ANALOG PROCESSOR

FIELD

This disclosure generally relates to analog processors such as quantum processors, and associated systems, devices, methods, and articles.

BACKGROUND

Quantum Devices

Quantum devices are structures in which quantum mechanical effects are observable. Quantum devices include circuits in which current transport is dominated by quantum mechanical effects. Such devices include spintronics, and superconducting circuits. Both spin and superconductivity are quantum mechanical phenomena. Quantum devices can be used for measurement instruments, in computing machinery, and the like.

Quantum Computation

A quantum computer is a system that makes direct use of at least one quantum-mechanical phenomenon, such as, superposition, tunneling, and entanglement, to perform operations on data. The elements of a quantum computer are qubits. Quantum computers can provide speedup for certain classes of computational problems such as computational problems simulating quantum physics.

Quantum Annealing

Quantum annealing is a computational method that may be used to find a iow-energy state of a system, typically preferably the ground state of the system. Similar in concept to classical simulated annealing, the method relies on the underlying principle that natural systems tend towards lower energy states because lower energy states are more stable. While classical annealing uses classical thermal fluctuations to guide a system to a low-energy state, quantum annealing may use quantum effects, such as quantum tunneling, as a source of delocalization to reach an energy minimum more accurately and/or more quickly than classical annealing.

A quantum processor may be designed to perform quantum annealing and/or adiabatic quantum computation. An evolution Hamiltonian can be constructed that is proportional to the sum of a first term proportional to a problem Hamiltonian and a second term proportional to a delocalization Hamiltonian, as follows:

// , ⁽ X A(f)H _p + // ( { )// _;

where H _E is ihe evolution Hamiltonian, H _P is the problem Hamiltonian, H ₀ is the delocalization Hamiltonian, and A(t), B(t) are coefficients that can control the rate of evolution, and typically lie in the range [0,1]

In some implementations, a time-varying envelope function can be placed on the problem Hamiltonian A suitable delocalization Hamiltonian is given by:

where N represents the number of qubits, of is the Pauli x-matrix for the i ^th qubit and D _έ is the single qubit tunnel splitting induced in the i ^th qubit. Here, the of terms are examples of“off-diagonal” terms.

A common problem Hamiltonian includes a first component proportional to diagonal single qubit terms and a second component proportional to diagonal multi-qubit terms, and may be of the following form:

where N represents the number of qubits, of is the Pauli z-matrix for the i ^th qubit, hi and / _έ , are dimensionless local fields for the qubits, and couplings between qubits, respectively, and e Is some characteristic energy scale for H _P . Here, ihe of and of of terms are examples of diagonal terms.

The former is a single-qubit term, and the latter a two-qubit term.

Throughout this specification, the terms“problem Hamiltonian” and“final Hamiltonian” are used interchangeably unless the context dictates otherwise. Certain states of the quantum processor are, energetically preferred, or simply preferred by the problem Hamiltonian. These include the ground states but may include excited states.

Hamiltonians such as H _D and H _P in the above two equations, respectively, may be physically realized in a variety of different ways. A particular example is realized by an implementation of superconducting qubits.

BRIEF SUMMARY

A method of operation of a hybrid computing system that comprises an analog processor and at least one digital processor, the analog processor and the at least one digital processor communicatively coupled to one another, the analog processor comprising a plurality of analog devices, the plurality of analog devices characterized by at least one programmable parameter, the at least one programmable parameter programmable by the digital processor, may be summarized as including: embedding, by the at least one digital processor, a computational problem on the analog processor to generate an embedded problem; causing, by the at least one digital processor, a first repeated running of the embedded problem on the analog processor to generate a first plurality of candidate solutions to the computational problem; returning to the at least one digital processor the first plurality of candidate solutions to the computational problem; determining, by the at least one digital processor, a value for at least one statistical feature of the first plurality of candidate solutions to the computational problem; adjusting the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from an expected value of the at least one statistical feature, the expected value of the at least one statistical feature inferred from the structure of the embedded problem; and causing, by the at least one digital processor, a second repeated running of the embedded problem on the analog processor to generate a second plurality of candidate solutions to the computational problem.

Embedding, by the at least one digital processor, a computational problem on the analog processor to generate an embedded problem may include embedding, by the at least one digital processor, a computational problem on a quantum processor. Embedding, by the at least one digital processor, a computational problem on a quantum processor may include embedding, by the at least one digital processor, a computational problem on a superconducting quantum processor.

Adjusting the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from an expected value of the at least one statistical feature may include adjusting the at least one programmable parameter of a plurality of superconducting flux qubits and superconducting coupling devices in the superconducting quantum processor. Adjusting the at least one programmable parameter of a plurality of superconducting flux qubits and superconducting coupling devices in the superconducting quantum processor may include adjusting at least one of a flux, a flux bias offset, a coupling strength and an anneal offset.

Embedding, by the at least one digital processor, a computational problem on the analog processor to generate an embedded problem may include embedding, by the at least one digital processor, an optimization problem on the analog processor.

Adjusting the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from an expected value of the at least one statistical feature, the expected value of the at least one statistical feature inferred from the structure of the embedded problem may include adjusting the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from an expected value of the at least one statistical feature, the expected value of the at least one statistical feature inferred from one or more symmetries of the embedded problem. Adjusting the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from an expected value of the at least one statistical feature, the expected value of the at least one statistical feature inferred from the structure of the embedded problem may include adjusting the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from an expected value of the at least one statistical feature, the expected value of the at least one statistical feature inferred from one or more graph automorphisms of the embedded problem.

Determining, by the at least one digital processor, a value for at least one statistical feature of the first plurality of candidate solutions to the computational problem may include determining, by the at least one digital processor, a value for at least one of a magnetization and a spin-spin correlation.

Embedding, by the at least one digital processor, a computational problem on the analog processor to generate an embedded problem may include embedding, by the at least one digital processor, a computational problem on a topology comprising a repeating lattice.

Embedding, by the at least one digital processor, a computational problem on a topology comprising a repeating lattice may include embedding, by the at least one digital processor, a computational problem on a topology comprising at least one of a triangular lattice and a square lattice.

The method of operation of the hybrid computing system may further include: returning to the at least one digital processor the second plurality of candidate solutions to the computational problem; determining, by the at least one digital processor, a value for at least one statistical feature of the second plurality of candidate solutions to the computational problem; and adjusting the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from the expected value of the at least one statistical feature.

A hybrid computing system may be summarized as including: at least one analog processor comprising a plurality of analog devices; at least one digital processor-based device communicatively coupled to the at least one analog processor; and at least one non-iransitory computer-readable storage medium that stores processor-executable instructions, which when executed causes at least one processor-based device to: embed a computational problem on the analog processor to generate an embedded problem; cause by a first repeated running of the embedded problem on the analog processor to generate a first plurality of candidate solutions to the computational problem; return to the digital processor the first plurality of candidate solutions to the computational problem; determine a value for at least one statistical feature of the first plurality of candidate solutions to the computational problem; adjust the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from an expected value of the at least one statistical feature, the expected value of the at least one statistical feature inferred from the structure of the embedded problem; and cause a second repeated running of the embedded problem on the analog processor to generate a second plurality of candidate solutions to the computational problem.

The analog processor may include a quantum processor. The quantum processor may include a superconducting quantum processor.

The at least one programmable parameter of the plurality of analog devices may include at least one programmable parameter of a plurality of superconducting flux qubits and superconducting coupling devices in the superconducting quantum processor.

The processor-executable instructions, which when executed causes at least one processor-based device to adjust the at least one programmable parameter of a plurality of superconducting flux qubits and superconducting coupling devices in the superconducting quantum processor may include instructions, which when executed causes at least one processor- based device to adjust at least one of a flux, a flux bias offset, a coupling strength and an anneal offset.

The computational problem may include an optimization problem. The structure of the embedded problem may include one or more symmetries of the embedded problem. The structure of the embedded problem includes one or more graph automorphisms of the embedded problem.

The at least one statistical feature of the first plurality of candidate solutions may include at least one of a magnetization and a spin-spin correlation.

The analog processor may include a topology comprising a repeating lattice in some implementations, the repeating lattice is at least one of a triangular lattice and a square lattice.

The hybrid computing system may further include at least one non-transitory computer-readable storage medium that stores processor- executable instructions, which when executed causes at least one processor- based device to: return to the digital processor the second plurality of candidate solutions to the computational problem; determine a value for at least one statistical feature of the second plurality of candidate solutions to the computational problem; and adjust the at least one programmable parameter of the plurality of analog devices in the analog processor to at least partially compensate for deviations from the expected value of the at least one statistical feature.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

in the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not necessarily drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn are not necessarily intended to convey any information regarding the actuai shape of the particular elements, and have been selected for ease of recognition in the drawings.

Figure 1 is a schematic diagram of an example hybrid computing system including a digital computer coupled to an analog computer, in accordance with the present systems, devices, articles, and methods.

Figure 2 is a schematic diagram of a portion of an exemplary superconducting quantum processor designed for quantum annealing (and/or adiabatic quantum computing) components from which may be used to implement the present systems and devices, in accordance with the present systems, devices, articles, and methods.

Figure 3 is a schematic diagram illustrating an example implementation of a ceil of a quantum processor (such as the quantum processor of Figure 1), in accordance with the present systems, devices, articles, and methods.

Figure 4 is a schematic diagram illustrating an example implementation of a topology of a quantum processor (such as the quantum processor of Figure 1), in accordance with the present systems, devices, articles, and methods.

Figure 5A is a schematic diagram illustrating a portion of an example triangular lattice, in accordance with the present systems, devices, articles, and methods.

Figure 5B is a schematic diagram illustrating an extended portion of an example triangular lattice, in accordance with the present systems, devices, articles, and methods.

Figure 6 is a flow chart illustrating an example implementation of a method of operation of an analog processor, in accordance with the present systems, devices, articles, and methods. DETAILED DESCRIPTION

General Comments

In the following description, some specific details are included to provide a thorough understanding of various disclosed embodiments. One skilled In the relevant art, however, will recognize that embodiments may be practiced without one or more of these specific details, or with other methods, components, materials, etc. In other instances, well-known structures associated with quantum processors, such as quantum devices, couplers, and control systems Including microprocessors and drive circuitry have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the present methods. Throughout this specification and the appended claims, the words“element” and“elements” are used to encompass, but are not limited to, ail such structures, systems, and devices associated with quantum processors, as well as their related programmable parameters.

Unless the context requires otherwise, throughout the specification and claims which follow, the word“comprise” and variations thereof, such as,“comprises” and“comprising” are to be construed in an open, inclusive sense, that is as“including, but not limited to.”

Reference throughout this specification to“one embodiment”“an embodiment”,“another embodiment”,“one example”,“an example”,“another example”,“one implementation”,“another implementation”, or the like means that a particular referent feature, structure, or characteristic described in connection with the embodiment, example, or implementation is included in at least one embodiment, example, or implementation. Thus, the appearances of the phrases“in one embodiment”,“in an embodiment”,“another embodiment” or the like in various places throughout this specification are not necessarily all referring to the same embodiment, example, or implementation. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments, examples, or implementations. It should be noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a problem-solving system including "a quantum processor" includes a single quantum processor, or two or more quantum processors. It should also be noted that the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise.

The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

Glossary

Automorphism: An automorphism of a graph is a form of symmetry in which the graph is mapped onto itself while preserving the edge- vertex connectivity. An automorphism of a graph G = (!/ ^' , £ ^' ) (with a vertex set V and an edge set E) is a permutation s of V, such that a pair of vertices (V _lt V ₂ ) form an edge if and only if the pair (sq^, sq^)) also form an edge.

Chimera topology: A Chimera graph of size C _s is an s x s grid of Chimera ceils (also referred to in the present application as unit tiles or unit cells), each containing a complete bipartite graph on 8 vertices (a K _AA graph). Each vertex is connected to its four neighbors inside the cell as well as two neighbors (north/south or east/west) outside the cell. Every vertex has degree 6 excluding boundary vertices.

Edge-transitive graph: An edge-transitive graph is a graph G such that, given any pair of edges there is an automorphism of G that maps edge edge E ₂ .

Frustration: A system is frustrated when the global ordering of the system is not compatible with the local ordering of its particles. In frustrated magnetic systems, for example, the localized magnetic moments, or spins, interact through competing exchange interactions that cannot be

simultaneously satisfied in the Ising model, for example, frustration means there is at least one that provides a positive contribution to the ground state energy, i.e. , at least one satisfiability constraint on the /i, _: and J, _j values that is violated.

Superconducting Quantum Processor for Quantum Annealing

A superconducting quantum processor can be designed for quantum annealing (and/or adiabatic quantum computing - see below) components from which may be used to implement the present systems and methods. The superconducting quantum processor can comprise a plurality of superconducting qubits and at least one coupler providing a tunable afa coupling (diagonal coupling) between qubits.

The quantum processor can include a plurality of interfaces that are used to configure and control the state of quantum processor. Each of interfaces can be realized by a respective inductive coupling structure, e.g , inductor, as part of a programming subsystem and/or an evolution subsystem.

in the operation of the quantum processor, interfaces can be used to couple a flux signal into a respective compound Josephson junction of the qubits, thereby realizing a tunable tunneling term (the D, term) in the system Hamiltonian. This coupling provides the off-diagonal s ^c terms of the

Hamiltonian, and these flux signals are examples of“delocalization signals”.

Similarly, interfaces can be used to apply a flux signal into a respective qubit loop of the qubits, thereby realizing the h _t terms in the system Hamiltonian. This coupling provides the diagonal a ^z terms in the system Hamiltonian. Furthermore, an interface can be used to couple a flux signal into a coupler, thereby realizing the ) _i} term(s) in the system Hamiltonian. This coupling provides the diagonal ofaf terms in the system Hamiltonian.

The quantum processor can include readout devices to read out the final states of the qubits. Examples of superconducting qubits include superconducting flux qubits, superconducting charge qubits, and the like. Adiabatic Quantum Computing

One model of quantum computing is adiabatic quantum computing. Adiabatic quantum computing can be suitable for solving hard optimization problems, for example. Adiabatic quantum computing may be considered a special case of quantum annealing. In adiabatic quantum computation, the system ideally begins and remains in its ground state throughout an adiabatic evolution. Those of skill in the art will appreciate that quantum annealing systems and methods may generally be implemented on an adiabatic quantum computer. Throughout this specification and the appended claims, any reference to quantum annealing is intended to encompass adiabatic quantum computation unless the context requires otherwise.

Operation of a quantum computer such as a quantum annealer can include one or more calibration activities. See, for example, Perdomo-Ortiz A. et a!.,“Determination and correction of persistent biases in quantum annealers”, Scientific Reports 6:18828 (2018) which describes adjustment of local fields (the h _t terms in the system Hamiltonian) in the trivial instance of full symmetry and two-qubit systems. A shortcoming of existing systems and methods, such as the approach described by Ortiz, is that they are not applicable to non-triviai problems, and non-trivial symmetries of the problem, and are unable to correct for multi-body cross-talk, for example.

See also, for example US Patent Application

Publication No. US2017/0017894 entitled“Systems and methods for improving the performance of a quantum processor to reduce intrinsic/control errors” which describes systems and methods for improving the performance of an analog processor such as a quantum processor by implementing a calibration correction for local bias values of qubits that belong to a logical qubit. The method can include determining whether the logical qubit exhibits a bias toward a basis state (e.g., a bias toward +1 or -1), and adjusting one or more local bias values to at least partially compensate for the bias exhibited by the logical qubit.

Hybrid Computing System Comprising a Quantum Processor

A hybrid computing system can include a digital computer communicatively coupled to an analog computer. In some implementations, the analog computer is a quantum computer and the digital computer is a classical computer.

The digital computer can include a digital processor that can be used to perform classical digital processing tasks described in the present systems and methods. The digital computer can include at least one system memory which can be used to store various sets of computer-readable or processor-readable instructions, application programs and/or data.

The quantum computer can include a quantum processor that includes programmable elements such as qubits, couplers, and other devices. The quantum computer can include a readout system, and the readout system can be operable to read out qubits, and communicate results to the digital computer. The quantum computer can include a qubit control system and a coupler control system. The qubit and the coupler control systems can control the qubits and the couplers, respectively. In some implementations, the qubit and the coupler control systems can be used to implement quantum annealing on the analog computer.

Figure 1 illustrates an example hybrid computing system 100 including a digital computer 102 coupled to an analog computer 104. in some implementations, the analog computer 1 Q4 is a quantum computer and the digital computer 102 is a classical computer.

The exemplary digital computer 102 includes at least one digital processor 106, and each digital processor 106 may include one or more central processor units (not shown in Figure 1 ). Only one digital processor 106 is shown in Figure 1 Digital processor(s) 106 may be used to perform classical digital processing tasks described in the present systems and methods. In other implementations, digital computer 102 can include more than one digital processor. Those skilled in the relevant art will appreciate that the present systems and methods can be practiced with other digital computer

configurations, including hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, personal computers ("PCs"), network PCs, mini-computers, mainframe computers, and the like, when properly configured or programmed to form special purpose machines, and/or when communicatively coupled to control an analog computer, for instance a quantum computer.

Digital computer 102 will at times be referred to in the singular herein, but this is not intended to limit the application to a single digital computer. The present systems and methods can also be practiced in distributed computing environments, where tasks or sets of instructions are performed or executed by remote processing devices, which are linked through a communications network in a distributed computing environment computer- or processor-readable instructions (also referred to in the present application as program modules), application programs and/or data, may be stored in both local and remote memory storage devices (e.g , non-transitory computer- or processor-readable media).

Digital computer 102 may include at least one digital processor 106, at least one system memory 108, and at least one system bus 1 10 that provides communicative coupling between various system components, for example between system memory 108 and digital processor(s) 106.

Digital processor(s) 106 may be any logic processing unit, for example with one or more cores, for instance one or more central processing units ("CPUs”), graphics processing units ("GPUs"), digital signal processors ("DSPs"), application-specific integrated circuits ("ASICs"), field-programmable gate arrays ("FPGAs"), etc.

Unless described otherwise, the construction and operation of the various blocks shown in Figure 1 are of conventional design. As a result, such blocks need not be described in further detail herein, as they will be understood by those skilled in the relevant art.

Digital computer 102 may include a user Input/output subsystem 1 12. In some implementations, the user input/output subsystem includes one or more user input/output components such as a display 1 14, mouse 1 16, and/or keyboard 1 18. System bus 1 10 can employ any known bus structures or architectures, including a memory bus with a memory controller, a peripheral bus, and a local bus. System memory 108 may include non-volatile memory, such as read-only memory ("ROM"), static random-access memory ("SRAM"), Flash NAND; and volatile memory such as random-access memory ("RAM") (not shown), all of which are examples of non-transitory computer- or processor-readable media.

A basic input/output system ("BIOS") 120, which can form part of the ROM, contains basic routines that help transfer information between elements within digital computer 102, such as during startup.

Digital computer 102 may also include other non-volatile memory 122. Non-volatile memory 122 may take a variety of forms, including: a hard disk drive for reading from and writing to a hard disk, an optical disk drive for reading from and writing to removable optical disks, and/or a magnetic disk drive for reading from and writing to magnetic disks, all of which are examples of non-transitory computer- or processor-readable media. The optical disk can be a CD-ROM or DVD, while the magnetic disk can be a magnetic floppy disk or diskette. Non-volatile memory 122 may communicate with digital processor via system bus 1 10 and may include appropriate interfaces or controllers 124 coupled to system bus 1 10. Non-volatile memory 122 may serve as long-term storage for computer- or processor-readable instructions, data structures, or other data (also called program modules) for digital computer 102.

Although digital computer 102 has been described as employing hard disks, optical disks and/or magnetic disks, those skilled in the relevant art will appreciate that other types of non-volatile computer-readable media may be employed, such magnetic cassettes, flash memory cards, Flash, ROMs, smart cards, etc., all of which are further examples of non-transitory computer- or processor-readable media. Those skilled in the relevant art will appreciate that some computer architectures conflate volatile memory and non-volatile memory. For example, data in volatile memory can be cached to non-volatile memory. Or a solid-state disk that employs integrated circuits to provide non volatile memory. Some computers place data traditionally stored on disk in memory. As well, some media that are traditionally regarded as volatile can have a non-volatile form, e.g , Non-Volatile Dual in-line Memory Module variation of Dual In-Line Memory Modules.

Various sets of computer-readable or processor-readable instructions (also referred to in the present application as program modules), application programs and/or data can be stored in system memory 108. For example, system memory 108 may store an operating system 126, and a set of computer-or processor-readable server instructions (i.e., server modules) 128. in some implementations, server module 128 includes instructions for communicating with remote clients and scheduling use of resources including resources on the digital computer 102 and analog computer 104. For example, a Web server application and/or Web client or browser application for permitting digital computer 102 to exchange data with sources via the Internet, corporate intranets, or other networks, as well as with other server applications executing on server computers.

In some implementations, system memory 108 may store other sets of computer-readable or processor-readable instructions 130 such as calculation instructions, analog computer interface instructions and the like.

While shown in Figure 1 as being stored in system memory 108, server instructions 128, other instructions 130, and other data (not shown in Figure 1) can also be stored elsewhere including in non-volatile memory 122 or one or more other non-transitory computer-readable or processor-readable media.

The analog computer 104 can be provided in an isolated environment (not shown in Figure 1 ). For example, where analog computer

18 104 is a quantum computer, the environment shields the interna! elements of the quantum computer from heat, magnetic field, and the like, and other external noise (not shown in Figure 1 ) and/or which cools the analog processor to temperatures (i e., critical temperature) at or below which the circuitry of analog processor 104 becomes superconductive. In contrast, the digital computer 102 will typically operate at much higher temperatures ( e.g ., room temperature) at which superconductivity does not occur and/or may employ materials that are not superconductive even at or below the critical temperature. Analog computer 104 includes an analog processor 132 Examples of analog processor 132 include quantum processors such as those described below with reference to Figure 2.

A quantum processor includes programmable elements such as qubits, couplers, and other devices. The qubits can be read out via readout system 134 The readouts can be fed to various sets of computer-readable or processor-readable instructions for digital computer 102, including server module 128, or other modules 130 stored in non-volatile memory 122, returned over a network or the like. The qubits can be controlled via qubit control system 136. The couplers can be controlled via coupler control system 138. In some implementations, qubit control system 136 and coupler control system 138 are used to implement quantum annealing on analog processor 132, as described in the present application.

In some implementations, digital computer 102 can operate in a networking environment using logical connections to at least one client computer system in some implementations, digital computer 102 is coupled via logical connections to at least one database system. These logical connections may be formed using any means of digital communication, for example, through a network, such as a local area network ("LAN") or a wide area network ("WAN") including, for example, the Internet. The networking environment may include wired or wireless enterprise-wide computer networks, intranets, extranets, and/or the Internet. Other embodiments may include other types of communication networks such as telecommunications networks, cellular networks, paging networks, and other mobile networks. The information sent or received via the logical connections may or may not be encrypted. When used in a LAN networking environment, digital computer 102 may be connected to the LAN through an adapter or network interface card (“NIC”) (communicatively linked to system bus 1 10). When used in a WAN networking environment, digital computer 102 may include an interface and modem (not shown), or a device such as NIC, for establishing communications over the WAN. Non-networked communications may additionally, or alternatively, be employed.

Exemplary Superconducting Quantum Processor for Quantum Annealing

Figure 2 is a schematic diagram of a portion of an exemplary superconducting quantum processor 200 designed for quantum annealing (and/or adiabatic quantum computing) components from which may be used to implement the present systems and devices. The portion of superconducting quantum processor 200 shown in Figure 2 includes two superconducting qubits 202, and 204. Also shown is a tunable coupling (diagonal coupling) via coupler 206 between qubits 202 and 204 (i.e., providing 2-local interaction). While the portion of quantum processor 200 shown in Figure 2 includes only two qubits 202, 204 and one coupler 206, those of skill in the art will appreciate that quantum processor 200 may include any number of qubits and any number of couplers coupling information between them.

The portion of quantum processor 200 shown in Figure 2 may be implemented to physically realize quantum annealing and/or adiabatic quantum computing. Quantum processor 200 includes a plurality of interfaces 208-216 that are used to configure and control the state of quantum processor 200.

Each of interfaces 208-216 may be realized by a respective inductive coupling structure, as illustrated, as part of a programming subsystem and/or an evolution subsystem. Such a programming subsystem and/or evolution subsystem may be separate from quantum processor 200, or it may be included locally (i.e., on-chip with quantum processor 200). In the operation of quantum processor 200, interfaces 208 and 212 may each be used to couple a flux signal into a respective compound Josephson junction 218 and 220 of qubits 202 and 204, thereby realizing a tunable tunneling term (the D* term) in the system Hamiltonian. This coupling provides the off-diagonal s ^c terms of the Hamiltonian and these flux signals are examples of“delocalization signals”.

In some implementations, the tunneling term is selected to make a first portion of the qubits on the quantum processor more classical relative a second portion of the qubits. For example, qubit 202 may be a hidden unit in a Boltzmann machine and have a smaller tunneling term relative to qubit 204.

Similarly, interfaces 210 and 212 may each be used to apply a flux signal into a respective qubit loop of qubits 202 and 204, thereby realizing the hi terms in the system Hamiltonian. This coupling provides the diagonal s ^z terms in the system Hamiltonian. Furthermore, interface 218 may be used to couple a flux signal into coupler 208, thereby realizing the / _έ; term(s) in the system Hamiltonian. This coupling provides the diagonal afaf terms in the system Hamiltonian.

In Figure 2, the contribution of each of interfaces 208-216 to the system Hamiltonian is indicated in boxes 208a-216a, respectively. As shown, in the example of Figure 2, the boxes 2G8a-216a are elements of time-varying Hamiltonians for quantum annealing and/or adiabatic quantum computing.

Throughout this specification and the appended claims, the term “quantum processor” is used to generally describe a collection of physical qubits (e.g., qubits 202 and 204) and couplers (e.g., coupler 206). The physical qubits 2Q2 and 204 and the coupler 206 are referred to as the“programmable elements” of the quantum processor 200 and their corresponding parameters (e.g., the qubit h _t values and the coupler values) are referred to as the “programmable parameters” of the quantum processor. In the context of a quantum processor, the term“programming subsystem” is used to generally describe the interfaces (e.g ,“programming interfaces” 210, 212, and 216) used to apply the programmable parameters to the programmable elements of the quantum processor 200 and other associated control circuitry and/or instructions.

As previously described, the programming interfaces of the programming subsystem may communicate with other subsystems which may be separate from the quantum processor or may be included locally on the processor. As described in more detail later, the programming subsystem may be configured to receive programming instructions in a machine language of the quantum processor and execute the programming instructions to program the programmable elements in accordance with the programming instructions. Similarly, in the context of a quantum processor, the term“evolution subsystem” generally includes the interfaces (e.g.,“evolution interfaces” 208 and 214) used to evolve the programmable elements of the quantum processor 200 and other associated control circuitry and/or instructions. For example, the evolution subsystem may include annealing signal lines and their corresponding interfaces (208, 214) to the qubits (202, 204).

Quantum processor 200 also includes readout devices 222 and 224, where readout device 222 is associated with qubit 202 and readout device 224 is associated with qubit 204. in some embodiments, such as shown in Figure 2, each of readout devices 222 and 224 includes a DC-SQUID inductively coupled to the corresponding qubit. In the context of quantum processor 200, the term“readout subsystem” is used to generally describe the readout devices 222, 224 used to read out the final states of the qubits (e.g., qubits 202 and 204) in the quantum processor to produce a bit string. The readout subsystem may also include other elements, such as routing circuitry (e.g., latching elements, a shift register, or a multiplexer circuit) and/or may be arranged in alternative configurations (e.g., an XY-addressable array, an XYZ- addressable array, etc.). Qubit readout may also be performed using alternative circuits, such as that described in PCT Patent Publication

WO2012064974.

While Figure 2 illustrates only two physical qubits 2Q2, 204, one coupler 206, and two readout devices 222, 224, a quantum processor (e.g., processor 200) may employ any number of qubits, couplers, and/or readout devices, including a larger number (e.g , hundreds, thousands or more) of qubits, couplers and/or readout devices. The application of the teachings herein to processors with a different (e.g., larger) number of computational components should be clear to those of ordinary skill in the art.

Examples of superconducting qubits include superconducting flux qubits, superconducting charge qubits, and the like. In a superconducting flux qubit, the Josephson energy dominates or is equal to the charging energy. In a charge qubit it is the reverse. Examples of flux qubits that may be used include RF-SQUIDs, which include a superconducting loop interrupted by one

Josephson junction, persistent current qubits, which include a superconducting loop interrupted by three Josephson junctions, and the like.

The qubits and coupling devices in a quantum processor may be arranged according to an architecture into a topology such that a certain number of qubits may be laid out in a sub-topology of qubits (hereinafter,“sub topology”). A sub-topology is a portion of a quantum processor topology comprising qubits and coupling devices. A plurality of sub-topologies may be repeated or tiled (or otherwise directly communicatively coupled to one another) over an area of a quantum processor to produce a certain quantum processor topology.

in some implementations, each sub-topology in a topology is identical to each other sub-topology in the same topology in other

implementations, one or more sub-topologies in the topology comprise a different configuration of qubits and coupling devices than another sub-topology in the same topology.

Calibration of an Analog Processor

An analog processor can be calibrated to provide a desired level of performance on one or more types of problems. Conventionally, an analog processor is calibrated to provide a desired level of performance on a broad variety of problems. For some applications, it can be desirable for the devices of an analog processor to be behave uniformly. The analog processor can be a quantum processor. For example, for some applications, it can be desirable for qubits, qubit chains, couplers between qubits, and couplers between qubit chains of the quantum processor to exhibit uniform behavior.

in one implementation, a graph is embedded on an analog processor, and devices in the analog processor are expected to exhibit uniform behavior, through an automorphism of the graph. An automorphism may result from a structure of the analog processor, for example from a symmetry of the analog processor, or from a symmetry of devices in the analog processor.

In another implementation, different embeddings of the problem on an analog processor are used, and the embeddings are expected to exhibit uniform behavior (i.e , have the same or at least similar statistics), through an automorphism of the graph. An automorphism may result from a structure of the analog processor, for example from a symmetry of the analog processor, or from a symmetry of devices in the analog processor.

in yet another implementation, a graph can be embedded on one physical instance of a quantum processor, and an automorphism of the graph can be embedded on another physical instance of the quantum processor. Since the physical instances are logically equivalent to each other, through the automorphism, the physical instances can be expected to exhibit the same behavior as each another.

in yet another implementation, a graph is embedded multiple times on a quantum processor with different embeddings. Through

automorphism of the embedded graphs, statistics between embeddings can be expected to exhibit at least similar characteristics.

Chimera Topology Example

The systems and methods described in the present application can be used to improve calibration and/or performance of an analog processor where devices in the analog processor are expected individually and/or collectively to exhibit uniform behavior. One reason for expecting uniform behavior can be the structure of the analog processor, for example the topology of the analog processor, and, in some examples, symmetries in the topology of the analog processor.

The Chimera topology, described below with reference to Figure 3, is an example of a topology of an analog processor. The Chimera topology is an example of a topology with automorphisms. The systems and methods described in the present application can, for example, be used to improve calibration and/or performance of a quantum processor having a Chimera topology.

Figure 3 is a schematic diagram illustrating an example implementation of a ceil 300 of a quantum processor (such as quantum processor 132 of Figure 1 ), in accordance with the present systems, devices, articles, and methods. Cell 300 includes qubits 302a, 302b, 302c, and 3Q2d (collectively 302) and qubits 304a, 304b, 304c, and 3Q4d (collectively 304). Qubits 302 are laid out vertically in Figure 3, and qubits 304 are laid out horizontally in Figure 3 A person of skill in the art will appreciate that while four qubits are illustrated both horizontally and vertically, this number is for example only, and other implementations may comprise more than four qubits or less than four qubits.

Qubits 302 and 304 may be superconducting qubits. Each qubit of qubits 302 may include a respective loop of superconducting material 306 (only one called out in Figure 3) interrupted by at least one respective

Josephson junction 308 (only one called out in Figure 3) Each qubit of qubits 304 may include a respective loop of superconducting material 310 (only one called out in Figure 3) interrupted by at least one respective Josephson junction 312 (only one called out in Figure 3).

Couplers 314a, 314b, 314c, and 314d (collectively 314, only four called out in Figure 3) can provide communicative coupling between qubits 302 and 3Q4. Each qubit of qubits 302 can be coupled to each qubit of qubits 304 through four respective couplers from couplers 314 in a region proximate to where a portion of each qubit of qubits 302 crosses a portion of a qubit of qubits 304 Each coupler of couplers 314 may be a respective loop of

superconducting material wherein the loop or superconducting material may define a perimeter to a coupling region. Each coupler of couplers 314 may be a respective loop of superconducting material interrupted by at least one respective Josephson junction wherein the loop or superconducting material may define a perimeter to a coupling region wherein coupling occurs along the perimeter by having a current-carrying wire, such as loop of superconducting material (e.g , loop 306 of qubit 302a or loop 310 of qubit 304a) run parallel in some manner to a coupler of couplers 314 to allow flux from current within the loop of superconducting material to induce a current to flow in a coupler of couplers 314 and vice versa. Couplers 314 may be tunable in that the communicative coupling provided by couplers 314 between two respective qubits of qubits 302 and 304 can be changed during the operation of an analog processor. The coupling may change during computation. The coupling may change between computations to embed a problem into the analog processor. The coupling may be expressed as a coupling strength.

Figure 4 is a schematic diagram illustrating an example implementation of a topology 400 of a quantum processor (such as quantum processor 132 of Figure 1 ), in accordance with the present systems, devices, articles, and methods. Topology 400 comprises sub-topologies 402, 404, 406, 408, 410, and 412. Couplers 414, 416, 418, 420, 422, 424, and 426 providing communicative coupling between sub-topologies may also be located within topology 400. Sub-topologies 408 and 404 can be communicatively coupled to sub-topologies 410 and 406 through couplers 414, 416, and 418 such that a 2 x K ₈ graph can be embedded into topology 400. Each K _a graph, or portion thereof, may be coupled to variables from another K ₈ graph. For example, one K ₈ graph may be embedded into sub-topologies 402, 404, and 408 and another K _b graph may be embedded into sub-topologies 406, 410, 412. Couplers 414, 416, 418, 420, 422, 424, and 426 may be controllable such that both ferromagnetic couplings, anti-ferromagnetic couplings, zero couplings and transverse couplings may be created between pairs of adjacent qubits.

In an example implementation, a graph has a Chimera topology (see Glossary) with a Chimera ceil in which a coupling strength / _i - = 1 for all qubit pairs {i, /}, and a flux bias h _t = 0 for all qubits {i}. The graph is edge- transitive (see Glossary). There is a permutation of qubits that maps a pair of coupled qubits {/,/} to another pair of coupled qubits {k, i}.

Ideally, both couplers should be frustrated with the same probability (see Glossary for a definition of frustration). If one coupler is frustrated less than the other, then the coupling strength of at least one of the two couplers can be adjusted to improve the degree of homogeneity of the statistics of frustration.

Since the flux bias is set to zero, the expected average spin for each qubit should be zero. If an average spin is greater than or less than zero by more than a predetermined threshold, then a flux bias offset can be applied to the qubit to cause the average spin to be within the predetermined threshold from zero or at least closer to zero than without the flux bias offset.

in an example scenario of simulation of a repeating lattice (for example a square lattice or a triangular lattice), ideally the logical qubits and the logical couplers should behave in the same way. In practice, for a given set of calibration parameters for the quantum processor, systematic errors can be observed, and the systematic errors can interact with each other in a chaotic manner. The present application describes systems and methods for refining a calibration to improve the degree of homogeneity of the statistics of frustration for the logical couplers, and to cause the average net magnetization of the logical qubits (also referred to in the present application as qubit chains) to be closer to the expected value of zero.

Embedded Triangular Lattice

In an example implementation, the repeating lattice is a triangular lattice. For the triangular lattice, the probability a given coupler is frustrated (at a low temperature) Ideally has a value of one-third, and the magnetization of a given qubit ideally has a value zero. At a low temperature, the system can be in a ground state or a state that is close to the ground state. At a low temperature, a solution can be a low-energy solution. A low-energy solution is less likely to be observed at a higher temperature.

in the example implementation, the systems and methods described in the present application for refining the calibration of the quantum processor are implemented using one or more iterations, each iteration including an adjustment of a coupling strength / _έ ,·, a local bias h, and/or a flux bias offset to improve the degree of homogeneity of the statistics of frustration. A local bias value can vary as a function of progress through an anneal. A flux bias offset can be applied as a constant value.

Figure 5A is a schematic diagram illustrating a portion SOGa of an example triangular lattice, in accordance with the present systems, devices, articles, and methods. Portion 500a comprises three vertices 502, 504, and 506, and three edges 508, 510, and 512. in one implementation of an analog processor to which portion 500a belongs, vertices 502, 504, and 508 are qubits, and edges 508, 510, and 512 are couplers.

Figure 5B is a schematic diagram illustrating an extended portion 500b of an example triangular lattice, in accordance with the present systems, devices, articles, and methods. Extended portion 500b comprises vertices 514, 516, and 518, and edges 520, 522, and 524 (for clarity, only three vertices and three edges called out in Figure 5B).

A triangular lattice can have one or more symmetries. For example, each triangle in the lattice can have exactly one frustrated edge (in a low energy solution), and, except for triangles subject to non-periodic boundary conditions, the three edges of the triangle can be equivalent to each other by rotation.

Referring to Figure 5A, the triangle formed by vertices 502, 504, and 506, and edges 508, 510, and 512 can have exactly one frustrated edge,

28 and (provided the triangle is not subject to non-periodic boundary conditions) edges 508, 510, and 512 can be equivalent to each other by rotation.

In one Implementation, vertices 502, 504, and 508 are qubits in a quantum processor, and can be associated with a spin vector in the example illustrated in Figure 5A, vertex 502 has spin“up”, and vertex 504 has spin “down”. In the same implementation, edges 508, 510, and 512 are couplers, each with anti-ferromagnetic (AFM) coupling in the illustrated example of Figure 5A, the spin of vertex 506 is indeterminate, i e., cannot be selected to satisfy the constraints imposed by the spins of 502 and 504, and the AFM coupling of 508, 510, and 512.

In another implementation, a lattice can be a torus or a half-torus. The half-torus can be opened to form a cylinder. The toroidal or half-toroidal lattice can be a topology of an analog processor such as a quantum processor. The lattice can include couplers with rotational symmetry, and is an example of a topology with automorphisms. The systems and methods described in the present application can, for example, be used to improve calibration and/or performance of a quantum processor having a toroidal or half-toroidal topology.

in another implementation, adjustments can be made to anneal offsets. In an example implementation of a quantum processor, one subset of qubits can have different tunneling characteristics to another subset of qubits in this example, it can be beneficial to include adjustments to anneal offsets to at least improve and/or homogenize statistics. For example, the quantum processor can include an integrated circuit with horizontally-oriented qubits and vertically-oriented qubits, the horizontally-oriented qubits having different tunneling characteristics from the vertically-oriented qubits. Anneal offsets can be adjusted using the systems and methods described in the present application to at least improve and/or homogenize statistics.

In other implementations, at least one other suitable parameter can be adjusted and/or applied to improve and/or homogenize statistics.

In yet another implementation, the processor is first run with only the chain couplings active, using flux offsets to balance the chains to degeneracy. The processor is then run with the problem Hamiltonian, and adjustments made to flux bias offsets and/or coupling strengths in accordance with the systems and methods described in the present application.

in one implementation, the systems and methods described in the present application can be used to improve or refine a calibration in which two devices are expected to behave at least approximately identically.

in an example implementation, the calibration can be refined using an iterative method. The following acts are iterated until an exit criterion is reached.

First, a computational problem is run on an analog processor such as a quantum processor. The computational problem can be embedded on the analog processor, and can be run repeatedly multiple times it can be beneficial for the multiple runs to be independent of each other. In some implementations, it can be beneficial to allow sufficient time to elapse between each run, for example, to at least reduce dependencies or correlations between runs. In one implementation, it can be beneficial to allow sufficient time to elapse between runs to allow at least a degree of spin-bath depolarization to occur.

A processor (e.g , hardware circuitry) next determines magnetizations and correlations for logical qubits and logical couplers. The expected (ideal) values are zero and minus one-third (-1/3), respectively. The processor then computes and applies one or more flux bias offsets and/or one or more coupling strengths. A flux bias offset adjustment can be calculated to cause a magnetization to be zero. A coupling strength adjustment can be calculated to cause a correlation to be -1/3.

Figure 6 is a flow chart illustrating an example implementation of a method 600 of operation of an analog processor, in accordance with the present systems, devices, articles, and methods. Method 600 of Figure 6 comprises a plurality of acts. One or more of these acts may be performed by (or via) one or more circuits, for instance one or more processors, e.g , digital processors, and analog processors such as quantum processors, or a hybrid computer (for example hybrid computer 100 of Figure 1) that includes both digital and analog processors. For the purposes of the description of Figure 6, the acts are assumed to be performed by a hybrid computer comprising a quantum processor. Those of skill in the art will appreciate that alternative implementations may omit certain acts and/or include additional acts.

Method 600 starts at 602, for example in response to submission of a computational problem or in response to an invocation by another method. At 6Q4, the digital processor of the hybrid computer sends a computational problem to the analog processor to be run, the hybrid computer embeds the computational problem on the analog processor to generate an embedded problem, runs the embedded problem on the analog processor, and returns the results to the digital processor. The problem can be run repeatedly on the analog processor multiple times. The number of runs can be between ten (10) and ten thousand (10,000). The number of runs can be selected to cause systematic biases to be detectable using statistics of the results.

At 606, the hybrid computer determines one or more statistics of the results. The statistics can include magnetizations and spin-spin

correlations, for example.

At 608, the hybrid computer computes and applies values of at least one programmable parameter of the analog processor. The

programmable parameters can include local fields, flux bias offsets, coupling strengths, and/or anneal offsets, for example.

At 610, the hybrid computer determines whether an exit criterion has been met. The exit criterion can be a single criterion or a combination of more than one criterion. Example criteria can include thresholds based on a number of iterations performed by the hybrid computer, a magnitude of a deviation of the statistics from expected statistics, a degree of improvement in the homogeneity of the statistics, a rate of convergence, and/or an execution time for method 600.

In response to determining an exit criterion has been met (YES) at 610, method 600 ends at 612. in response to determining an exit criterion has not been met (NO) at 610, control of method 600 returns to 604 where method 600 begins a further iteration of acts 604, 606, 608, and 610.

Statistics can include first-order statistics such as the magnetization or average magnetization. Statistics can include second-order statistics such as spin-spin correlations of coupled pairs of qubits. Statistics can include higher-order statistics such as higher-order effects between devices. For example, statistics can include the degree of spin-spin correlation between one or more sets of three qubits arranged in a triangular lattice. Statistics can include other suitable statistical measures.

In one implementation, an adjustment to a flux bias offset can be at least approximately proportional to a measured deviation of the

magnetization from its expected value. An adjustment to a coupling strength can be at least approximately proportional to a measured deviation of the correlation from its expected value

in one implementation, the flux bias offset adjustment and/or the coupling strength adjustment is sufficiently small to avoid overcompensation and/or ringing effects.

In one implementation, the number of iterations (the number of times method 60Q performs acts 604, 6Q6, 608, and 610) is approximately ten (10).

in one implementation, a damping parameter is included in the method described above. A benefit of the damping parameter is improved stability and/or performance of the method, for example in reducing ringing effects and/or reducing the number of iterations needed to achieve a desired outcome. A damping parameter can be used to control the size of adjustments to a flux bias offset and/or a coupling strength during each iteration.

The exit criterion can be one of the following or another exit criterion or a combination of criteria: a) when the number of iterations equals a predetermined maximum number of iterations, b) when the time equals or exceeds a predetermined time limit, c) when a magnitude of a measured deviation from the expected (ideal or nominal) value is less than a predetermined threshold, d) when the improvement in the homogeneity of the statistics is less than a predetermined threshold, e) when the rate of convergence in the improvement in the homogeneity of the statistics is less than a predetermined threshold, and/or f) another suitable criterion.

A refinement of the calibration in accordance with the systems and methods described in the present application can be performed as needed in some implementations, the refinement is typically stable and may persist over time. An occasional adjustment (also referred to in the present application as a shim) may be needed to improve the statistics.

The above description of illustrated embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Although specific embodiments of and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art. The teachings provided herein of the various embodiments can be applied to other analog processors, not necessarily the exemplary quantum processors generally described above.

The various embodiments described above can be combined to provide further embodiments. To the extent that they are not inconsistent with the specific teachings and definitions herein, ail of the US patent application publications, US patent applications, U.S. patents, International patent applications, foreign patents, and foreign patent applications referred to in this specification and/or listed in the Application Data Sheet that are commonly assigned to D-Wave Systems Inc. are incorporated herein by reference, in their entirety, including but not limited to: US Patent 7,984,012; US Patent

8,244,882; US Patent 8, 174,305; PCT Patent Publication WO2Q12064974; US Patent Application Publication No. US2015/0032994; PCT Patent Application Publication No. WO2017075246; US Patent Application Publication No.

2015/363708; US Patent Application No. 15/448361 ; and US Provisional Patent Application No. 62/620282. Aspects of the embodiments can be modified, if necessary, to employ systems, circuits and concepts of the various patents, applications and publications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of the above-detailed description in general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.