Quantum measurement methods and systems

Abstract

Methods and apparatus for measuring quantum systems involve: applying first optical pulses to the quantum system and monitoring for a first resulting photon after each first optical pulse; in response to detecting one or more first resulting photons, performing a π operation on the quantum system; and, after performing the π operation, applying second optical pulses to the quantum system and monitoring for a second resulting photon after each second optical pulse. The first and second optical pulses comprise a wavelength corresponding to a transition from a first basis state of the quantum system to an excited state. A measurement result is determined based on results of the monitoring.

Description

Cross-Reference to Related Application

[0001] This application claims priority from US application No. 63 / 623613 filed 22 January 2024 and entitled QUANTUM MEASUREMENT METHODS AND SYSTEMS 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 / 623613 filed 22 January 2024 and entitled QUANTUM MEASUREMENT METHODS AND SYSTEMS which is hereby incorporated herein by reference for all purposes. Field

[0002] This invention relates to making measurements on quantum systems. The invention has example applications in quantum informatics processing. Background

[0003] In quantum informatics processing it is typical to associate different quantum states of a physical quantum system with different values. For example, the quantum system may comprise a spin % particle, e.g. an electron, which has two basis spin states which may be represented as |T) and |i) (spin up and spin down respectively). For example, the spin up state may be associated with the value “1” or “TRUE” and the spin down state may be associated with the value “0” or “FALSE” or vice versa. At a given time, the quantum system may be in either one of these basis spin states or in a quantum superposition of these basis spin states.

[0004] In quantum informatics processing it may be necessary to make a measurement on the quantum system. There is a need for fast, high fidelity measurement protocols that have minimal impact on the coherence of quantum states of neighbouring quantum systems in which information is stored.

[0005] In an ideal example case, one could make the measurement by applying a stimulus to the quantum system. The stimulus may be chosen such that a response to the stimulus depends on the quantum state of the quantum system. For example, applying the stimulus may result in the response of a photon being emitted if the quantum system is in the state |T) and the response that no photon being emitted if the quantum system is in the state |i) (or vice versa). In this ideal example one could obtain a reliable measurement of the state of the quantum system as it was immediately prior to application of the stimulus based on whether or not application of the stimulus resulted in detection of a photon or did not result in detection of a photon.

[0006] In the real world, the measurement approach given in the above example is unreliable because a photon may not be detected (e.g. because of photon losses or inefficiency of a photon detector) even when the quantum system is in the state |T) and a photon may be detected (e.g. a “dark count” or a false detection caused by noise) even when the quantum system is in the state |i).

[0007] In some cases it can be possible to increase the reliability of a real world measurement by repeating a measurement multiple times and determining measurement results by combining results of the individual measurements. This approach can work but is associated with problems that include: measurements taking undesirably long times; measurements taking an unpredictable amount of time; and measurements causing undesirably large degradation of the fidelity of quantum states of other quantum systems that are local to or coupled to the quantum system on which the measurements are performed.

[0008] There is a need for new methods and apparatus for making reliable measurements on quantum systems. Summary

[0009] This invention has a number of aspects. These include, without limitation: methods for measuring quantum systems, apparatus for measuring quantum systems, control systems for use in apparatus for measuring quantum systems, tangible computer readable media storing machine executable instructions that, when executed by a data processor, cause the data processor to perform or coordinate performance of a method for measuring quantum systems. The methods and apparatus may, for example, be applied to measure a spin state of a quantum spin.

[0010] One aspect of the invention provides methods for performing measurements on quantum systems having quantum basis states |0) and |1). The methods comprise: applying one or more first optical pulses to a quantum system and performing first monitoring for a first resulting photon after each of the one or more first optical pulses; in response to the first monitoring detecting one or more first resulting photons, performing a it operation on the quantum system; after performing the it operation, applying one or more second optical pulses to the quantum system; and performing second monitoring for a second resulting photon after each of the one or more second optical pulses. The it operation transforms the basis states according to: |0) i—> |1) and |1) >—> |0). Each of the one or more first optical pulses and each of the one or more second optical pulses comprises a wavelength corresponding to a transition from a first one of the basis states of the quantum system to a higher energy state. The methods determine a measurement result based on results of the first monitoring or if the second monitoring was performed, based on results of the first monitoring and the second monitoring. The measurement result indicating whether the quantum spin was in the first one of the basis states or a second one of the basis states.

[0011] In some embodiments the quantum system comprises a quantum spin and the quantum basis states |0) and |1) correspond to spin states of the quantum spin. In some embodiments performing the it operation comprises flipping the quantum spin. In some embodiments the transition is a spin preserving transition.

[0012] In some embodiments performing the it operation comprises applying a microwave or radio frequency pulse to the quantum system. In some embodiments the microwave pulse is a two-colour it pulse.

[0013] In some embodiments if the first monitoring fails to detect any first resulting photons after a predetermined number of the first optical pulses, determining the measurement result comprises setting the measurement result to indicate that the quantum spin was in the second basis state.

[0014] In some embodiments the methods comprise performing the it operation in response to the first monitoring having detected two or more first resulting photons.

[0015] In some embodiments the methods comprise determining a number of the second optical pulses to apply to the quantum system based on a number of the first optical pulses applied to the quantum system prior to performing the it operation on the quantum system.

[0016] In some embodiments the methods comprise ceasing to apply the second optical pulses to the quantum system in response to the sum of the number of first optical pulses applied to the quantum system and the number of second optical pulses applied to the quantum system reaching a predetermined number. In some embodiments the methods comprise the predetermined number is in the range of 5 to 100.

[0017] In some embodiments the quantum system is an electron spin. In some embodiments the methods comprise the electron spin is an electron spin of a luminescent centre in a crystalline substance. In some embodiments, the luminescent centre is a radiation damage centre. In some embodiments the luminescent centre is a T-centre or is selected from the group consisting of a T-centre, an l-centre, an M-centre and a G-centre.

[0018] In some embodiments, if the first monitoring detects one or more first resulting photons and the second monitoring detects one or more second resulting photons, the method comprises indicating that the measurement result is unreliable.

[0019] In some embodiments the methods comprise, prior to applying the one or more first optical pulses to the quantum system, controlling the quantum system to reduce a lifetime of the excited state.

[0020] In some embodiments, controlling the quantum system to reduce the lifetime of the excited state comprises performing one or more of the following: controlling a resonance of an optical resonator that is optically coupled to the quantum system to match a wavelength corresponding to the transition; and controlling the wavelength corresponding to the transition to match the resonance of the optical resonatorthat is optically coupled to the quantum system.

[0021] In some embodiments, the methods comprise evaluating one or more criteria based on results of the first monitoring and determining whether to proceed to perform the it operation on the quantum system or to proceed to determine the measurement result without performing the it operation based on the criteria. In some embodiments, the criteria comprise a maximum number of the first optical pulses to be applied. In some embodiments, the maximum number of the first optical pulses to be applied to the quantum system is based upon a probability that the first monitoring will detect the first resulting photon if the quantum system is in the first basis state when the first optical pulse is applied to the quantum system.

[0022] In some embodiments, the methods comprise determining a level of confidence of the measurement result.

[0023] In some embodiments, the quantum system is coupled to one or more additional quantum systems that have quantum states in which information has been stored.

[0024] In some embodiments, the quantum system is a broker quantum system and the one or more additional quantum systems comprise a client quantum system.

[0025] Another aspect of the invention provides a control system configured to perform the method described above. Another aspect of the invention provides an apparatus comprising the control system and the quantum system.

[0026] Another aspect of the invention provides apparatus for making a measurement on a quantum system having quantum basis states |0) and |1). The apparatus comprises a control system configured to perform a measurement on the quantum system by: causing an optical pulse generator to apply one or more first optical pulses to the quantum system; receiving results of first monitoring fora first resulting photon after each of the one or more first optical pulse; in response to the results of the first monitoring indicating detection of one or more first resulting photons, causing a it operation to be applied to the quantum system. The it operation transforms the basis states according to: |0) >—> |1) and |1) >—> |0). The control system is configured to, after the it operation has been applied to the quantum system, cause the optical pulse generator to apply one or more second optical pulses to the quantum system and, receive results of second monitoring for a second resulting photon after each of the one or more second optical pulses. In some embodiments the control system is configured to cause application of the it operation to the quantum system by controlling a microwave generator to generate microwave energy. The microwave energy may comprise one or more wavelengths corresponding to one or more microwave transitions of the quantum system. In some embodiments, the microwave energy comprises a two-colour it pulse. In some embodiments, the apparatus further comprises the microwave generator.

[0027] In some embodiments, the control system is further configured to determine a measurement result based on results of the first monitoring or, if the second monitoring was performed, based on results of the first monitoring and the second monitoring, the measurement result indicating whether the quantum system was in a first one of the basis states or a second one of the basis states.

[0028] In some embodiments, the quantum system comprises a quantum spin, the quantum basis states |0) and |1) correspond to spin states of the quantum spin and wherein each of the one or more first optical pulses and each of the one or more second optical pulses comprises a wavelength corresponding to a transition from a first one of the basis states of the quantum system to a higher energy state.

[0029] In some embodiments, performing the it operation comprises flipping the quantum spin. In some embodiments, wherein the transition is a spin preserving transition.

[0030] In some embodiments, the control system is configured to, if the first monitoring fails to detect any first resulting photons after a predetermined number of the first optical pulses, set the measurement result to indicate that the quantum spin was in the second basis state.

[0031] In some embodiments, the control system is configured to cause the microwave generator to generate microwave energy to apply the it operation to the quantum system in response to the first monitoring having detected two or more first resulting photons.

[0032] In some embodiments, the control system is configured to determine a number of the second optical pulses to apply to the quantum system based on a number of the first optical pulses applied to the quantum system prior to performing the it operation on the quantum system.

[0033] In some embodiments, the control system is configured to cause the application of the second optical pulses to the quantum system to cease in response to the sum of the number of first optical pulses applied to the quantum system and the number of second optical pulses applied to the quantum system reaching a predetermined number. In some embodiments, the predetermined number is in the range of 5 to 100.

[0034] In some embodiments, the apparatus comprises the quantum system. In some embodiments, the quantum system is an electron spin. In some embodiments, the electron spin is an electron spin of a luminescent centre in a crystalline substance.

[0035] In some embodiments, the luminescent centre is a radiation damage centre. In some embodiments, the luminescent centre is a T-centre. In some embodiments, the luminescent centre is selected from the group consisting of a T-centre, an l-centre, an M-centre and a G-centre. In some embodiments, the control system is configured to indicate that the measurement result is unreliable in response to the first monitoring detecting one or more first resulting photons and the second monitoring detecting one or more second resulting photons.

[0036] In some embodiments, the control system is configured to, prior to causing application of the one or more first optical pulses to the quantum system, control the quantum system to reduce a lifetime of the excited state. In some embodiments, controlling the quantum system to reduce the lifetime of the excited state comprises performing one or more of the following: controlling a resonance of an optical resonator that is optically coupled to the quantum system to match a wavelength corresponding to the transition; and controlling the wavelength corresponding to the transition to match the resonance of the optical resonatorthat is optically coupled to the quantum system.

[0037] In some embodiments, the control system is configured to evaluate one or more criteria based on results of the first monitoring and to determine whether to proceed to perform the it operation on the quantum system or to proceed to determine the measurement result without performing the it operation based on the evaluation of the one or more criteria. In some embodiments, the criteria comprise a maximum number of the first optical pulses to be applied to the quantum system. In some embodiments, the maximum number of the first optical pulses to be applied the quantum system is based upon a probability that the first monitoring will detect the first resulting photon if the quantum system is in the first basis state when the first optical pulse is applied to the quantum system.

[0038] In some embodiments, the control system is configured to determine a level of confidence of the measurement result.

[0039] In some embodiments, the quantum system is coupled to one or more additional quantum systems that have quantum states in which information has been stored. The coupling may, for example, comprise hyperfine coupling.

[0040] In some embodiments, the apparatus further comprises the photon detector.

[0041] In some embodiments, the apparatus further comprises the optical pulse generator.

[0042] In some embodiments, the quantum system is a broker quantum system and the one or more additional quantum systems comprise a client quantum system.

[0043] Another aspect of the invention provides methods comprising any new and inventive steps, acts, combination of steps or acts or sub-combination of steps and / or acts as described herein.

[0044] Another aspect of the invention provides apparatus comprising any new and inventive feature, combination of features or sub-combination of features as described herein.

[0045] Further aspects and example embodiments are illustrated in the accompanying drawings and / or described in the following description.

[0046] It is emphasized that the invention relates to all combinations of the above features, even if these are recited in different claims, illustrated in different drawings and / or described in different paragraphs or sections. Brief Description of the Drawings

[0047] The accompanying drawings illustrate non-limiting example embodiments of the invention.

[0048] Fig. 1 is a flowchart illustrating a method for measuring a quantum system according to an example embodiment.

[0049] Fig. 1A is a schematic energy level diagram for an example class of quantum systems on which the method of Fig. 1 may be performed.

[0050] Fig. 1B is a block diagram that schematically illustrates an apparatus for measuring quantum systems according to an example embodiment.

[0051] Fig. 2 is an energy level diagram for a T-centre.

[0052] Fig. 3 illustrates the structure of a T-centre. Detailed Description

[0053] 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.

[0054] Fig. 1 is a flowchart that illustrates a method 10 for making a measurement on a quantum system according to an example embodiment. Fig. 1A is a schematic energy level diagram for a quantum system of a type on which measurements may be performed using method 10. Fig. 1B is a schematic block diagram illustrating apparatus 16 that may be used to perform method 10.

[0055] Method 10 applies optical pulses to a quantum system. The optical pulses have the property that, depending on the quantum state of the quantum system at the start of method 10, application of an optical pulse will either cause emission of a photon or not cause emission of a photon.

[0056] The term “optical” is used herein to describe any wavelengths within the range of infrared to ultraviolet (e.g. light having wavelengths in the range of about 100 nm to about 1000 pm). Optical wavelengths include wavelengths within the optical telecommunication bands (O, E, S, C, L and U bands) which collectively span about 1260 nm to 1675 nm.

[0057] The quantum system is a matter-based quantum system (as opposed to a photon). The quantum system may be of any type that has quantum states and a transition as generally illustrated in Fig. 1A. For example, the quantum system may comprise an elementary particle such as an electron, a composite particle such as an atomic nucleus or an ion, a quasi-particle such as a hole, or a matter based quantum system of another type e.g. a quantum dot. In some embodiments, the quantum system is disposed in a solid matrix (e.g. a crystalline substance), for example, silicon, silicon carbide, diamond or the like. In some such embodiments the quantum system may, for example, be associated with an impurity atom, a defect and / or a luminescent centre in the solid material. Examples of defects and luminescent centres include T-centres, l-centres, M-centres and G-centres in silicon and NV centres in diamond. In some embodiments the quantum system comprises a trapped ion. The trapped ion may, for example comprise Ytterbium (i?iYb+) or another type of trapped ion or trapped atom, which has spin states that exhibit state-selective fluorescence.

[0058] The quantum system has a quantum state that can be represented as one of or a combination of plural basis states. The basis states may be referred to as “computational states”. The basis states may be denoted generally, for example, by |0) and |1). The generalized basis states correspond to physical states of the quantum system. Different types of quantum system (e.g. quantum spins, trapped ions, trapped neutral atoms, quantum dots, impurity atoms or ions, superconducting circuits etc.) have different physical states that may be used as quantum basis states. For example where the quantum system comprises an intrinsic spin the generalized basis states may respectively correspond to the spin orientation being spin up and spin down, or vice versa. Other examples of physical states that may correspond to basis states in certain types of quantum systems include: electron number (e.g. |0) may correspond to no electron present in or at a structure, orbital or level, and |1) may correspond to one electron present in or at the structure, orbital or level or vice versa), charge (e.g. |1) may correspond to an extra Cooper pair being present in a superconducting charge qubit and |0) may correspond to the extra Cooper pair not being present or vice versa), current direction (e.g. |1) may correspond to current flowing in one direction around a current loop and |0) may correspond to current flowing in the opposite direction around the current loop).

[0059] An operation that when applied to a quantum system maps the basis states as follows: |0) i—> |1) and 11) i—* |0) may be called a “it operation” or “pi operation” since, when the quantum state of the quantum system is represented by a vector direction in the Bloch sphere, the it operation rotates the vector direction by n radians.

[0060] Implementation of a it operation on a specific quantum system can depend on the nature of the physical states of the quantum system that correspond to the basis states |0) and |1) as well as how each of the basis states are mapped to those physical states. For example, in the case where the basis states correspond to spin states of a quantum system made up of a quantum spin, the correspondence between basis states and physical states of the quantum spin could be: |0) = |T) and |1) = |i) (or vice versa). Where the quantum system is a quantum spin (also where the quantum system is of certain other types) the it operation may be performed by applying an electromagnetic it pulse to the quantum system. For some quantum systems the electromagnetic it pulse can be provided by a microwave pulse (which may be delivered as a single pulse or by a series of microwave pulses that collectively operate as a it pulse). In some embodiments, the it operation on a quantum system is implemented by application of an X gate or a Y gate to the quantum system.

[0061] In block S11A, method 10 applies a first optical pulse to the quantum system. Block S11B performs first monitoring for a first photon resulting from the application of the first optical pulse of block S11 A. Block S11B may perform the first monitoring during a time window. The time window of block 11B does not need to commence immediately after the end of the first optical pulse of block 11 A. In some embodiments the time window commences after a delay sufficient to configure apparatus to detect the photon. Depending on the construction of the apparatus it may be necessary or desirable to deactivate or isolate a photon detector during the application of the first optical pulse and to reactivate and / or reconnect the photon detector after the first pulse has ended. Start and end times for the time window may be selected so that the time window includes times at which emission of the resulting photon is most probable.

[0062] Block S11C applies one or more criteria to determine whether to proceed to block S11E or to deliver more first optical pulses to the quantum system. If the criteria are satisfied (“YES” result at block S11C) method 10 proceeds to block S11E. Otherwise (“NO” result at block S11C), method 10 returns to block S11A via loop S11D.

[0063] Block S11E applies one or more criteria to determine whether to proceed to block S12A or to skip blocks S12Ato S12D and proceed to block S14. If the criteria are satisfied (“YES” result at block S11E) method 10 continues to block S12A. Otherwise (“”NO” result at block S11E), method 10 proceeds to block S14.

[0064] Block S12A performs a it operation on the quantum system (e.g. if the quantum system is in basis state |0) at the start of block S12A the operation of block S12A causes the quantum system to be in basis state |1) at the end of block S12A).

[0065] Block S12B applies a second optical pulse to the quantum system. The second optical pulse can be the same as or functionally equivalent to the first optical pulse of block S11A.

[0066] Block S12C performs second monitoring for a second photon resulting from the application of the second optical pulse of block S12B during a time window following block S12B. The time window of block 12C does not necessarily commence immediately after the end of the second optical pulse. In some embodiments the time window commences after a delay sufficient to configure apparatus to detect the photon. Depending on the construction of the apparatus it may be desirable to deactivate or isolate a photon detector during the application of the second optical pulse and to reactivate and / or reconnect the photon detector after the second pulse has ended. Start and end times for the time window may be selected so that the time window includes times at which emission of the resulting second photon is most probable.

[0067] Block S12D applies one or more criteria to determine whether to proceed to block S14 or whether more second optical pulses should be delivered to the quantum system. If the criteria are satisfied (“YES” result at block S12D) method 10 proceeds to block S14. Otherwise (“NO” result at block S12D) method 10 returns to block S12B via loop S12E.

[0068] Block S14 determines a measurement result for the quantum system based on results of the first monitoring in block S11B and, if applicable, the results of the second monitoring in block S12C. The measurement result is output to a control system. The control system may, for example, use the measurement result: as a basis for making one or more decisions in executing a quantum informatics program; to support a process such as quantum teleportation; as part of a quantum error correcting protocol; as a basis for determining a result of a computation; and / or in other ways to facilitate quantum informatics processing.

[0069] Method 10 may be applied, for example, to a quantum system 17 (see e.g. Fig. 1B) which has an energy level structure according to the simplified energy level diagram shown in Fig. 1A. Quantum system 17 includes a ground state 12 which is split into non-degenerate states 12A and 12B. Basis states |0) and |1) may, for example, correspond to ground state 12A and 12B respectively (or vice versa).

[0070] Quantum system 17 has an excited state 14. Excited state 14 may be split into plural energy levels (not shown). Quantum system 17 may be excited to excited state 14 from state 12A via transition 15A. A quantum system having an energy level structure that includes the features illustrated in Fig. 1B can exhibit state-selective fluorescence.

[0071] Transition 15A may be caused to occur, for example, by delivering an optical pulse to quantum system 17 where the optical pulse comprises a wavelength corresponding (by way of the Planck equation - E = hv where E is the energy difference of transition 15A, h is Planck’s constant and v is the frequency of the optical pulse) to the difference E between the energy of quantum system 17 when in state 12A and the energy of quantum system 17 when in excited state 14 immediately following transition 15A. The duration and intensity of the optical pulse may be chosen as known in the art to make it sufficiently likely that application of the pulse will cause quantum system 17 to undergo transition 15A.

[0072] In some embodiments transition 15A corresponds to an energy difference E that corresponds to an optical wavelengths lying within an optical telecommunication band. This can be advantageous in light of the wide range of optical components (optical fibers, optical switches, etc.) that are engineered for wavelengths in the optical telecommunication bands and are readily commercially available.

[0073] A transition 15B from state 12B to excited state 14 may also exist but is not required. Where transition 15B is available, transitions 15A and 15B require different amounts of energy (corresponding to different optical wavelengths). Wavelengths of optical pulses applied to quantum system 17 may be selected to cause transition 15A but not transition 15B (or vice versa).

[0074] One example of a quantum system that can have energy levels and transitions as shown in Fig. 1A is a quantum spin, for example an unpaired electron. In some embodiments, the quantum spin comprises a spin % particle. In this case, states 12A and 12B may correspond to the quantum system being spin down and spin up respectively. Where quantum system 17 comprises a quantum spin, it is advantageous that the transition being used in method 10 (e.g. transition 15A or 15B) is a spin-preserving transition. A spin-preserving transition is beneficial because spinpreserving transitions tend to have a higher probability of occurring than crosstransitions which do not preserve spin. Also, where the transition used is spinpreserving there is generally a higher probability that the excited state will decay by way of the transition to the same spin state that the quantum system was in prior to the optical pulse of block S11A or S12B.

[0075] Once elevated to excited state 14 via transition 15A, quantum system 17 will typically decay back to state 12A, emitting a photon in the process.

[0076] Therefore, if an optical pulse corresponding to transition 15A is delivered to quantum system 17 when quantum system 17 is in state 12A then it is expected that a photon having a wavelength corresponding to transition 15A will be emitted within a time window following application of the optical pulse and that after emission of the photon quantum system 17 will be again in state 12A. The length of the time window depends on the lifetime of excited state 14 of quantum system 17 which, in turn, depends on characteristics of quantum system 17 and its environment.

[0077] If an optical pulse corresponding to transition 15A is delivered to quantum system 17 when quantum system 17 is in state 12B, then it is expected that no photon corresponding to transition 15A will be emitted within the time window following application of the optical pulse and that after emission of the photon quantum system 17 will remain in state 12B.

[0078] When method 10 is performed on a quantum system 17, the result of the first monitoring of block S11B depends on the quantum state of quantum system 17 at the start of method 10. If quantum system 17 is initially in state 12B, then application of the optical pulse in block S11A ought to result in emission of no photons. Blocks S11A and S11B may be repeated multiple times and each of these times the first monitoring of block S11B would ideally not detect any photon. In the real world there can be a possibility that a photon may be detected by the first monitoring of block S11B, for example, because of a dark count of a photon detector occurring in the time window of block S11B or the optical pulse of block S11A exciting the quantum system 17 byway of transition 15B, if transition 15B is available. With modern detectors, the likelihood of a dark count in a time window having a duration on the order of a decay time of a typical quantum system 17 is low. If the energies of transitions 15A and 15B differ enough then the likelihood that an optical pulse designed to excite quantum system 17 by transition 15A will excite quantum system 17 by transition 15B instead is low.

[0079] If quantum system 17 is initially in state 12A then application of the optical pulse in block S11A ought to result in emission of one photon. In the real world, the first monitoring of block 11B may fail to detect any photon in block S11B, for example, because the photon was lost before reaching a detector, the quantum system 11 decayed from excited state 14 by a path that does not yield the expected photon, the detector failed to detect the photon etc. Depending on factors such as the optical path from quantum system 17 to a photon detector, the likelihood that a photon emitted as a result of the optical pulse of block S11A will not be detected can be significant. In some cases the probability that the photon will be detected is less than 50% or some lower probability such as less than 10%. Consequently, it may be necessary to repeat loop S11D two or more times before detecting a photon even if quantum system 17 is initially in state 12A.

[0080] Because of real world factors, including those discussed above, it may not be possible to obtain a measurement result that indicates the quantum state of quantum system 17 to a desired level of confidence based on a single instance of a photon being detected in the first monitoring performed in block S11B. By selecting criteria to be applied in blocks S11C, S11E and / or S12D method 10 may be adapted to measure the quantum state of quantum system 11 to a desired level of confidence.

[0081] In a simple example, block S11C repeats loop S11D until any one or both of two conditions are satisfied. The first condition is based on a number of times that block S11B detected a photon in the time window. The second condition is based on a number of times that blocks S11A and S11B have been performed (i.e. the number of iterations of loop S11D).

[0082] In some embodiments, the first condition causes method 10 to proceed to block S11E if any instance of the first monitoring of block S11B detected a photon. In some embodiments the first condition causes method 10 to proceed to block S11E if for some number of times that block S11A was executed (the number may, for example, be 2, 3 or more) the first monitoring of block S11B detected a photon.

[0083] In some embodiments, the second condition sets a maximum number of times that blocks S11A and S11B will be performed. This maximum number may be chosen based on parameters such as the probability that a photon emitted as a result of an optical pulse of block S11A will be detected by the first monitoring in the immediately subsequent block S11B. This maximum number may be set to higher values where the probability that a photon that is emitted as a result of an optical pulse in block S11A will not be detected by the first monitoring in the immediately following block S11B is greater. In some embodiments the maximum number is in the range of 5 to 50 or 4 to 200. In some embodiments, the maximum number is in the range 5 to 15 (e.g. 10).

[0084] The criteria of block S11E may be selected to skip blocks S12A, S12B, S12C and S12D if no instance of the first monitoring of block S11B detected a photon. Note that the logic of blocks S12C and S12E may be combined.

[0085] In some embodiments the criteria of block S12D comprises a maximum number of times to perform blocks S12B and S12C (i.e. a maximum number of iterations of loop S12E). In some embodiments the criteria of block S12D comprises a maximum number for the sum of the numbers of iterations of loops 11D and 12E (where each time that blocks S11A and S11B are performed is one iteration of loop S11D and each time that blocks S12C and S12D are performed is one iteration of loop 12E). When the criteria of block S12D are satisfied, block S12D causes method 10 to proceed to block S14.

[0086] In some embodiments the criteria of one or more of blocks S11C, S11E and S12D are adaptive (e.g. block S11C may have a criterion which determines a number of iterations of loop S11D to complete based on a number of and / or a pattern of photon detections by the first monitoring of block S11B).

[0087] With the example criteria discussed above, block S14 may receive different outcomes. In a first example outcome, loop S11D may be iterated the maximum number of times permitted by block S11C with no photons being detected in any instance of block S11B. Block S11E may then cause method 10 to proceed to block S14. In this case, block S14 may determine that quantum system 17 was in state 12B.

[0088] Block S14 may optionally determine a level of confidence in the measurement result. In this case, the level of confidence may be based at least in part on the value of the maximum number of iterations of loop S11D. In some embodiments the level of confidence may be based in part on additional information such as information indicative of a probability that any first photon resulting from application of the first optical pulse of block S11A would be detected by the first monitoring of block S11B.

[0089] In a second example outcome, loop S11D may be iterated one or more times until a photon is detected by the first monitoring of block S11B in one iteration of loop S11D. The criteria of block S11C may cause method 10 to proceed to block S12A if any photons have been detected by the first monitoring of block S11B. Method 10 may then proceed to block S12A which applies a it operation to quantum system 17. Method 10 may then perform one or more iterations of loop S12E with no photon being detected in the second monitoring of any instance of block S12C. In this case, block S14 may determine that quantum system was in state 12A.

[0090] Block S14 may optionally determine a level of confidence in the measurement result. In this case the level of confidence may depend at least in part on the number of iterations of loop S12E in which the second monitoring of block S12C did not detect any photon. In some embodiments the level of confidence may be based in part on additional information such as one or more of: information indicative of a probability that any first photon resulting from application of the first optical pulse of block S11A would be detected by the first monitoring of block S11B, information indicative of a probability that any second photon resulting from application of the second optical pulse of block S12B would be detected by the second monitoring of block S12C, a number of iterations of loop S11D in which a first photon was not detected, and a number of iterations of loop S11D in which a first photon was detected.

[0091] In the real world it is also possible to have an outcome in which the first monitoring of block S11B detects a photon in one or more iterations of loop S11D and the second monitoring of block S12C detects a photon in one or more iterations of loop S11E. In some embodiments, in the event of such an outcome, block S14 may determine that the results of method 10 are not consistent with any reliable measurement of the state of quantum system 17 being obtained.

[0092] In some embodiments block S14 comprises determining whether a number of photons detected is or is not below a threshold. For example in the first example outcome presented above, zero photons are detected and in the second example outcome presented above, one photon is detected. With the criteria stated for the first and second outcomes the threshold may be one (1) and block S14 may output the measurement result that quantum system 17 was in state 12B if the number of photons is less than 1 and the measurement result that the quantum system was in state 12A if the number of photons is not less than the threshold.

[0093] Block S14 may be configured based on the criteria used in block S11C. For example, method 10 may be implemented in a way that requires detection of a photon in the first monitoring of block S11B in each of two iterations of loop S11D to conclude that quantum system 17 is probably in state 12A. Where such criteria are used, block S14 may output: the measurement result that quantum system 17 was in state 12A if two photons were detected in aggregate in the first monitoring of block S11B and the second monitoring of block S12C; and the measurement result that quantum system was in state 12B if zero photons were detected in aggregate in the first monitoring and the second monitoring. For other outcomes (e.g. only one photon was detected in aggregate in the first monitoring and the second monitoring or more than two photons were detected in aggregate in the first monitoring and the second monitoring) then block S14 may take some other action such as outputting a null result.

[0094] In some embodiments, block S14 implements a logic truth table that takes as inputs results of first monitoring in instances of block S11B and instances of second monitoring by block S12C, if performed. Each pattern of inputs to the logic truth table may correspond to a corresponding measurement output. In some embodiments, block S14 includes additional functionality such as calculating and outputting a confidence level for a measurement.

[0095] It can be appreciated that method 10 beneficially provides a method for obtaining a measurement of the quantum state of a quantum system 17 to a desired level of confidence. Method 10 is particularly beneficial in embodiments in which quantum system 17 is coupled to another quantum system 17A (see Fig. 1B) that has a quantum state that contains useful quantum information. In such cases, evolution of the phase of the quantum state of quantum system 17A occurs at a rate that is affected by the quantum state of the coupled quantum system 17. Each time that quantum system 17 is optically cycled (e.g. in each iteration of loop S11D or loop S12E in which quantum system 17 is excited), quantum system 17 may spend an unknown amount of time in excited state 14. This risks decoherence (acquisition of an unknown phase) of the quantum state of optical system 17A which can make using or recovering the quantum information encoded in the quantum state of quantum system 17A impossible. Each time that quantum system 17 is excited to excited state 14 can add unpredictably to the phase of the quantum state of the coupled quantum system 17A (e.g. a nuclear spin) and adds to the risk that the coupled quantum system will be subject to an error (e.g. a phase error and / or a flip error).

[0096] In some embodiments quantum systems 17 and 17A make up a broker-client system in which quantum system 17 acts as a broker to store and / or retrieve and / or transfer quantum information in quantum system 17A, which acts as a client. Use of method 10 to perform a measurement on broker quantum system 17 can facilitate accurate measurement while reducing the risk that the fidelity of quantum information stored in client quantum system 17A will be degraded (by decoherence) as a result of the measurement process.

[0097] In an example case where a quantum system 17 is coupled to one or more quantum systems 17A that may store quantum information, quantum system 17 is provided by an electron spin and coupled quantum system(s) 17A are provided by one or more nuclear spins. In such an example the electron spin may be used as a broker quantum system and the nuclear spin(s) may be used as client quantum system(s). The electron spin may, for example, may be associated with a defect or luminescent centre in a crystalline substance, such as a T-centre, an l-centre an M-centre or a G-centre in silicon, a NV centre in diamond or a defect in silicon carbide.

[0098] It can be seen that method 10 may allow measurement of the quantum state of a quantum system 17 while reducing or minimizing an amount of time that the quantum system spends in an excited state (e.g. state 14), therefore reducing a rate of decoherence of any coupled quantum systems 17A. Method 10 can achieve this result by using criteria in block S11C which cause method 10 to proceed to block S12A when enough (one or more) photons have been detected in the first monitoring of block S11B to conclude that quantum system 17 is most likely in state 12A. The it pulse of block S12A causes quantum system 17 to be in state 12B. Subsequent iterations of loop S12E in which the second monitoring of block S12C increase confidence that quantum system 17 was in state 12A at the start of method 10 but the iterations of loop S12E do not excite quantum system 17 and therefore do not risk decay from the excited state to a different ground state or introduce the decoherence of quantum system(s) 17A that accompanies exciting quantum system 17.

[0099] Method 10 can advantageously detect cases in which dark counts provide false indications that quantum system 17 is in state 12A as described above.

[0100] Method 10 can be advantageously performed rapidly because the optical pulses delivered in blocks S11A and S12B can be of the same wavelength. Therefore, method 10 may be performed without the need to switch between different wavelengths, which can be time consuming (especially where switching wavelengths comprises tuning optical components such as optical resonators and / or parameters that affect energy levels of quantum system 17).

[0101] Fig. 1B shows apparatus 16 according to an example embodiment. In apparatus 16, a quantum system 17 and optionally one or more quantum systems 17A that are coupled to quantum system 17 are optically coupled to a photon detector 18 capable of detecting single photons by an optical path 19. Photon detector 18 may, for example comprise a superconducting nanowire single photon detector (“SNSPD”).

[0102] In this example, optical path 19 includes an optical resonator 19A and an optical waveguide 19B. Optical resonator 19A may be provided, for example, by an optical cavity such as a photonic cavity. Waveguide 19B may be provided, for example, by an integrated waveguide, an optical fiber or the like. In some embodiments optical path 19 passes through one or more optical switches (not shown) which are configurable to provide the optical connection of optical resonator 19A to photon detector 18. Optical resonator 19A may have and / or may be controlled to have a resonance that corresponds to an optical wavelength of photons that may be emitted as a result of exposing quantum system 17 to optical pulses on blocks S11B and S12C of method 10 (e.g. photons carrying energies corresponding to transition 15A). Optical resonator 19A may beneficially improve collection of photons emitted in transitions of quantum system 17, reduce photon loss, and decrease the lifetime of excited states of quantum system 17 (via the Purcell effect).

[0103] A control system 18A controls an optical pulser 18B to deliver optical pulses to quantum system 17. The optical pulses may be delivered to quantum system 17 by way of optical path 19, or another optical path, free space etc. Control system 18A also controls a microwave pulse generator 18C. Microwave pulse generator 18C may be configured to generate that microwave pulses having duration, amplitude and wavelength selected to flip the state of quantum system 17 by way of one or more microwave transitions (see e.g. Fig. 2). Microwave pulse generator 18C may be controlled, for example, to implement block S12A of method 10 by emitting one or more microwave pulses which result in the spin of quantum system 17 being flipped.

[0104] In apparatus 16, control system 18Aalso controls a resonator control 18D which is operable to vary the resonant wavelength and / or Q factor of optical resonator 19A. For example, resonance control 18D may be operable to tune optical resonator 19A to a wavelength corresponding to the desired transition. This may reduce a lifetime of the excited state by the Purcell effect. The tuning may be accomplished, for example, where optical resonator 19A is formed of an electro-optic material by applying an electric field across optical resonator 19A. Another example way to control the optical resonator is by applying a stress to a piezoelectric material.

[0105] In some embodiments, resonator control 18D is not present. In some such embodiments, multiple pairs of an optical resonator 19A that is coupled to a corresponding quantum system 17 are provided and one of the pairs for which the optical resonator has a resonant wavelength that corresponds to a transition of the quantum system 17 that will be exploited in method 10.

[0106] Control system 18A also controls a qubit controller 18E which is operable to tune the energy levels of quantum system 17 (and to thereby alter wavelengths of light corresponding to transitions 15A and / or 15B). Qubit controller 18E may, for example, operate by setting strength or other characteristics of an electric field and / or a magnetic field at the location of quantum system 17 and / or setting a magnitude of a strain in a substrate in which quantum system 17 is supported.

[0107] Control system 18A is also connected to receive signals from photon detector 18 and to process the signals to obtain measurement results for quantum system 17 according to method 10. In some embodiments control system 18A is configured to compute a level of confidence in the measurement results. The level of confidence may be used internally by control system 18A and / or output for information and / or output for use by other systems. For example, in some embodiments control system 18A may make decisions regarding execution of a quantum informatics program or a quantum circuit which are conditional on a measurement results and levels of confidence associated with the measurement results. As another example, in some embodiments control system 18A may output measurement results to another system that makes decisions regarding execution of a quantum informatics program or a quantum circuit which are conditional on a measurement results and levels of confidence associated with the measurement results.

[0108] Control system 18A may have any of a wide variety of constructions. Functions of control system 18A may be provided by a single unit or module or may be distributed among plural units or modules. In some embodiments, control system 18A comprises a data collecting component that acquires results of first monitoring and second monitoring, when performed, and a data processing component. The data processing component may operate to determine measurement results based at least in part on the results of the first monitoring and / or the second monitoring. The data processing component may also be configured to perform additional data processing (e.g. controlling execution of a quantum informatics program or a quantum circuit based at least in part on the measurement results).

[0109] In some such embodiments the data collecting component and the data processing component are integrated together in a single unit or module. In some such embodiments the data collecting component and the data processing component are separate or separable and may be provided in the form of separately vendible units or modules that may be but are not necessarily co-located.

[0110] Control system 18A may, for example comprise 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”). 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 control circuit for a device may implement methods as described herein (e.g. method 10) by executing software instructions in a program memory accessible to the processors.

[0111] In some embodiments, apparatus 16 is configurable to make measurements (e.g. according to method 10) on any of a plurality of quantum systems 17. In some such embodiments, parameters that affect operation of method 10 (e.g. parameters of criteria for determining whether or not to perform another iteration of loop S11D or loop S12E) may be specified separately for different ones of the quantum systems 17 or for different groups of the quantum systems 17.

[0112] For example, a parameter specifying a maximum number of iterations of loop S11D to be performed may be set based on a probability that a photon emitted from a particular quantum system 17 as a result of application of an optical pulse in block S11A will be detected in the first monitoring of block S11B. This probability may differ among quantum systems 17, for example as a result of differences in the lossiness of optical paths from different quantum systems 17 to different detectors and / or the efficiencies with which photons from different quantum systems 17 are coupled into the optical paths. This probability may be determined, for example, by performing a calibration procedure in which the quantum system in question is placed into a state where a photon should be emitted each time the quantum system is optically cycled, optically cycling the quantum system multiple times, and calculating the proportion of optical cycles in which a photon was detected.

[0113] In some applications, method 10 is applied to make measurements of plural or multiple quantum systems 17. In some such embodiments method 10 is implemented in a way that allows different parameters to be used for different ones of the quantum systems and / or for different combinations of quantum systems and photon detectors. Apparatus 16 may be configured to retrieve and use different parameters for different quantum systems 17. For example one or more of the following parameters may optionally be set for individual quantum systems 17 or defined groups of quantum systems 17: • maximum number of iterations of loop S11D - this number may be based upon a probability that the first monitoring will detect the first resulting photon if the quantum system is in the first basis state when the first optical pulse is applied to the quantum system. This number may be made larger for quantum systems 17 for which photons emitted in response to optical pulses of block S11A have a lower probability of detection by a particular photon detector than for quantum systems 17 for which the photons have a higher probability of detection; • which transition to use (i.e. the wavelength of optical pulses provided by blocks S11B and S12C - for example, a transition may be selected based on brightness of available transitions which may be different for different quantum systems); • values defining tuning of a corresponding optical resonator; • values for controlling energy levels of the quantum system (e.g. by controlling magnetic fields, electric fields and / or strain at a location of the quantum system 17); and • parameters specifying which of a plurality of detectors to use for measuring particular quantum systems 17.

[0114] 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 (e.g. method 10 ora portion thereof). 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.

[0115] In some embodiments, the invention may be implemented in software. For greater clarity, “software” includes any instructions executed on a processor, and may include (but is not limited to) firmware, resident software, microcode, code for configuring a configurable logic circuit, applications, apps, and the like. Both processing hardware and software may be centralized or distributed (or a combination thereof), in whole or in part, as known to those skilled in the art. For example, software and other modules may be accessible via local memory, via a network, via a browser or other application in a distributed computing context, or via other means suitable for the purposes described above.

[0116] In some embodiments, quantum system 17 is provided by the unpaired electron of a T-centre. A T-centre is a location where a silicon atom in a silicon crystal has been replaced by two carbon atoms and a hydrogen atom. Fig. 2 is an energy level diagram for the unpaired electron of a T-centre. Fig. 3 shows the structure of a T-centre 30. A T-centre comprises two carbon atoms 31A and 31B and a hydrogen atom 32 that is bonded to carbon atom 31B. Carbon atom 31A has one unpaired electron. The spin state of the unpaired electron of carbon atom 31A may be used as a qubit and measured using method 10.

[0117] T-centres incorporate several spins that have quantum states that may be used to store information. These include: one unpaired electron spin; one hydrogen nuclear spin; and two carbon nuclear spins. It is possible to store quantum information in quantum states of any of these spins.

[0118] In some embodiments, quantum system 17 is the unpaired electron spin of a T-centre. From Fig. 2 it can be seen that the energy level structure of the unpaired electron of a T-centre has the overall structure shown in Fig. 1A with some additional details.

[0119] In the presence of a magnetic field Bo, the ground state of the T-centre (T) is split into two states, 22A and 22B. These two states are further split to provide four states, 22A-1,22A-2, 22B-1 and 22B-2. which correspond to the four possible combinations of electron spin (which can be up Qe) or down (J,e)) and nuclear spin -for example the spin of the hydrogen nucleus of the T-centre (which can be up (ft N) or down (U N)). The excited state TX0 is split into two states defined by a hole spin which can be up or down (), / 7). The ground states may be used as computational states (e.g. to store information). The excited states may be used for initialization and measurement.

[0120] A T centre has plural transitions that may be used in method 10. For example, optical pulses delivered in blocks S11A and S12B may have wavelengths selected to correspond to transition 25A or transition 25B and photons detected in the first monitoring of block S11B and S12C have wavelengths corresponding to the selected transition. Where a quantum system 17 includes plural transitions that may be used in method 10, the best results are typically obtained by using the “brightest” transition, which is the transition that provides the most probable decay from an excited state of the quantum system 17 to a ground state of the quantum system 17.

[0121] Where quantum system 17 is a T-centre, the brightest transition is usually either transition 25A or transition 25B, which are both spin preserving. However, for some T-centres, transition 25A is brighter than transition 25B and for other T-centres the opposite is true.

[0122] The it operation of block S12A may, for example, comprise applying a it pulse (e.g. an X gate or a Y gate) to the T-centre. This may be done by applying one or more microwave it pulses that act on both microwave transition 27-1 between states 22A-1 and 22B-1 and microwave transition 27-2 between states 22A-2 and 22B-2. The microwave it pulse(s) may, for example, be provided by: a single microwave pulse that acts simultaneously on both of transitions 27-1 and 27-2 (e.g. a microwave pulse comprising wavelengths corresponding to each of transitions 27-1 and 27-2 -which may be called a “two-colour pulse”); a series of shorter microwave pulses that act simultaneously on transitions 27-1 and 27-2 and collectively have the effect of a microwave it pulse; or applying separate microwave pulses that act on transitions 271 and 27-2 respectively (in each case the pulses may comprise a single it pulse ora series of pulses that collectively act as a it pulse).

[0123] A non-limiting example application for method 10 is implementing measurement gates for quantum circuits. For example, method 10 may be applied for making measurements in a physical circuit for implementing a teleported CNOT (tCNOT) gate between two qubits that are respectively provided by first and second nuclear spins which are each associated with an electron spin. A protocol for implementing the tCNOT gate may involve making measurements on the electron spins. Decoherence of the nuclear spins caused by such measurements can be one of the main sources of infidelity when effecting a tCNOT gate between two nuclear spins. Use of method 10 to make the measurements to implement a tCNOT gate can increase the fidelity of the tCNOT gate.

[0124] As mentioned above, method 10 is not limited to T-centres or any other specific type of quantum system. Method 10 may, for example, be used with any of wide variety of quantum systems which have: • A first ground state having a first optical transition to a first excited state; • A second ground state; and • A transition between the first and second ground states that does not involve exciting the quantum system to any excited state; wherein the first and second ground states are long-lived enough to function as computational states, and the probability that the first excited state will decay to the first ground state via the first transition is significantly greater than the combined probabilities that the first excited state will decay from the first excited state to the second ground state via any other transition(s).

[0125] For quantum systems in which two distinct transitions (e.g. 15A and 15B) are available, (e.g. for a T-centre) method 10 may be varied by omitting block S12A, making the first optical pulses delivered in block S11A have a wavelength that corresponds to a first one of the transitions and making the second optical pulses delivered in block S12B have a wavelength that corresponds to a second one of the transitions. In some embodiments the modified method includes retuning an optical resonator associated with the quantum system and / or adjusting energy levels of the quantum system (e.g. by Stark shifting). In some embodiments of the modified method the first optical pulses and the second optical pulses have wavelengths that are the same or nearly the same and between the last performance of block S11A and the first performance of block S12B the energy levels of the quantum system are shifted such that before the shift the first optical pulses correspond to the first one of the transitions and after the shift, the second optical pulses correspond to the second one of the transitions

[0126] Where a component (e.g. an optical path, other optical component, photon detector, processor, device, software, data, 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.

[0127] It is not always the case that a first resulting photon has the same wavelength as the first optical pulse or a second resulting photon has the same wavelength as the second optical pulse. For some quantum systems, decay of an excited state to a ground state can occur by way of one or more energy levels that have energies between energies of the ground and excited states. In such embodiments the first resulting photon may have a wavelength longer than that of the first optical pulse and / or a second resulting photon may have a wavelength shorter than that of the second optical pulse. Interpretation of Terms

[0128] 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, ora 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 any appropriate 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.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] 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 of the present invention.

[0134] Any aspects described above in reference to apparatus may also apply to methods and vice versa.

[0135] 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.

[0136] 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.

[0137] 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

1. A method for performing a measurement on a quantum system having quantum basis states |0) and 11>, the method comprising:applying one or more first optical pulses to the quantum system and performing first monitoring for a first resulting photon after each of the one or more first optical pulses;in response to the first monitoring detecting one or more first resulting photons, performing a it operation on the quantum system, the it operation transforming the basis states according to: |0) >—> |1) and |1) >—> |0);after performing the it operation, applying one or more second optical pulses to the quantum system and performing second monitoring for a second resulting photon after each of the one or more second optical pulses;wherein each of the one or more first optical pulses and each of the one or more second optical pulses comprises a wavelength corresponding to a transition from a first one of the basis states of the quantum system to a higher energy stateanddetermining a measurement result based on results of the first monitoring or if the second monitoring was performed, based on results of the first monitoring and the second monitoring, the measurement result indicating whether the quantum spin was in the first one of the basis states or a second one of the basis states.

2. The method according to claim 1, wherein the quantum system comprises a quantum spin and the quantum basis states |0) and |1) correspond to spin states of the quantum spin.

3. The method according to claim 2, wherein performing the it operation comprises flipping the quantum spin.

4. The method according to claim 2 or 3, wherein the transition is a spin preserving transition.

5. The method according to any of claims 1 to 4, wherein performing the it operation comprises applying a microwave or radio frequency pulse to the quantum system.

6. The method according to claim 5, wherein the microwave pulse is a two-colour it pulse.

7. The method according to any of claims 1 to 6, wherein, if the first monitoringfails to detect any first resulting photons after a predetermined number of the first optical pulses, determining the measurement result comprises setting the measurement result to indicate that the quantum spin was in the second basis state.

8. The method according to any of claims 1 to 7, comprising performing the it operation in response to the first monitoring having detected two or more first resulting photons.

9. The method according to any of claims 1 to 8, comprising determining a number of the second optical pulses to apply to the quantum system based on a number of the first optical pulses applied to the quantum system prior to performing the it operation on the quantum system.

10. The method according to any of claims 1 to 9, comprising ceasing to apply the second optical pulses to the quantum system in response to a sum of the number of first optical pulses applied to the quantum system and the number of second optical pulses applied to the quantum system reaching a predetermined number.

11. The method according to claim 10, wherein the predetermined number is in the range of 5 to 100.

12. The method according to any of claims 1 to 11, wherein the quantum system is an electron spin.spin of a luminescent centre in a crystalline substance.

14. The method according to claim 13, wherein the luminescent centre is a T-centre.

15. The method according to claim 13, wherein the luminescent centre is selected from the group consisting of a T-centre, an l-centre, an M-centre and a G-centre.

16. The method according to any of claims 1 to 15, wherein if the first monitoring detects one or more first resulting photons and the second monitoring detects one or more second resulting photons, the method comprises indicating that the measurement result is unreliable.

17. The method according to any of claims 1 to 16 comprising, prior to applying the one or more first optical pulses to the quantum system, controlling the quantum system to reduce a lifetime of the excited state.

18. The method of claim 17, wherein controlling the quantum system to reduce the lifetime of the excited state comprises performing one or more of the following: controlling a resonance of an optical resonator that is optically coupled to the quantum system to match a wavelength corresponding to the transition; and controlling the wavelength corresponding to the transition to match the resonance of the optical resonatorthat is optically coupled to the quantum system.

19. The method according to any of claims 1 to 18, wherein the method comprises evaluating one or more criteria based on results of the first monitoring and determining whether to proceed to perform the it operation on the quantum system or to proceed to determine the measurement result without performing the it operation based on the criteria.

20. The method according to claim 19, wherein the criteria comprise a maximum number of the first optical pulses to be applied.

21. The method according to claim 20, wherein the maximum number of the first optical pulses to be applied to the quantum system is based upon a probability that the first monitoring will detect the first resulting photon if the quantum system is in the first basis state when the first optical pulse is applied to the quantum system.

22. The method according to any of claims 1 to 21, comprising determining a level of confidence of the measurement result.

23. The method according to any of claims 1 to 22, wherein the quantum system is coupled to one or more additional quantum systems that have quantum states in which information has been stored.

24. The method according to claim 23, wherein the quantum system is a broker quantum system and the one or more additional quantum systems comprise a client quantum system.

25. A control system configured to perform the method of any one of claims 1 to 24.

26. Apparatus comprising:the control system of claim 25; and the quantum system.

27. Apparatus for making a measurement on a quantum system having quantum basis states |0) and 11>, the apparatus comprising:a control system configured to perform a measurement on the quantum system by:causing an optical pulse generator to apply one or more first optical pulses to the quantum system;receiving results of first monitoring for a first resulting photon after each of the one or more first optical pulses;in response to the results of the first monitoring indicating detection of one or more first resulting photons, causing a it operation to be applied tothe quantum system, the tt operation transforming the basis states according to: |0) >—> |1) and |1) >—> |0);after the tt operation has been applied to the quantum system, causing the optical pulse generator to apply one or more second optical pulses to the quantum system and, receive results of second monitoring fora second resulting photon after each of the one or more second optical pulses.

28. The apparatus according to claim 27 wherein the control system is further configured to determine a measurement result based on results of the first monitoring or, if the second monitoring was performed, based on results of the first monitoring and the second monitoring, the measurement result indicating whether the quantum system was in a first one of the basis states or a second one of the basis states.

29. The apparatus according to claim 27 or 28, wherein the quantum system comprises a quantum spin, the quantum basis states |0) and |1) correspond to spin states of the quantum spin and wherein each of the one or more first optical pulses and each of the one or more second optical pulses comprises a wavelength corresponding to a transition from a first one of the basis states of the quantum system to a higher energy state.

30. The apparatus according to claim 29, wherein performing the tt operation comprises flipping the quantum spin.

31. The apparatus according to claim 29 or 30, wherein the transition is a spin preserving transition.

32. The apparatus according to any one of claims 27 to 31, wherein causing the tt operation to be applied to the quantum system comprises causing a microwave generator to generate microwave energy to apply the tt operation to the quantum system.a two-colour it pulse.

34. The apparatus according to claim 32 or 33, wherein the control system is configured to cause the microwave generator to generate the microwave energy to apply the it operation to the quantum system in response to the first monitoring having detected two or more first resulting photons.

35. The apparatus according to any of claims 32 to 34 further comprising the microwave generator.

36. The apparatus according to any of claims 27 to 35, wherein the control system is configured to, if the first monitoring fails to detect any first resulting photons after a predetermined number of the first optical pulses, set the measurement result to indicate that the quantum spin was in the second basis state.

37. The apparatus according to any of claims 27 to 36, wherein the control system is configured to determine a number of the second optical pulses to apply to the quantum system based on a number of the first optical pulses applied to the quantum system prior to performing the it operation on the quantum system.

38. The apparatus according to any of claims 27 to 37, wherein the control system is configured to cause the application of the second optical pulses to the quantum system to cease in response to a sum of the number of first optical pulses applied to the quantum system and the number of second optical pulses applied to the quantum system reaching a predetermined number.

39. The apparatus according to claim 38, wherein the predetermined number is in the range of 5 to 100.

40. The apparatus according to any of claims 27 to 39, comprising the quantum system, wherein the quantum system is an electron spin.spin of a luminescent centre in a crystalline substance.

42. The apparatus according to claim 41, wherein the luminescent centre is a T-centre.

43. The apparatus according to claim 41, wherein the luminescent centre is selected from the group consisting of a T-centre, an l-centre, an M-centre and a G-centre.

44. The apparatus according to any of claims 27 to 43, wherein the control system is configured to indicate that the measurement result is unreliable in response to the first monitoring detecting one or more first resulting photons and the second monitoring detecting one or more second resulting photons.

45. The apparatus according to any of claims 27 to 44, wherein the control system is configured to, prior to causing application of the one or more first optical pulses to the quantum system, control the quantum system to reduce a lifetime of the excited state.

46. The apparatus of claim 45, wherein controlling the quantum system to reduce the lifetime of the excited state comprises performing one or more of the following: controlling a resonance of an optical resonator that is optically coupled to the quantum system to match a wavelength corresponding to the transition; and controlling the wavelength corresponding to the transition to match the resonance of the optical resonatorthat is optically coupled to the quantum system.

47. The apparatus according to any of claims 27 to 46, wherein the control system is configured to evaluate one or more criteria based on results of the first monitoring and to determine whether to proceed to perform the it operation on the quantum system or to proceed to determine the measurement result without performing the it operation based on the evaluation of the one or more criteria.maximum number of the first optical pulses to be applied to the quantum system.

49. The apparatus according to claim 48, wherein the maximum number of the first optical pulses to be applied the quantum system is based upon a probability that the first monitoring will detect the first resulting photon if the quantum system is in the first basis state when the first optical pulse is applied to the quantum system.

50. The apparatus according to any of claims 27 to 49, wherein the control system is configured to determine a level of confidence of the measurement result.

51. The apparatus according to any of claims 27 to 50, wherein the quantum system is coupled to one or more additional quantum systems that have quantum states in which information has been stored.

52. The apparatus according to claim 51, wherein the quantum system is a broker quantum system and the one or more additional quantum systems comprise a client quantum system.

53. The apparatus of any of claims 27 to 52 further comprising the photon detector.

54. The apparatus of any of claims 27 to 53 further comprising the optical pulse generator.

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