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, 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, methods and systems are described for dynamically partitioning and virtualizing a monolithic quantum-classical hybrid computing resource into multiple different and independently-saleable, as a resource to a user, parcels. These parcels may comprise configurations of qubits and qubit-qubit links on one or more quantum processor units for use by users for running computer programs.

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, 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: 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 computer system includes a low-latency communication link between a classical computer and a quantum computing resource. In some cases, a quantum machine image operates on a classical computer system. The quantum machine image includes a virtualized execution environment for quantum programs. The quantum machine image is engaged with a quantum processing unit of a quantum computing system. A quantum program is communicated over a low-latency communication pathway from the classical computer system to the quantum computer system. The quantum program is executed at the quantum computer system.

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, 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, an integrated quantum circuit includes a first substrate and a second substrate. The first substrate includes a first surface and a recess formed in the first substrate along the first surface. The recess has a recess surface and is configured to enclose a quantum circuit element. The first substrate includes a first electrically-conductive layer disposed on the first surface and covering at least a portion of the recess surface. The first electrically-conductive layer includes a first superconducting material. The second substrate includes a second surface and a quantum circuit element. The second substrate includes a second electrically-conductive layer on the second surface that includes a second superconducting material. The first substrate is adjacent the second substrate to enclose the quantum circuit device within the recess. The first electrically-conductive layer of the first substrate is electrically-coupled to the second electrically-coupled layer of the second substrate.

Abstract: In a general aspect, a tunable qubit device is identified that exhibits a frequency-dependent energy relaxation process in a quantum processor cell. The frequency-dependent energy relaxation process is produced by a material defect in the quantum processor cell. A first qubit frequency associated with a first relaxation time of the tunable qubit device is identified and a second qubit frequency associated with a second relaxation time of the tunable qubit device is identified. The second relaxation time is shorter than the first due to the frequency-dependent energy relaxation process produced by the material defect. The tunable qubit device is operated at the first qubit frequency while processing quantum information in the quantum processor cell. The tunable qubit device is tuned from the first qubit frequency to the second qubit frequency. A qubit state of the qubit device is reset by the frequency-dependent energy relaxation process produced by the material defect.

Abstract: In some aspects, a hybrid quantum-classical computing platform may comprise: a first quantum processor unit (QPU); a second QPU; and a shared classical memory, the shared classical memory being connected to both the first QPU and the second QPU, wherein the shared classical memory is configured to share data between the first QPU and the second QPU. In some embodiments, the first QPU operates at a higher repetition rate and/or clock rate than the second QPU and the second QPU operates with a higher fidelity than the first QPU.

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: A compiler translates programs for execution on a quantum processing system. To facilitate portability of quantum programs across differently configured quantum processors, the compiler accepts a specification of the quantum processor as input along with a quantum program for compilation. A specification may include information about the type of each qubit device in the quantum processor, the number of qubits, the qubit topology, coherence times of individual qubits, and operations that the quantum processor supports. The compilation process may include manipulating operations of the input program to generate equivalent operations that can be performed by the quantum gates and qubit devices on the quantum processor for which the program is being compiled.

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 a general aspect, an integrated microwave circuit is disclosed for processing quantum information. The integrated microwave circuit includes a substrate having a first surface and a second surface opposite the first surface. The substrate is formed of a silicon oxide material having a loss tangent no greater than 1×10?5 at cryogenic temperatures at or below 120 K. The integrated microwave circuit also includes qubit circuitry disposed on the first surface that includes a Josephson junction. A ground plane is disposed on the first surface or the second surface. In some variations, the silicon oxide material is fused silica. In other variations, the silicon oxide material is crystalline quartz.

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 some aspects, a flexible cable may comprise: a flexible strip with first and second parallel surfaces and first and second ends, said flexible strip being electrically insulating; a metal stripline within said flexible strip; first and second metallic grounding planes on said first and second surfaces, respectively; and a first circuit board mechanically attached to at least one of said first end of said flexible strip and said first and second metallic grounding planes at said first end, said first circuit board being mechanically stiff, said metal stripline being electrically connected to electrical circuitry on said first circuit board.