Abstract: In a general aspect, a quantum logic gate is performed in a quantum computing system. In some cases, a pair of qubits are defined in a quantum processor; the pair of qubits can include a first qubit defined by a first qubit device in the quantum processor and a second qubit defined by a tunable qubit device in the quantum processor. A quantum logic gate can be applied to the pair of qubits by communicating a control signal to a control line coupled to the tunable qubit device. The control signal can be configured to modulate a transition frequency of the tunable qubit device at a modulation frequency, and the modulation frequency can be determined based on a transition frequency of the first qubit device.

Abstract: In a general aspect, a qubit device includes two circuit loops. In some aspects, a first circuit loop includes a first Josephson junction, a second circuit loop includes a second Josephson junction, and the first and second loops are configured to receive a magnetic flux that defines a transition frequency of a qubit device. In some aspects, a quantum integrated circuit includes an inductor connected between a first circuit node and a second circuit node; the first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and the second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node.

Abstract: In a general aspect, calibration is performed in a quantum computing system. In some cases, domains of a quantum computing system are identified, where the domains include respective domain control subsystems and respective subsets of quantum circuit devices in a quantum processor of the quantum computing system. Sets of measurements are obtained from one of the domains and stored in memory. Device characteristics of the quantum circuit devices of the domain are obtained based on the set of measurements, and the device characteristics are stored in a memory of the control system. Quantum logic control parameters for the subset of quantum circuit devices of the domain are obtained based on the set of measurements and stored in memory.

Abstract: In a general aspect, quantum computing system performance is tested. Systems and methods for testing hardware in a quantum computing system are described. The methods may include certification/decertification of data produced by the quantum computing system, detection of faults, correction of errors and/or recalibration/replacement of the quantum computing system or a quantum computing subsystem.

Abstract: In a general aspect, a qubit device includes two circuit loops. In some aspects, a first circuit loop includes a first Josephson junction, a second circuit loop includes a second Josephson junction, and the first and second loops are configured to receive a magnetic flux that defines a transition frequency of a qubit device. In some aspects, a quantum integrated circuit includes an inductor connected between a first circuit node and a second circuit node; the first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and the second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node.

Abstract: In a general aspect, a microwave quantum circuit includes an on-chip impedance matching circuit. In some cases, a microwave quantum circuit includes a dielectric substrate, a quantum circuit device on the substrate, and an impedance matching circuit device on the substrate. The quantum circuit device includes a Josephson junction, and the impedance matching circuit device is coupled to the quantum circuit device on the substrate.

Abstract: In a general aspect, a quantum logic gate is performed in a quantum computing system. In some cases, a pair of qubits are defined in a quantum processor; the pair of qubits can include a first qubit defined by a first qubit device in the quantum processor and a second qubit defined by a tunable qubit device in the quantum processor. A quantum logic gate can be applied to the pair of qubits by communicating a control signal to a control line coupled to the tunable qubit device. The control signal can be configured to modulate a transition frequency of the tunable qubit device at a modulation frequency, and the modulation frequency can be determined based on a transition frequency of the first qubit device.

Abstract: In a general aspect, calibration is performed in a quantum computing system. In some cases, domains of a quantum computing system are identified, where the domains include respective domain control subsystems and respective subsets of quantum circuit devices in a quantum processor of the quantum computing system. Sets of measurements are obtained from one of the domains and stored in memory. Device characteristics of the quantum circuit devices of the domain are obtained based on the set of measurements, and the device characteristics are stored in a memory of the control system. Quantum logic control parameters for the subset of quantum circuit devices of the domain are obtained based on the set of measurements and stored in memory.

Abstract: In a general aspect, calibration is performed in a quantum computing system. In some cases, domains of a quantum computing system are identified, where the domains include respective domain control subsystems and respective subsets of quantum circuit devices in a quantum processor of the quantum computing system. Sets of measurements are obtained from one of the domains and stored in memory. Device characteristics of the quantum circuit devices of the domain are obtained based on the set of measurements, and the device characteristics are stored in a memory of the control system. Quantum logic control parameters for the subset of quantum circuit devices of the domain are obtained based on the set of measurements and stored in memory.

Abstract: In a general aspect, a qubit device includes two circuit loops. In some aspects, a first circuit loop includes a first Josephson junction, a second circuit loop includes a second Josephson junction, and the first and second loops are configured to receive a magnetic flux that defines a transition frequency of a qubit device. In some aspects, a quantum integrated circuit includes an inductor connected between a first circuit node and a second circuit node; the first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and the second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node.

Abstract: In a general aspect, a quantum logic gate is performed in a quantum computing system. In some cases, a pair of qubits are defined in a quantum processor; the pair of qubits can include a first qubit defined by a first qubit device in the quantum processor and a second qubit defined by a tunable qubit device in the quantum processor. A quantum logic gate can be applied to the pair of qubits by communicating a control signal to a control line coupled to the tunable qubit device. The control signal can be configured to modulate a transition frequency of the tunable qubit device at a modulation frequency, and the modulation frequency can be determined based on a transition frequency of the first qubit device.

Abstract: In a general aspect, a quantum computing system includes ancilla qubit devices. In some aspects, a quantum computing system includes a quantum processor cell that includes a superconducting quantum circuit system. The superconducting quantum circuit system includes a tunable-frequency primary qubit device; a flux-bias device coupled to the tunable-frequency primary qubit device; and a fixed-frequency ancilla qubit device. The fixed-frequency ancilla qubit device is connected only to the tunable-frequency primary qubit device in the superconducting quantum circuit system. The quantum computing system also includes a control system communicably coupled to the quantum processor cell. The control system is configured to apply a parametrically-activated two-qubit quantum logic gate to the tunable-frequency primary qubit device and the fixed-frequency ancilla qubit device by sending, to the flux-bias device, a radio-frequency control signal that modulates the tunable-frequency primary qubit device.

Abstract: In a general aspect, calibration is performed in a quantum computing system. In some cases, domains of a quantum computing system are identified, where the domains include respective domain control subsystems and respective subsets of quantum circuit devices in a quantum processor of the quantum computing system. Sets of measurements are obtained from one of the domains and stored in memory. Device characteristics of the quantum circuit devices of the domain are obtained based on the set of measurements, and the device characteristics are stored in a memory of the control system. Quantum logic control parameters for the subset of quantum circuit devices of the domain are obtained based on the set of measurements and stored in memory.

Abstract: In a general aspect, a qubit device includes two circuit loops. In some aspects, a first circuit loop includes a first Josephson junction, a second circuit loop includes a second Josephson junction, and the first and second loops are configured to receive a magnetic flux that defines a transition frequency of a qubit device. In some aspects, a quantum integrated circuit includes an inductor connected between a first circuit node and a second circuit node; the first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and the second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node.

Abstract: In a general aspect, a quantum logic gate is performed in a quantum computing system. In some cases, a pair of qubits are defined in a quantum processor; the pair of qubits can include a first qubit defined by a first qubit device in the quantum processor and a second qubit defined by a tunable qubit device in the quantum processor. A quantum logic gate can be applied to the pair of qubits by communicating a control signal to a control line coupled to the tunable qubit device. The control signal can be configured to modulate a transition frequency of the tunable qubit device at a modulation frequency, and the modulation frequency can be determined based on a transition frequency of the first qubit device.

Abstract: In a general aspect, calibration is performed in a quantum computing system. In some cases, domains of a quantum computing system are identified, where the domains include respective domain control subsystems and respective subsets of quantum circuit devices in a quantum processor of the quantum computing system. Sets of measurements are obtained from one of the domains and stored in memory. Device characteristics of the quantum circuit devices of the domain are obtained based on the set of measurements, and the device characteristics are stored in a memory of the control system. Quantum logic control parameters for the subset of quantum circuit devices of the domain are obtained based on the set of measurements and stored in memory.

Abstract: In a general aspect, a computing system is configured to execute a quantum approximate optimization algorithm. In some aspects, a control system identifies a pair of qubit devices in a quantum processor. The quantum processor includes a connection that provides coupling between the pair of qubit devices. ZZ coupling between the pair of qubit devices is activated to execute a cost function defined in the quantum approximate optimization algorithm. The cost function is associated with a maximum cut problem, and the ZZ coupling is activated by allowing the pair of qubits to evolve under a natural Hamiltonian for a time period ?. One or more of the pair of qubit devices is measured to obtain an output from an execution of the quantum approximate optimization algorithm.

Abstract: In a general aspect, a quantum computing method is described. In some aspects, a control system in a quantum computing system assigns subsets of qubit devices in a quantum processor to respective cores. The control system identifies boundary qubit devices residing between the cores in the quantum processor and generates control sequences for each respective core. A signal delivery system in communication with the control system and the quantum processor receives control signals to execute the control sequences, and the control signals are applied to the respective cores in the quantum processor.

Abstract: In a general aspect, a microwave quantum circuit includes an on-chip impedance matching circuit. In some cases, a microwave quantum circuit includes a dielectric substrate, a quantum circuit device on the substrate, and an impedance matching circuit device on the substrate. The quantum circuit device includes a Josephson junction, and the impedance matching circuit device is coupled to the quantum circuit device on the substrate.

Abstract: In a general aspect, a qubit device includes two circuit loops. In some aspects, a first circuit loop includes a first Josephson junction, a second circuit loop includes a second Josephson junction, and the first and second loops are configured to receive a magnetic flux that defines a transition frequency of a qubit device. In some aspects, a quantum integrated circuit includes an inductor connected between a first circuit node and a second circuit node; the first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and the second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node.