Abstract: In some aspects, a quantum computing system includes a multi-dimensional array of qubit devices. Coupler devices reside at intervals between neighboring pairs of the qubit devices in the multi-dimensional array. Each coupler device is configured to produce an electromagnetic interaction between one of the neighboring pairs of qubit devices. In some cases, each qubit device has a respective qubit operating frequency that is independent of an offset electromagnetic field experienced by the qubit device, and the coupling strength of the electromagnetic interaction provided by each coupler device varies with an offset electromagnetic field experienced by the coupler device. In some cases, readout devices are each operably coupled to a single, respective qubit device to produce qubit readout signals that indicate the quantum state of the qubit device.

Abstract: In some aspects, a heterogeneous computing system includes a quantum processor unit and a classical processor unit. In some instances, variables defined by a computer program are stored in a classical memory in the heterogeneous computing system. The computer program is executed in the heterogeneous computing system by operation of the quantum processor unit and the classical processor unit. Instructions are generated for the quantum processor by a host processor unit based on values of the variables stored in the classical memory. The instructions are configured to cause the quantum processor unit to perform a data processing task defined by the computer program. The values of the variables are updated in the classical memory based on output values generated by the quantum processor unit. The classical processor unit processes the updated values of the variables.

Abstract: In a general aspect, hybrid computing systems and hybrid computing methods are described. In some cases, a program to be executed in a hybrid computing system is identified. The hybrid computing system includes a control system that includes a classical processor. The hybrid computing system includes a quantum processor that defines qubits. By operation of the control system, a set of events to execute the program is identified. By operation of the control system, an event schedule that includes resource schedules for the respective qubits is generated. The event schedule is executed in the hybrid computing system. The event schedule, when executed in the hybrid computing system, coordinates operation of the quantum processor and the classical processor.

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, 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 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: A quantum computing system that includes a quantum circuit device having at least one operating frequency; a first substrate having a first surface on which the quantum circuit device is disposed; a second substrate having a first surface that defines a recess of the second substrate, the first and second substrates being arranged such that the recess of the second substrate forms an enclosure that houses the quantum circuit device; and an electrically conducting layer that covers at least a portion of the recess of the second substrate.

Abstract: In a general aspect, a superconducting quantum circuit system is modeled. In some aspects, a graph representing a quantum circuit system is generated. The graph includes vertices and edges; the edges represent circuit elements of the quantum circuit system, and the vertices represent physical connections between the circuit elements. Inverse inductances, conductances, capacitances, and junction inverse inductances are assigned to respective edges of the graph based on a lumped-element approximation of the quantum circuit system. A coordinate system is determined based on the graph, and a matrix representation of the system is determined based on the coordinate system. A Hamiltonian for the quantum circuit system is determined using the matrix representation, and the quantum circuit system is simulated based on the Hamiltonian.

Abstract: In a general aspect, a quantum logic gate can be performed by tuning a coupler device. One or more coupler control signals can be received at a coupler device in a quantum processor cell. In some instances, in response to the coupler control signals, a coupler operating frequency of the coupler device changes toward a qubit operating frequency of a qubit device, and a phase shift arises in a quantum state of the qubit device due to an interaction between the qubit device and the coupler device. In some instances, in response to the control signals, the coupler operating frequency changes toward a first qubit operating frequency of a first qubit device, then changes toward a second qubit operating frequency of a second qubit device, and a controlled-phase shift arises in a quantum state of the qubit devices due to interactions between the coupler device and the respective qubit devices.

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 control sequence is generated for a quantum information processor. In some aspects, a quantum computation to be performed by a quantum information processor is identified. The quantum information processor includes data qubits and is configured to apply entangling quantum logic operations to respective pairs of the data qubits. A graph representing the quantum information processor is defined. The graph includes vertices and edges; the vertices represent the data qubits, and the edges represent the entangling quantum logic operations. A quantum logic control sequence is generated based on the graph. The quantum logic control sequence includes a sequence of quantum logic operations configured to perform the quantum computation when executed by the quantum information processor.

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, control of a quantum superconducting circuit is analyzed. In some implementation, a parameter set for a control signal for a superconducting quantum circuit is received. The parameters set can include initial voltage amplitudes for respective time segments of the control signal. A first subset of time segments is selected for improving a quality measure of a quantum logic operation produced by delivery of the control signal in the superconducting quantum circuit. New voltage amplitudes are calculated for one or more segments in the first subset, such that the new voltage amplitudes improve the quality measure. The parameter set is updated to include the new voltage amplitudes for the first subset while preserving the initial voltage amplitudes for a second subset of the time segments.

Abstract: A quantum computing system includes a quantum circuit device, a substrate having a first surface on which the quantum processing device is disposed, and one or more vias each extending through the substrate. The vias include a material that is a superconducting material during operation of the quantum computing system.

Abstract: In a general aspect, control of a quantum superconducting circuit is analyzed. In some implementation, a parameter set for a control signal for a superconducting quantum circuit is received. The parameters set can include initial voltage amplitudes for respective time segments of the control signal. A first subset of time segments is selected for improving a quality measure of a quantum logic operation produced by delivery of the control signal in the superconducting quantum circuit. New voltage amplitudes are calculated for one or more segments in the first subset, such that the new voltage amplitudes improve the quality measure. The parameter set is updated to include the new voltage amplitudes for the first subset while preserving the initial voltage amplitudes for a second subset of the time segments.

Abstract: In some aspects, a quantum simulation method includes generating a set of models representing a quantum system. The set of models includes subsystem models representing respective fragments of the quantum system. The quantum system is simulated by operating the set of models on a computer system that includes a classical processor unit and multiple unentangled quantum processor units (QPUs), and the unentangled QPUs operate the respective subsystem models. In some examples, density matrix embedding theory (DMET) is used to compute an approximate ground state energy for the quantum system.

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 some aspects, a quantum computing system includes a multi-dimensional array of qubit devices. Coupler devices reside at intervals between neighboring pairs of the qubit devices in the multi-dimensional array. Each coupler device is configured to produce an electromagnetic interaction between one of the neighboring pairs of qubit devices. In some cases, each qubit device has a respective qubit operating frequency that is independent of an offset electromagnetic field experienced by the qubit device, and the coupling strength of the electromagnetic interaction provided by each coupler device varies with an offset electromagnetic field experienced by the coupler device. In some cases, readout devices are each operably coupled to a single, respective qubit device to produce qubit readout signals that indicate the quantum state of the qubit device.

Abstract: In some aspects, a quantum information processing circuit includes a lumped-element device on the surface of a dielectric substrate. The lumped-element device can include a capacitor pad and an inductive transmission line. The capacitor pad can be capacitively coupled to another capacitor pad. The inductive transmission line can reside in an interior clearance area defined by an inner boundary of the capacitor pad. The lumped-element device can be, for example, a resonator device or a filter device. The inductive transmission line can be, for example, a meander inductor.

Abstract: In a general aspect, a quantum information processing circuit is analyzed. In some implementations, a linear response function of a quantum information processing circuit is generated. A linear circuit model is generated based on the linear response function. A composite circuit model is generated by combining the linear circuit model and a nonlinear circuit model. An operating parameter of the quantum information processing circuit is computed by solving the composite circuit model. In some implementations, an electromagnetic structure solver determines the linear response function based on a circuit specification, a quantum circuit analysis tool calculates the operating parameters, and the circuit specification is modified based on the operating parameters.