Abstract: In a general aspect, iSWAP gates are calibrated on a superconducting quantum processing unit. In some aspects, initial values of control parameters to apply the iSWAP gate on a pair of qubits defined by first and second qubit devices of the superconducting quantum processing unit are identified. Phase tracking devices associated with the first and second qubit devices of the superconducting quantum processing unit are initialized. After initiating the phase tracking devices, shifts in local phases of the pair of qubits are determined based on one or more applications of the iSWAP gate to the pair of qubits. The one or more applications of the iSWAP gate to the pair of qubits is performed using the initial values of the control parameters. Updated values of one or more of the control parameters are generated based on the shifts in local phases and outputs of the phase tracking devices.

Abstract: In a general aspect, a modular quantum processing unit (QPU) includes quantum processor chips inter-connected by a cap structure. In some cases, a modular quantum processing unit includes a tunable-frequency coupler device, a first quantum processor chip, a second quantum processor chip, and a cap structure. The tunable-frequency coupler device includes a superconducting quantum interference device (SQUID) loop, a shunt capacitor, and a flux bias control line that controls a magnetic flux through the SQUID loop. The first quantum processor chip includes a first qubit device, the SQUID loop, the flux bias control line, and a first capacitive coupler device galvanically connected between the first qubit device and the tunable-frequency coupler device. The second quantum processor chip includes a second qubit device.

Abstract: A quantum computing system includes a cryostat to support a low-temperature vacuum environment during operation of the quantum computing system; a quantum processor positioned in the cryostat; a first electronic control module external to the cryostat; a second electronic control module within the cryostat; at least one optical transmission line connecting the first electronic control module external to the cryostat with the second electronic control module internal to the cryostat, the optical transmission line being configured to transmit optical signals to and from the second electronic control module during operation of the quantum computing system; and a plurality of signal lines connecting the second electronic control module with the quantum processor, a first subset of the signal lines being configured to transmit microwave signals to and from the quantum processor during operation of the quantum computing system.

Abstract: In a general aspect, an optimization problem is solved using a hybrid computing system. A classical processor unit receives a first data structure that represents the optimization problem. The classical processor unit executes a branch-and-bound process on the first data structure to generate values for a first subset of elements of a solution to the optimization problem. A second data structure is generated based on the first data structure and the first subset of elements. The second data structure represents a reduced version of the optimization problem. A quantum processor unit and a classical processor unit are used to execute a quantum approximate optimization algorithm (QAOA) on the second data structure to generate values for a second subset of the elements of the solution to the optimization problem. The first subset and second subset are combined to obtain the solution to the optimization problem.

Abstract: In a general aspect, hybrid quantum/classical algorithms are executed in a computing system. A first set of values representing a measurement of a reduced density matrix (RDM) is obtained. The first set of values is based on sampling quantum states generated by a quantum processor. A classical processor generates a second, different set of values to represent the measurement of the RDM. The second set of values is constructed based on the first set of values by a process that imposes one or more n-representability conditions on the second set of values to represent the measurement of the RDM.

Abstract: In a general aspect, a quantum processor has a modular architecture. In some aspects, a modular quantum processor includes first and second quantum processor chips and a cap structure. The first quantum processor chip is supported on a substrate layer and includes a first plurality of qubit devices. The second quantum processor chip is supported on the substrate layer and includes a second plurality of qubit devices. The cap structure is supported on the first and second quantum processor chips and includes a coupler device that provides coupling between at least one of the first plurality of qubit devices with at least one of the second plurality of qubit devices. In some instances, the coupler device is an active coupler device that is configured to selectively couple at least one of the first plurality of qubit devices with at least one of the second plurality of qubit devices.

Abstract: In a general aspect, user requests for access distributed quantum computing resources in a distributed quantum computing system are managed. In a general aspect, a job request for accessing a quantum computing resource is received. The job request includes a user id and a program. On authentication of a user associated with the job request, a job identifier is assigned to the job request, and a particular quantum computing resource is selected for the job request. The job request is individualized based on user permissions and pushed onto a queue to be processed for execution by the quantum computing resource.

Abstract: A system and method for randomly accessing pairs of quantum gates associated with any given quantum gate included in a set of quantum instructions allows for the reduction of unitary errors when executing the quantum instructions. The system generates a set of modified quantum instructions using the randomly accessed pair of quantum gates. The modified quantum instructions produce the same result as the unmodified quantum instructions when executed on a quantum processing system that does not introduce error when executing the instructions. Additionally, the modified quantum instructions produce a more accurate result with less error than the unmodified quantum instructions when executed on a quantum processing system that introduces error.

Abstract: In a general aspect, a quantum streaming kernel processes a data stream. In some aspects, an input stream of data is converted to an output stream of data by repeatedly receiving new portions of the input stream; encoding each new portion into an internal quantum state of a quantum processor; measuring a first part of the internal quantum state while maintaining coherence of a second part of the internal quantum state; and producing the output stream of data based on the measurements. In some cases, a history of the input stream is preserved by the coherence of the internal quantum state, and the measurements contain information based on the history of the input stream.

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, signals are converted between regimes in a quantum computing system. In some cases, a quantum computing system includes: a quantum processing unit, a control system, a transmission medium, and circuitry. The quantum processing unit includes a superconducting circuit, which includes a plurality of qubit devices. The control system includes a signal generator configured to generate a first control signal and encode qubit control information in the first control signal. The transmission medium is configured to couple the signal generator with a signal conversion system. The signal conversion system is configured to: receive the first control signal generated by the signal generator; and generate a second control signal based on the qubit control information encoded in the first control signal. The circuitry is configured to deliver the second control signal to the plurality of qubit devices.

Abstract: In a general aspect, a quantum error-correction technique includes applying a first set of two-qubit gates to qubits in a lattice cell, and applying a second, different set of two-qubit gates to the qubits in the lattice cell. The qubits in the lattice cell include data qubits and ancilla qubits, and the ancilla qubits reside between respective nearest-neighbor pairs of the data qubits. After the first and second sets of two-qubit gates have been applied, measurement outcomes of the ancilla qubits are obtained, and the parity of the measurement outcomes is determined.

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 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, input data for a computer process are preprocessed by a preprocessor unit that includes a quantum processor. In some aspects, a preprocessor unit obtains input data for a computer process that is configured to run on a computer processing unit. Randomized parameter values are computed for variable parameters of a quantum logic circuit based on the input data. A classical processor in the preprocessor unit computes the randomized parameter values from the input data and a set of random numbers. A quantum processor in the preprocessor unit produces quantum processor output data by executing the quantum logic circuit having the randomized parameter values assigned to the variable parameters. Preprocessed data generated based on the quantum processor output data are then provided as the input for the computer process configured to run on the computer processing unit.

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 surface code syndrome measurement is performed on a superconducting quantum processing unit. In some implementations, the superconducting quantum processing unit is caused to apply a quantum error correction code including X-type and Z-type stabilizer check patches. Each of the X-type and Z-type stabilizer check patches includes a stabilizer check qubit device and data qubit devices of the superconducting quantum processing unit. Applying the quantum error correction code includes iteratively twirling the data qubit devices in a stabilizer check patch; and evolving the stabilizer check qubit device in the stabilizer check patch and the data qubit devices in the stabilizer check patch under an interaction Hamiltonian. The interaction Hamiltonian includes a plurality of terms interactions between the stabilizer check qubit device in the stabilizer check patch and a respective one of the data qubit devices in the stabilizer check patch.

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, parametric amplification is performed in a quantum computing system. In some cases, a traveling wave parametric amplifier (TWPA) includes a plurality of Josephson junctions connected in series. The plurality of Josephson junctions includes a first Josephson junction, which includes a first superconducting electrode on a surface of a substrate, a second superconducting electrode that overlaps the first superconducting electrode, and a barrier sandwiched between overlapping sections of the first and second superconducting electrodes. The barrier defines a footprint with a tapered shape over the surface of the substrate.

Abstract: In a general aspect, a machine learning process is performed using data-parallel quantum processing. In some cases, a machine learning model is operated in a hybrid computing system. The hybrid computing system includes a quantum computing resource and a classical computing resource. The quantum computing resource includes quantum processing unit (QPU) sublattices, each including a subset of qubit devices.