Google Inc.

Yuezhen Niu

Vadim Smelyanskiy

Sergio Boixo Castrillo

Hartmut Neven

Masoud Mohseni

Yu Chen

Zhang Jiang

Charles Neill

Alexander Nikolaevich Korotkov

Kostyantyn Yevgenovych Kechedzhi

Trevor Johnathan Mccourt

Xiao Mi

Systems and methods for composite quantum gate calibration for a quantum computing system are provided. In some implementations, a method includes accessing a unitary gate model describing a composite quantum gate. The unitary gate model includes a plurality of gate parameters. The method includes implementing the composite quantum gate for a plurality of gate cycles on the quantum system to amplify the plurality of gate parameters. The method includes obtaining a measurement of a state of the quantum system after implementing the composite quantum gate for the plurality of gate cycles. The method includes determining at least one of the plurality of gate parameters based at least in part on the measurement of the state of the quantum system. The method includes calibrating the composite quantum gate for the quantum computing system based at least in part on the plurality of gate parameters.

Composite quantum gate calibration

Errors that affect a quantum computer can be efficiently measured and characterized by placing the quantum computer in a highly-entangled state such as a Greenberger-Horne-Zeilinger (GHZ) state, accumulating quantum errors in the highly entangled state, and then measuring the accumulated errors. In some approaches, the error characterization includes measuring parity oscillations of the GHZ state and fitting a quantum error model to a power spectrum of the parity oscillations. The fitted quantum error model can be used to select a suitable fault-tolerant error correction scheme for the quantum computer given its environmental noise.

Characterization of time-correlated quantum errors through entanglement

Methods, systems and apparatus for implementing a quantum gate on a quantum system comprising a second qubit coupled to a first qubit and a third qubit. In one aspect, a method includes evolving a state of the quantum system for a predetermined time, wherein during evolving: the ground and first excited state of the second qubit are separated by a first energy gap ω; the first and second excited state of the second qubit are separated by a second energy gap equal to a first multiple of ω minus qubit anharmoniticity η; the ground and first excited state of the first qubit and third qubit are separated by a third energy gap equal to ω−η; and the first and second excited state of the first qubit and third qubit are separated by a fourth energy gap equal to the first multiple of the ω minus a second multiple of η.

Three qubit entangling gate through two-local Hamiltonian control

Methods, systems, and apparatus for implementing a unitary quantum gate on one or more qubits. In one aspect, a method includes the actions designing a control pulse for the unitary quantum gate, comprising: defining a universal quantum control cost function, wherein the control cost function comprises a qubit leakage penalty term representing i) coherent qubit leakage, and ii) incoherent qubit leakage across all frequency components during a time dependent Hamiltonian evolution that realizes the unitary quantum gate; adjusting parameters of the time dependent Hamiltonian evolution to vary a control cost according to the control cost function such that leakage errors are reduced; generating the control pulse using the adjusted parameters; and applying the control pulse to the one or more qubits to implement the unitary quantum gate.

Universal control for implementing quantum gates

The present disclosure provides systems and methods to measure quantum gate fidelity through swap spectroscopy. In particular, aspects of the present disclosure are directed to the derivation and use of a physical model that models non-Markovian quantum dynamics of interactions between one or more qubits of a quantum gate and one or more two-level-system (TLS) defects during operation of the quantum gate.

Systems and methods to measure quantum gate fidelity through swap spectroscopy

Methods, systems, and apparatus for designing a quantum control trajectory for implementing a quantum gate using quantum hardware. In one aspect, a method includes the actions of representing the quantum gate as a sequence of control actions and applying a reinforcement learning model to iteratively adjust each control action in the sequence of control actions to determine a quantum control trajectory that implements the quantum gate and reduces leakage, infidelity and total runtime of the quantum gate to improve its robustness of performance against control noise during the iterative adjustments.

Quantum computation through reinforcement learning

Methods, systems and apparatus for implementing a quantum gate on a quantum system comprising a second qubit coupled to a first qubit and a third qubit. In one aspect, a method includes evolving a state of the quantum system for a predetermined time, wherein during evolving: the ground and first excited state of the second qubit are separated by a first energy gap &#x3c9;; the first and second excited state of the second qubit are separated by a second energy gap equal to a first multiple of &#x3c9; minus qubit anharmoniticity&#x2212;; the ground and first excited state of the first qubit and third qubit are separated by a third energy gap equal to &#x3c9;&#x2212;; and the first and second excited state of the first qubit and third qubit are separated by a fourth energy gap equal to the first multiple of the &#x3c9; minus a second multiple of.

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El Segundo, CA, United States of America

Yuezhen Niu