BACKGROUND

Field

This disclosure generally relates to systems, devices, methods, and articles for quantum computation, and, in particular, for improving the performance and efficiency of quantum annealing.

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, where electronic spin is used as a resource, 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

Quantum computation and quantum information processing are active areas of research and define classes of vendible products. 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 quantum binary digits, known as qubits. Quantum computers hold the promise of providing exponential speedup for certain classes of computational problems such as computational problems simulating quantum physics. Useful speedup may exist for other classes of problems.

One model of quantum computing is adiabatic quantum computing. Adiabatic quantum computing can be suitable for solving hard optimization problems, for example. Further details on adiabatic quantum computing systems, methods, and apparatus are described, for example, in U.S. Pat. No. 7,135,701 entitled “ADIABATIC QUANTUM COMPUTATION WITH SUPERCONDUCTING QUBITS”; and U.S. Pat. No. 7,418,283 “ADIABATIC QUANTUM COMPUTATION WITH SUPERCONDUCTING QUBITS”.

Quantum Annealing

Quantum annealing is a computational method that may be used to find a low-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. In quantum annealing, thermal effects and other noise may be present. The final low-energy state may not be the global energy minimum.

Adiabatic quantum computation 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. Thus, 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.

Quantum annealing uses quantum mechanics as a source of delocalization, sometimes called disorder, during the annealing process.

The foregoing examples of the related art, and limitations related thereto, are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.

BRIEF SUMMARY

Methods and systems to passively and/or actively mitigate the effects of spin-bath polarization are described and illustrated.

Passive mitigation can be realized by, for example a) reducing the time for which the quantum processor stores state information in the body of each qubit; b) copying the state of the QFP into the next shift register stage to eliminate or at least reduce a contribution to spin-bath polarization caused by galvanic coupling of each qubit and the respective QFP; c) modifying the read-out protocol to eliminate or at least reduce a qubit- or QFP-sourced spin-bath polarization by storing qubit state information in the QFP; d) introducing a sufficient delay between one annealing cycle and the next annealing cycle to allow the spin-bath polarization to decay i.e., allow enough time for spin-bath depolarization; e) adding a passive delay between anneals that is equal to the schedule duration for anneal schedules that differ from a standard forward anneal pattern; f) modifying a duty cycle and programming protocols to minimize, or at least reduce, on-chip heating, and, in particular, to minimize, or at least reduce, fluctuations in temperature; g) employing a fabrication process that reduces the spin bath polarization signal; h) raising the temperature to reduce the degree of spin-bath polarization; i) replace galvanic coupling of one or more of the qubits to their respective QFPs by non-galvanic coupling; and/or j) replace at least some of the wiring used for inductance by one or more Josephson junctions.

Active mitigation can be realized by, for example: 1) performing an annealing cycle, 2) while the qubit barrier is high (i.e., beyond the freeze-out point), reading out each qubit by shifting the final state of each qubit to a QFP, such that the spin orientation of the QFP reflects the spin orientation of the respective qubit; 3) setting the compound Josephson junction (CJJ) bias of a buffer QFP between a qubit and its respective QFP to zero to cause the buffer QFP to behave as an antiferromagnetic (AFM) coupler, and keeping the QFP latched, and with the buffer QFP behaving as an AFM coupler, persistent current in the QFP can generate a flux bias in the qubit via the buffer QFP; 4) raising the qubit barrier, and, in so doing, cause the qubit state (spin orientation) to be the opposite of the QFP, and the opposite of its final state after the annealing cycle; 5) keeping the qubit latched for a sufficiently long time to eliminate, or at least reduce, the spin-bath polarization; 6) lower the qubit barrier, and/or 7) reading out the final qubit state after the annealing cycle from the QFP. In some implementations, each of the qubit CJJ bias lines are operated separately, and in sequence, to at least reduce the likelihood of swamping the bias from the QFP with a bias from one or more neighboring qubits.

A method of operation of a hybrid computing system comprising a quantum processor may be summarized as including: performing a first quantum annealing cycle by the quantum processor; and actively mitigating by the quantum processor a spin-bath polarization.

The quantum processor may include a plurality of superconducting qubits, and performing a first quantum annealing cycle by the quantum processor may include performing a first quantum annealing cycle by the quantum processor to generate a first state of a qubit of the plurality of superconducting qubits.

The method of operation of a hybrid computing system comprising a quantum processor may further include reading out the first state of the qubit.

The method of operation of a hybrid computing system comprising a quantum processor may further include performing a second quantum annealing cycle by the quantum processor after actively mitigating by the quantum processor the spin-bath polarization.

Performing a second quantum annealing cycle by the quantum processor after actively mitigating by the quantum processor the spin-bath polarization may include performing a second quantum annealing cycle by the quantum processor after ceasing actively mitigating by the quantum processor the spin-bath polarization. Reading out the first state of the qubit may include: raising a tunneling barrier of the qubit; and shifting the first state of the qubit to a quantum flux parametron (QFP). Shifting the first state of the qubit to a QFP may include causing the spin orientation of the QFP to be the same as the spin orientation of the qubit.

The method of operation of a hybrid computing system comprising a quantum processor may further include: causing a buffer QFP to behave as an antiferromagnetic (AFM) coupler, the buffer QFP providing AFM coupling between the qubit and the QFP; raising the tunneling barrier of the qubit; and lowering the tunneling barrier of the qubit after a determined duration.

Causing a buffer QFP to behave as an antiferromagnetic (AFM) coupler may include setting a compound Josephson junction (CJJ) bias of a buffer QFP to zero. Actively mitigating a spin-bath polarization may include at least partially depolarizing by the quantum processor the spin-bath polarization. At least partially depolarizing the spin-bath polarization may include: flipping the first state of the qubit of the quantum processor to a second state; and latching the qubit in the second state for a determined duration. Flipping the first state of the qubit of the quantum processor to a second state may include flipping the first state of the qubit of the quantum processor to a second state opposite the first state. Flipping the first state of the qubit of the quantum processor to a second state opposite the first state may include: reading out the first state of the qubit; and causing the state of the qubit to be the opposite state of the first state. Performing an annealing cycle by the quantum processor to generate a first state of a qubit of the quantum processor may include performing an annealing cycle by the quantum processor to generate a first spin orientation of a qubit of the quantum processor. Flipping the first state of the qubit of the quantum processor to a second state may include flipping the first state of the qubit of the quantum processor to a second spin orientation, the second spin orientation opposite to the first spin orientation.

A hybrid computing system may be summarized as including: a quantum processor, and a control system including circuitry communicatively coupled to control and to readout from the quantum processor, the control system which in response to executable control logic performs any of the above methods.

