments, circuit controller 50A is configured for managing (initiating, tracking, controlling) implementation of a quantum circuit by quantum systems 11 . The quantum circuit may, for example, include initializing quantum systems 11 in particular ways, applying gates to quantum systems 11 , making measurements on quantum systems 11 , generating entanglement of certain quantum systems 11 and so on.

[0156] Circuit controller 50A may interact with other controllers of control system 50 by event messaging. The event messaging may, for example, use the same messaging protocol that is used for higher latency messages 81 .

[0157] Circuit controller 50A may transmit messages to other controllers of control system 50 which include commands for setting instructions to be executed by the other controllers to, for example, implement a quantum circuit. Circuit controller 50A may also transmit messages to other controllers which include commands for setting instructions to be executed by the other controllers to manage entanglement generation as described herein. The instructions may, for example, cause the other controllers to optically connect selected quantum systems 11 to ports of detector units 12, deliver optical and / or microwave control signals 13A, 15B to execute a selected entanglement protocol, communicating the state of the entanglement / detection events back to circuit controller 50A, and / or collect and store log data during runtime.

[0158] Fig. 5A is a functional block diagram showing main elements of an example detector controller 50C. Entanglement generation success logic 50C-1 receives signals output by detector units 12 at inputs 50C-2.

[0159] Entanglement success detection logic 50C-1 is configured to determine whether the pattern(s) of signals presented at inputs 50C-2 indicates a successful entanglement attempt. If so, entanglement success detection logic 50C-1 initiates fast trigger 32 which generates signal 33. Signal 33 is distributed to one or more select controllers via signal line 50C-3.

[0160] Entanglement success detection logic 50C-1 also triggers message transmission interface 50C-4 to generate a higher latency message 81 . In some embodiments, message transmission interface 50C-4 retrieves information that directly or indirectly identifies the quantum systems 11 involved in the successful entanglement attempt from EA data 50C-5. Message transmission interface 50C-4 formats and transmits a message 81 that includes that information. In some embodiments, message transmission interface 50C-4 retrieves a pre-prepared message body from message body(ies) 50C-6 and transmits that message body as higher latency message 81. Higher latency message 81 is output onto messaging fabric 50C-7 for delivery to select controllers (e.g. one or more optical select controllers 50F and / or one or more microwave select controllers 50G).

[0161] In some embodiments, entanglement success detection logic 50C-1 is configured to simultaneously monitor a plurality of concurrent entanglement attempts for one or more successful entanglement attempts. In some embodiments detector controller 50C includes plural entanglement success detection logic 50C-1 that are each connected to determine whether signals output from a respective one of plural detector units 12 corresponds to a heralding pattern that indicates a successful entanglement attempt.

[0162] Fig. 5B is a functional block diagram of an example select controller 50F. The arrangement of Fig. 5B is applicable to both optical select controllers 50F and microwave select controllers 50G. Select controller 50F is connected to receive signals 33 from a detector controller 50C on signaling line 50C-3 and to receive higher latency messages 81 via messaging fabric 50C-7.

[0163] Incoming signals 33 trigger timer 34 to start counting down for a set period. While timer 34 is counting down, inhibit logic 50F-1 is caused to deselect one or more quantum systems in response to a signal 50F-2 output by timer 34.

[0164] Incoming higher latency messages 81 are received via messaging fabric 50C- 7 at message receipt interface 50F-3. The timing is such that receipt of a higher latency message 81 is generally completed while timer 34 remains active (counting down).

[0165] In some embodiments, message receipt interface 50F-3 additionally passes the received higher latency message 81 to a higher level controller (e.g. higher level controller 31 B or circuit controller 50A - not shown in Fig. 5B). The higher level controller may use the higher latency messages 81 as feedback for monitoringprogress of and / or controlling a program 31 C that is being executed.

[0166] Inhibit logic 50F-1 determines whether any successful entanglement attempt corresponding to signal 33 involves one or more quantum systems 11 associated with select controller 50F.This may be done, for example, by comparing information in the higher latency message 81 to entanglement attempt data 50F-5.

[0167] In response to determining that a successful entanglement attempt corresponding to signal 33 does involve one or more quantum systems 11 associated with select controller 50F, inhibit logic 50F-1 may control an analog interface 50F-6 to impose signals on output lines 50F-7 that cause quantum systems 11 associated with select controller 50F to be deselected

[0168] In some embodiments, in response to determining that a successful entanglement attempt corresponding to signal 33 does involve one or more quantum systems 11 associated with select controller 50F, inhibit logic 50F-1 may cancel timer 34 by delivering a timer reset signal on line 50F-8. In response to cancelling timer 34 inhibit logic 50F-1 may select the quantum systems 11 associated with select controller 50F by controlling analog interface 50F-6 to impose signals on output lines 50F-7 that cause quantum systems 11 associated with select controller 50F to be selected.

[0169] In a method according to an example embodiment of the present technology, in response to receiving signals from detector(s) of a detector unit 12 indicating that an entanglement attempt succeeded, a detector controller (e.g. 50C) sends a low latency signal 33 (a hardware trigger) to plural qubit controllers (e.g. microwave select controllers 50F and optical select controllers 50G) that control whether or not entanglement attempts are performed on individual quantum systems 11 . The low latency signal starts a wait period (e.g. as determined by timers 34). Until the wait period expires, qubit controllers (e.g. optical select controllers 20F and / or microwave select controllers 20G) prevent entanglement attempts from being performed on the quantum systems 11 that they are associated with.

[0170] A higher latency message (e.g. message 81 shown in Figs. 8A and 8B) that includes information that indicates which quantum systems 11 were involved in the successful entanglement attempt or at least which group(s) those quantum systems 11 belong to is sent to the qubit controllers before the wait period expires. Each of the qubit controllers then determines from the higher latency message whether or not to configure the quantum systems that they are associated with to resume entanglementattempts. This decision may be based on: whether or not the qubit controller is associated with a quantum system 11 that is identified in the higher latency message (if not, the qubit controller configures the associated quantum systems to resume entanglement attempts - if so, the qubit controller continues to inhibit entanglement attempts on the associated quantum systems 11). In some embodiments, the qubit controllers are configured to resume entanglement attempts on the associated quantum systems if the higher latency signal is not received before the wait signal expires.

[0171] Advantageously, this method decouples the most frequent entanglement attempt outcome (failure) from the signaling latency of the control system. Consequently, successive entanglement attempts may be made at a high rate (shorter AT) while minimizing the risk of spoiling successful entanglement attempts. Further, since higher latency messages 81 are sent only in the event of a successful entanglement attempt (which is typically relatively rare), this approach can be implemented without creating excessive traffic on messaging channels.

[0172] Fig. 6 is a flowchart for a method 60 according to an example embodiment. Method 60 may, for example, be performed by apparatus as described herein, for example, apparatus 30. Block S61A executes an entanglement attempt (e.g. an entanglement attempt 20). In some embodiments, block 61 A executes plural entanglement attempts in parallel.

[0173] Block S61 B detects heralding signals. Block S61C determines whether the heralding signals indicate that the entanglement attempt succeeded. If the entanglement attempt did not succeed (NO result) then method 60 returns to block S61 A for another entanglement attempt. As long as the entanglement attempts of block S61A fail, method 60 may continuously loop around loop S62 which may result in a series of entanglement attempts separated by a time difference AT as shown, for example in Fig. 2.

[0174] If block S61C determines that the entanglement attempt did succeed (YES result) then method 60 proceeds to block S61 D which generates a low latency signal (e.g. signal 33). Block S61 E starts one or more timers (e.g. timer 34). The time elapsed between the end of blocks S61 B and S61 E is short enough that block S61 E starts the timer before a next entanglement attempt can be commenced (e.g. the time elapses between the end of block S61 B and block S61 E is less than AT). Since blocks S61 B to S61 E may be implemented by fast electronics, method 60 canaccommodate embodiments where AT is very short (e.g. 1 ps or less or significantly less than 1 ps).

[0175] In block S61 F, method 60 transmits a higher latency message that contains information that identifies the quantum systems that were successfully entangled. Block S61G takes steps to use, transfer and / or protect the successfully generated entanglement. After block S61G is completed, method 60 may return to block S61A to perform another entanglement attempt.

[0176] Fig 6A shows an example method 60A which may be executed by detector controller 50C (or any other controller that is operable to detect heralding patterns that indicate success and generate a fast trigger 32 and a higher latency message 81 in response to such detection. Where applicable, blocks of method 60A are identified with the same references as blocks of method 60 which perform the same or similar functions.

[0177] Fig. 7 is a flowchart that illustrates a method 70 according to an example embodiment. Method 70 may, for example, be performed by an optical select controller 50F or a microwave select controller 50G. Block S71 A receives a low latency signal (e.g. a signal 33 or the signal generated in step S61 D). Signal 33 indicates that an entanglement attempt has succeeded.

[0178] Block S71 B sets a timer (e.g. a timer 34). Block S71C receives a higher latency message that includes information about quantum systems were entangled by the successful entanglement attempt (e.g. the higher latency message sent in S61 F). Block S71 D processes the higher latency message to determine whether the successful entanglement attempt indicated by the low latency signal of block S71A is relevant in the context in which method 70 is being executed. For example, by processing the higher latency message, block S71 D may determine whether or not the successful entanglement attempt created entanglement of any quantum systems in a particular group of quantum systems (e.g. a group of quantum systems associated with a particular controller). If the processing of block S71 D indicates that the low latency signal is not relevant (NO result in block S71 E) then method 70 proceeds to block S71 F which cancels the timer and then returns to wait for block S71 A. If the processing of block S71 D indicates that the low latency signal is relevant (YES result in block S71 E) then method 70 proceeds to block S71 G which takes steps to use, transfer and / or preserve the entanglement created in the successful entanglement attempt.

[0179] Figs. 8A and 8B are timing diagrams that illustrate example timing of messages and steps in an example embodiment. Fig. 8A is for the case where a successful entanglement attempt is not relevant to a particular optical select controller and microwave select controller. Fig. 8B is for the case where the successful entanglement attempt is relevant to a particular optical select controller and microwave select controller.

[0180] In the examples of Figs. 8A and 8B, lower latency signal 33 and higher latency message 81 are transmitted from a detector controller 50C. In other embodiments one or both of lower latency signal 33 and higher latency message 81 are transmitted from one or more other components of a control system.

[0181] In Figs 8A and 8B, a series of entanglement attempts 20 are performed (e.g. as illustrated in Fig. 2). As long as the entanglement attempts fail (heralding patterns indicating success are not detected at detector controller 50C) entanglement attempts 20 are executed in a continuing sequence (serial entanglement attempts are executed).

[0182] When a heralding pattern that indicates success of an entanglement attempt 20 is detected (e.g. by detector controller 50C), detector controller 50C transmits low latency signal 33. Low latency signal 33 may be output by a trigger, such as fast trigger 32. Low latency signal 33 may be a point-to-multipoint signal. For example signal 33 may be delivered to each optical select controller 50F and each microwave select controller 50G of a control system 50 (see Fig. 5). In another example, signal 33 is delivered to a subset of the optical select controllers 50F, or a subset of optical controllers 50F and a subset of microwave select controllers 50G. The subset of controllers to which signal 33 is delivered may be selected based on which quantum systems 11 were involved in the successful entanglement attempt. For example, the subset may exclude some or all controllers 50F and / or 50G that are not associated with the quantum systems 11 involved in the successful entanglement attempt.

[0183] Upon receipt of low latency signal 33, each of optical select controllers 50F and microwave select controllers 50G starts a corresponding timer (e.g. a timer 34). Until the corresponding timer times out, quantum systems that are associated with the optical select controller 50F or microwave select controller 50G are deselected (i.e. so that those quantum systems 11 do not respond to the optical pulses or microwave pulses that are used to perform entanglement attempts). Deselection of the associated quantum systems may prevent any entanglement of the associatedquantum systems from being destroyed by further optical and / or microwave pulses.

[0184] Detection of a heralding pattern that indicates success of an entanglement attempt also directly or indirectly causes generation of higher latency message 81 . Higher latency message 81 has a higher latency than low latency signal 33 in the sense that lower latency signal 33 is received (e.g. by optical select controllers 50F and microwave select controllers 50G, as applicable) before reception of higher latency message 81 is completed (e.g. by optical select controllers 50F and / or microwave select controllers 50G). In some cases, higher latency messages 81 have a latency of 250 ns or more.

[0185] Higher latency messages 81 include information that at least indicates directly or indirectly whether or not any quantum system associated with the optical select controller 50F or microwave select controller 50G was involved in a successful entanglement attempt associated with low latency signal 33. In some embodiments, higher latency messages 81 have sizes on the order of tens of bytes. For example, higher latency messages 81 may comprise a header, a footer and a data payload that includes information that identifies one or more entanglement requests corresponding to successful entanglement attempt (where each entanglement request may, for example correspond to a particular pair of quantum systems 11 to be subjected to entanglement attempts) and / or identifies entanglement attempts that succeeded (e.g. by directly identifying the entangled quantum systems 11 which correspond to successful entanglement attempts).

[0186] If the information of higher latency message 81 indicates that no quantum system associated with the optical select controller 50F or microwave select controller 50G was involved in a successful entanglement attempt associated with low latency signal 33 then the timer is cancelled as shown in Fig. 8A. Cancellation of the timer causes the quantum systems associated with the optical select controller 50F or microwave select controller 50G to be selected so that those quantum systems can once again participate in entanglement attempts.

[0187] If the information of higher latency message 81 indicates that one or more quantum systems associated with the optical select controller 50F or microwave select controller 50G was involved in a successful entanglement attempt associated with low latency signal 33 then the optical select controller 50F or microwave select controller 50G holds (e.g. continues) the deselection of the associated quantum systems. This helps to preserve the entanglement of the associated quantumsystems while steps are taken to exploit the entanglement (as shown in Fig. 8B).

[0188] In some embodiments, with the possible exception of circuit controller 50A, all controllers of control system 50 are hardware-based controllers. For example, each of the controllers may comprise a control-plane component, which may, for example be based on a System-on-Chip (SoC) together a data-plane component comprising a configurable logic device such as an FPGA connected to control and to receive signals from one or more analog signaling device. The data-plane component may include several cores, each of which may connect to a signaling interface of the one or more analog signaling device. Providing each controller with multiple cores allows each controller to send / receive multiple signals concurrently. This allows each hardware controller to control multiple resource instances in parallel e.g. one optical select controller 50F may control the analog signaling device to select or deselect optical signals for many quantum systems 11 concurrently. Similarly, one detector controller 50C may use multiple cores to process detection events for many optical detectors concurrently. A control system 50 may include plural instances of any type of hardware controller, as needed. The mapping between resources (e.g. detectors, quantum system selects, optical switches etc.) and hardware controllers may be defined when a quantum circuit is compiled.

[0189] In some embodiments, each of detector controller 50C, optical select controllers 50F and microwave select controllers 50G is implemented in a configurable logic device such as a FPGA. In some embodiments, the FPGA is configured to provide multiple cores or “soft CPUs”. Detector controller 50C, optical select controllers 50F and / or microwave select controllers 50G may be implemented in whole or in part by sets of instructions executing on such soft CPUs. Each such controller may control functions of apparatus as described herein (e.g. apparatus 30) by causing analog control interfaces to apply selected electrical current or voltage signals to perform functions such as selecting or deselecting a quantum system 11 or a group of quantum systems 11 in relation to optical or microwave pulses or configuring operating modes of detector units 12. In some embodiments, the FPGA is configured to provide a separate optical select controller for each quantum system 11 that is being used in entanglement attempts 20.

[0190] The instructions that configure each soft CPU may be altered to change operation of the apparatus 30. For example, a change to a different entanglement protocol for which a different heralding pattern indicates successful entanglementmay involve changing instructions in soft CPUs that implement detector controllers to check for the heralding pattern of the new entanglement protocol.

[0191] Control system 50 may, for example comprise a platform that complies with the standards of the pTCA (Micro Telecommunications Computing Architecture). Such platforms are advantageously, scalable, modular, and standardized. Controllers of control system 50 may be hosted on pTCA-compliant advanced mezzanine cards. For example, a detector controller 50C may be hosted on a pTCA-compliant advanced mezzanine card. Low latency signals 33 and higher latency messages 81 may be carried between cards on a backplane. Lower latency signals 33 may comprise pTCA-compliant hardware triggers.

[0192] In some embodiments, quantum systems 11 are hosted on one or more chips. For example, quantum systems 11 may comprise luminescence centres (e.g. T- centres, l-centres, M-centres of G-centres) in a layer of silicon on a chip.

[0193] Quantum systems 11 may be used as qubits that have two computational basis states that may be used to represent information. For example, two spin states of the spin of an electron in the ground state of a luminescence centre such as a T centre may be used as computational states. In other examples, computational basis states may be provided by states of the spin of a nucleus in a ground state of the luminescence centre or the spin of a hole in an excited state of the luminescence centre.

[0194] The silicon may be isotopically enriched silicon-28. In some embodiments, each chip includes one or more groups of quantum systems 11 together with optical channels operable to optically connect individual quantum systems 11 to corresponding detector units. The optical channels may, for example, comprise optical waveguides that are coupled to quantum systems 11 by optical resonators formed in or on the silicon. The optical resonators may, for example, comprise photonic cavities.

[0195] As is understood by those of skill in the art, it can be necessary or desirable to maintain at least some components of apparatus and systems as described herein at cryogenic temperatures. For example, quantum systems and / or single photon detectors may be operated at cryogenic temperatures. In some embodiment, quantum systems of apparatus and systems as described herein include cryogenic refrigeration systems that are operable to cool certain components of the systems and apparatus to cryogenic temperatures (e.g. temperatures below 100 Kelvin or 3Kelvin or temperatures below 1 Kelvin). Other components of the systems and apparatus as described herein may operate acceptably at non-cryogenic temperatures. For example, in some embodiments some optical switches, analog interfaces, portions of optical paths and / or controllers are operated in non-cryogenic environments such as room temperature environments. The quantum systems of an apparatus or system as described herein may all be in the same cryogenic environment or in plural separate cryogenic environments.

[0196] Where a component (e.g. a software module, processor, assembly, device, circuit, etc.) is referred to herein, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e. , that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention.

[0197] Control systems according to the invention may be implemented using specifically designed hardware, configurable hardware, programmable data processors configured by the provision of software (which may optionally comprise “firmware”) capable of executing on the data processors, special purpose computers or data processors that are specifically programmed, configured, or constructed to perform one or more steps in a method as explained in detail herein and / or combinations of two or more of these. Examples of specifically designed hardware are: logic circuits, application-specific integrated circuits (“ASICs”), large scale integrated circuits (“LSIs”), very large scale integrated circuits (“VLSIs”), and the like. Examples of configurable hardware are: one or more programmable logic devices such as programmable array logic (“PALs”), programmable logic arrays (“PLAs”), and field programmable gate arrays (“FPGAs”). Control systems according to some embodiments of the invention are implemented using configurable hardware, specifically designed hardware or combinations thereof. For example, the controllers of control system 50 (with the possible exception of circuit controller 50A) may each be provided by configurable hardware, specifically designed hardware or combinations thereof.

[0198] Examples of programmable data processors are: microprocessors, digital signal processors (“DSPs”), embedded processors, graphics processors, math coprocessors, general purpose computers, server computers, cloud computers,mainframe computers, computer workstations, and the like. For example, one or more data processors in a circuit controller 50A or a higher level control system may implement methods as described herein by executing software instructions in a program memory accessible to the processors.

[0199] The invention may also be provided in the form of a program product. The program product may comprise any non-transitory medium which carries a set of computer-readable instructions which, when executed by a data processor, cause the data processor to execute a method of the invention and / or cause configuration of configurable hardware according to the invention. Program products according to the invention may be in any of a wide variety of forms. The program product may comprise, for example, non-transitory media such as magnetic data storage media including floppy diskettes, hard disk drives, optical data storage media including CD ROMs, DVDs, electronic data storage media including ROMs, flash RAM, EPROMs, hardwired or preprogrammed chips (e.g., EEPROM semiconductor chips), nanotechnology memory, or the like. The computer-readable signals on the program product may optionally be compressed or encrypted.Interpretation of Terms

[0200] Unless the context clearly requires otherwise, throughout the description and the claims:• “comprise”, “comprising”, and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”;• “connected”, “coupled”, or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof;• “herein”, “above”, “below”, and words of similar import, when used to describe this specification, shall refer to this specification as a whole, and not to any particular portions of this specification;• “or”, in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list;• the singular forms “a”, “an”, and “the” also include the meaning of anyappropriate plural forms. These terms (“a”, “an”, and “the”) mean one or more unless stated otherwise;• “and / or” is used to indicate one or both stated cases may occur, for example A and / or B includes both (A and B) and (A or B);• “approximately” when applied to a numerical value means the numerical value ± 10%;• where a feature is described as being “optional” or “optionally” present or described as being present “in some embodiments” it is intended that the present disclosure encompasses embodiments where that feature is present and other embodiments where that feature is not necessarily present and other embodiments where that feature is excluded. Further, where any combination of features is described in this application this statement is intended to serve as antecedent basis for the use of exclusive terminology such as "solely," "only" and the like in relation to the combination of features as well as the use of "negative" limitation(s)” to exclude the presence of other features; and• “first” and “second” are used for descriptive purposes and cannot be understood as indicating or implying relative importance or indicating the number of indicated technical features.

[0201] Words that indicate directions such as “vertical”, “transverse”, “horizontal”, “upward”, “downward”, “forward”, “backward”, “inward”, “outward”, “left”, “right”, “front”, “back”, “top”, “bottom”, “below”, “above”, “under”, and the like, used in this description and any accompanying claims (where present), depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly.

[0202] Where a range for a value is stated, the stated range includes all sub-ranges of the range. It is intended that the statement of a range supports the value being at an endpoint of the range as well as at any intervening value to the tenth of the unit of the lower limit of the range, as well as any subrange or sets of sub ranges of the range unless the context clearly dictates otherwise or any portion(s) of the stated range is specifically excluded. Where the stated range includes one or both endpoints of the range, ranges excluding either or both of those included endpoints are also included in the invention.

[0203] Certain numerical values described herein are preceded by "about". In this context, "about" provides literal support for the exact numerical value that it precedes, the exact numerical value ±5%, as well as all other numerical values that are near to or approximately equal to that numerical value. Unless otherwise indicated a particular numerical value is included in “about” a specifically recited numerical value where the particular numerical value provides the substantial equivalent of the specifically recited numerical value in the context in which the specifically recited numerical value is presented. For example, a statement that something has the numerical value of “about 10” is to be interpreted as: the set of statements:• in some embodiments the numerical value is 10;• in some embodiments the numerical value is in the range of 9.5 to 10.5; and if from the context the person of ordinary skill in the art would understand that values within a certain range are substantially equivalent to 10 because the values with the range would be understood to provide substantially the same result as the value 10 then “about 10” also includes:• in some embodiments the numerical value is in the range of C to D where C and D are respectively lower and upper endpoints of the range that encompasses all of those values that provide a substantial equivalent to the value 10.

[0204] Specific examples of systems, methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to systems other than the example systems described above. Many alterations, modifications, additions, omissions, and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by: replacing features, elements and / or acts with equivalent features, elements and / or acts; mixing and matching of features, elements and / or acts from different embodiments; combining features, elements and / or acts from embodiments as described herein with features, elements and / or acts of other technology; and / or omitting combining features, elements and / or acts from described embodiments.

[0205] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any other described embodiment(s) without departing from the scope ofthe present invention.

[0206] Any aspects described above in reference to apparatus may also apply to methods and vice versa.

[0207] Any recited method can be carried out in the order of events recited or in any other order which is logically possible. For example, while processes or blocks are presented in a given order, alternative examples may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and / or modified to provide alternative or subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, simultaneously or at different times.

[0208] Various features are described herein as being present in “some embodiments”. Such features are not mandatory and may not be present in all embodiments. Embodiments of the invention may include zero, any one or any combination of two or more of such features. All possible combinations of such features are contemplated by this disclosure even where such features are shown in different drawings and / or described in different sections or paragraphs. This is limited only to the extent that certain ones of such features are incompatible with other ones of such features in the sense that it would be impossible for a person of ordinary skill in the art to construct a practical embodiment that combines such incompatible features. Consequently, the description that “some embodiments” possess feature A and “some embodiments” possess feature B should be interpreted as an express indication that the inventors also contemplate embodiments which combine features A and B (unless the description states otherwise or features A and B are fundamentally incompatible). This is the case even if features A and B are illustrated in different drawings and / or mentioned in different paragraphs, sections or sentences.

[0209] It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, omissions, and sub-combinations as may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

WHAT IS CLAIMED IS:1 . Control apparatus for use in a system for generating quantum entanglement, the control apparatus comprising entanglement success detection logic configured to: monitor output signals from one or more detectors for a heralding pattern indicating a successful entanglement attempt, wherein a successful entanglement attempt is an entanglement attempt that places a pair of quantum systems in an entangled quantum state; and in response to detecting a heralding pattern that indicates a successful entanglement attempt: cause a low latency signal to be output on one or more signal lines; and cause a higher latency message to be transmitted by a messaging interface, the higher latency message containing information that directly or indirectly identifies the pair of quantum systems entangled by the successful entanglement attempt.

2. The control apparatus according to claim 1 wherein the control apparatus is configured to simultaneously monitor a plurality of concurrent entanglement attempts for one or more successful entanglement attempts.

3. The control apparatus according to claim 2, wherein the control apparatus is configured to monitor a series of the plurality of concurrent entanglement attempts for the one or more successful entanglement attempts.

4. The control apparatus according to any of claims 1 to 3 wherein the signal line is connected to deliver the low latency signal to trigger at least one timer, an output of the at least one timer being connected to inhibit logic operative to deselect one or more quantum systems associated with the timer from participating in further entanglement attempts when the timer is active.

5. The control apparatus according to claim 4 wherein the timer is associated with a plurality of quantum systems and the inhibit logic is configured to cause each of the plurality of quantum systems to be inhibited from participating in further entanglement attempts while the timer is active.

6. The control apparatus according to claim 5 wherein the at least one timer comprises a plurality of timers, each of the plurality of timers having a respective output connected to a respective one of a plurality of inhibit logic, the respective inhibit logic operative to deselect a respective plurality of the quantum systems associated with the respective timer from participating in further entanglement attempts while the respective timer is active.

7. The control apparatus according to claim 6 wherein each of the plurality of inhibit logic is configured to cancel the respective timer if the information in the higher latency message that identifies the pair of quantum systems entangled by the successful entanglement attempt does not include any of the quantum systems associated with the respective timer.

8. The control apparatus according to any of the preceding claims wherein the low latency signal comprises a hardware trigger.

9. The control apparatus according to any of the preceding claims wherein the low latency signal is generated within 100 ns of the successful entanglement attempt.

10. The control apparatus according to any of the preceding claims wherein the control apparatus is configured to coordinate concurrently, for each of a plurality of pairs of quantum systems, serially executing entanglement attempts on each of the pairs of quantum systems using successive iterations of a heralded entanglement protocol.11 . The control apparatus according to claim 10 wherein the heralded entanglement protocol comprises initializing a state of each of the quantum systems in each iteration of the heralded entanglement protocol.

12. The control apparatus according to claim 10 or claim 11 wherein the heralded entanglement protocol is the BK protocol.

13. The control apparatus according to any of claims 10 to 12 wherein theheralded entanglement protocol comprises delivering optical pulses and / or microwave pulses to the quantum systems and the control apparatus is operative to selectively engage and disengage the quantum systems from the optical pulses and / or the microwave pulses.

14. The control apparatus according to claim 13 wherein the control apparatus comprises a plurality of analog interfaces connected to engage and disengage the quantum systems from the optical pulses and / or the microwave pulses by altering magnetic fields and / or electric fields at the quantum systems.

15. The control apparatus according to claim 13 or 14 wherein the quantum systems are divided into plural groups and the control apparatus is operable to engage and disengage the quantum systems of any one of the groups independently of the quantum systems belonging to other ones of the groups.

16. The control apparatus according to claim 15 comprising a timer associated with each of the groups, the timer connected to be triggered to be active for a period of time by the low latency signal wherein, the quantum systems of the group are disengaged from the optical pulses and / or the microwave pulses while the timer associated with the group is active.

17. The control apparatus according to claim 16 wherein the set period is in the range of about 150 ns to about 1 .5 ps.

18. The control apparatus according to any of the preceding claims wherein the entanglement success detection logic is connected to receive signals output by a plurality of single photon detectors and the monitoring comprises the entanglement success detection logic monitoring the output signals from the single photon detectors for the heralding pattern that indicates success of the entanglement attempt.

19. The control apparatus according to claim 18 wherein the single photon detectors are incorporated into a plurality of Bell state analyzers (BSAs) and the entanglement success detection logic is configured to monitor outputs of the single photon detectors of each of the BSAs for the heralding pattern.

20. The control apparatus according to claim 19 wherein the entanglement success detection logic comprises a separate logic unit associated with each of the BSAs, the separate logic units each configured to detect, in the output signals of the single photon detectors of the corresponding BSA, the heralding pattern that indicates success.21 . The control apparatus according to claim 20 wherein each of the logic units comprises a corresponding core configured in a configurable logic device.

22. A method for controlling a system for generating quantum entanglement, the method comprising: monitoring for successful entanglement attempts, wherein a successful entanglement attempt is an entanglement attempt that places a pair of quantum systems in an entangled quantum state; and in response to detecting a successful entanglement attempt: causing a low latency signal to be generated; and causing a higher latency message to be transmitted, the higher latency message containing information that directly or indirectly identifies the pair of quantum systems entangled by the successful entanglement attempt.

23. The method according to claim 22 wherein the low latency signal comprises a hardware trigger.

24. The method according to claim 22 wherein the low latency signal is generated within 100 ns of the successful entanglement attempt being heralded.

25. The method according to any of claims 22 to 24 comprising, concurrently, for each of a plurality of pairs of quantum systems, serially executing entanglement attempts on each of the pairs of quantum systems using successive iterations of a heralded entanglement protocol.

26. The method according to claim 25 wherein executing the heralded entanglement protocol comprises initializing a state of each of the quantum systemsin each iteration of the heralded entanglement protocol.

27. The method according to claim 25 or 26 wherein the heralded entanglement protocol is the BK protocol.

28. The method according to any of claims 25 to 27 wherein the successive entanglement attempts are separated by a time AT, wherein AT is 1 s or less.

29. The method according to any of claims 25 to 27 wherein the heralded entanglement protocol comprises delivering optical pulses and / or microwave pulses to the quantum systems and the method comprises selectively engaging and disengaging the quantum systems from the optical pulses and / or the microwave pulses.

30. The method according to claim 29 wherein disengaging the quantum systems from the optical pulses and / or the microwave pulses comprises altering magnetic fields and / or electric fields at the quantum systems.31 . The method according to claim 29 or claim 30 wherein the quantum systems are divided into plural groups and selectively engaging and disengaging the quantum systems from the optical pulses and / or microwave pulses comprises: engaging and disengaging the quantum systems of one of the groups independently of the quantum systems belonging to other ones of the groups.

32. The method according to claim 31 comprising, in response to the low latency signal, triggering a plurality of timers to be active for a period of time, each of the groups being associated with a corresponding one of the timers, the method comprising, for each of the groups, disengaging the quantum systems of the group from the optical pulses and / or the microwave pulses while the corresponding timer associated with the group is active.

33. The method according to claim 32 wherein the period of time is in the range of about 150 ns to about 1 .5 ps.

34. The method according to any of claims 25 to 30 comprising, in response to the low latency signal, disengaging the plurality of pairs of quantum systems from participating in further entanglement attempts for a time period.

35. The method according to claim 34, comprising within the time period, for one or more of the pairs of quantum systems for which the entanglement attempt succeeded, using the entanglement of the pair of quantum systems; transferring the entanglement of the pair of quantum systems; or protecting the entanglement of the pair of quantum systems.

36. The method according to claim 34 or claim 35, wherein each of the entanglement attempts comprises a sequence of optical pulses being delivered to the quantum systems of one of the pairs of quantum systems while an optical transition of each of the quantum systems is resonant with the optical pulses.

37. The method according to claim 36 wherein the optical pulses are delivered simultaneously to the quantum systems of the pairs of quantum systems.

38. The method according to claim 36 or 37, wherein the optical pulses are delivered in a continuous stream in which the sequence of optical pulses repeats with a period that is not more than 15% longer than the sequence of optical pulses.

39. The method according to any of claims 36 to 38 wherein disengaging the plurality of pairs of quantum systems from participating in further entanglement attempts for a time period comprises altering the optical transition of the quantum systems of the plurality of pairs of quantum systems so that the optical transition is not resonant with the optical pulses.

40. The method according to claim 39 wherein the higher latency message is transmitted to one or more controllers and the method comprises, by the one or more controllers, processing the higher latency message, and based on the processing selectively altering the transitions of some of the quantum systems other than the quantum systems of the one or more pairs of quantum systems for which theentanglement attempt succeeded to be resonant with the optical pulses.41 . The method according to claim 40 wherein the latency between occurrence of a heralding pattern that indicates success and completion of reception of the higher latency message at the one or more controllers is longer than a time AT between successive entanglement attempts.

42. The method according to claim 40 or 41 wherein the quantum systems each belong to one of a plurality of groups of the quantum systems, wherein the one or more controllers comprises a plurality of controllers, each of the plurality of controllers being associated with one of the groups of quantum systems and operable to control the transitions of the quantum systems belonging to the associated group of quantum systems, and wherein the method comprises: processing the higher latency message at each of the plurality of controllers, based on the processing determining whether the associated group of quantum systems includes any of the quantum systems of the pairs of quantum systems for which the entanglement attempt succeeded, and if the associated group of quantum systems does not include any of the quantum systems of the pairs of quantum systems for which the entanglement attempt succeeded, engaging the associated group of quantum systems to participate in further entanglement attempts.

43. The method according to any of claims 39 to 42 wherein the entanglement protocol includes delivering one or more microwave pulses to the quantum systems while a microwave transition of each of the quantum systems is resonant with the one or more microwave pulses.

44. The method according to claim 43 wherein inhibiting the plurality of pairs of quantum systems from participating in further entanglement attempts for a time period comprises altering the microwave transition of the quantum systems of the plurality of pairs of quantum systems so that the microwave transition is not resonant with the one or more microwave pulse.

45. The method according to any of claims 39 to 42 wherein altering the opticaltransition of the quantum systems of the plurality of pairs of quantum systems comprises varying electric fields at locations of the quantum systems.

46. The method according to any of claims 39 to 42 wherein altering the optical transition of the quantum systems of the plurality of pairs of quantum systems comprises varying strain in a crystalline lattice at locations of the quantum systems.

47. The method according to any of claims 22 to 46 wherein the probability that any one of the entanglement attempts will result in success does not exceed 5%.

48. The method according to any of claims 22 to 41 wherein the monitoring comprises monitoring the output signals from plural single photon detectors for a heralding pattern that indicates a successful entanglement attempt.

49. The method according to claim 48 wherein the single photon detectors are incorporated into a plurality of Bell state analyzers (BSAs) and the method comprises making Bell state measurements (“BSMs”) using the BSAs.

50. The method according to claim 49 comprising monitoring outputs of each of the BSAs for a successful entanglement attempt using a distinct logic unit associated with the BSA.51 . The method according to claim 22 comprising simultaneously monitoring a plurality of concurrent entanglement attempts for one or more successful entanglement attempts.

52. The method according to any of claims 22 to 24 or 51 comprising, in response to the low latency signal, deselecting one or more quantum systems from participating in further entanglement attempts for a period of time.

53. The method according to any of claims 22 to 24 or 51 comprising, in response to the low latency signal, deselecting a plurality of the quantum systems from participating in further entanglement attempts for a period of time.

54. The method according to claim 53 comprising engaging the plurality of the quantum systems prior to the end of the period of time in response to determining that the pair of quantum systems identified in the higher latency message as being entangled by the successful entanglement attempt does not include any of the plurality of the quantum systems.

55. Apparatus for generating quantum entanglement, the apparatus comprising: a plurality of quantum systems; a plurality of detector units, each of the detector units comprising a Bell state analyzer having first and second optical inputs optically coupled to receive photons from a pair of the quantum systems that are respectively optically coupled to the first and second inputs; one or more control units operable to deliver control signals to the quantum systems according to an entanglement protocol; one or more inhibit mechanisms operable to engage or disengage the quantum systems from the control signals; and a control system configured to: operate the control units to serially execute entanglement attempts on the pairs of quantum systems using successive iterations of a heralded entanglement protocol; monitor outputs of the Bell state analyzers for success of each of the entanglement attempts, wherein success comprises placing the corresponding pair of quantum systems in an entangled quantum state; in response to detecting success of any of the entanglement attempts, generate a low latency signal and a higher latency message; in response to the low latency signal, operate the one or more inhibit mechanisms to disengage the plurality of pairs of quantum systems from participating in further entanglement attempts for a time period; and during the time period receive and process the higher latency message.

56. The apparatus according to claim 55 comprising an optical switching network operable to optically couple to the first and second optical inputs of each of the Bell state analyzers a pair of the quantum systems.

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