Abstract: In a general aspect, a superconducting quantum processing circuit includes a tunable qubit device. The tunable qubit device includes a circuit loop configured to receive, during operation of the tunable qubit device, a magnetic flux that controls a qubit frequency of the tunable qubit device. In some aspects, the circuit loop consists of: a single non-linear circuit element and one or more linear circuit elements. The single non-linear circuit element is a Josephson junction connected between a first circuit node and a second circuit node. The one or more linear circuit elements includes a linear inductor connected in parallel with the Josephson junction between the first circuit node and the second circuit node.

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: Quantum operations can be simulated on a classical processing system using a quantum virtual machine (QVM). The QVM receives a quantum virtual state including a virtual wavefunction of n qubits. The virtual wavefunction is represented by probability amplitudes stored in a memory location of the classical processing system. The QVM simulates a received quantum operation by determining a set of virtual partial wavefunctions, accessing probability amplitudes for the virtual partial wavefunctions, and executing the quantum operation on the sub-bitstrings. The QVM can measure the result of the quantum operation, add noise, share the virtual wavefunction, or generate efficient machine instructions when simulating the quantum operation.

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 flexible cable communicates electromagnetic signals in a cryogenic system. In some aspects, the flexible cable includes a signal layer and a ground layer. The signal layer includes signal lines embedded in adhesive material. Each signal line includes a multilayer structure. The multilayer structure includes a layer of non-superconducting material and a layer of rhenium metal. The ground layer is laminated to the signal layer.

Abstract: In a general aspect, tuning the coupling strength between a qubit device and nearby control lines is described. In some implementations, a method includes identifying a design of first and second quantum processor wafers of a quantum processing system. The first quantum processor wafer includes a qubit device which includes two qubit electrodes and a SQUID loop. The second quantum processor wafer includes a control line which is configured to apply control signals to the qubit device and includes first and second control ports, a circuit loop inductively coupled to the SQUID loop, and conductive traces connected between the circuit loop and the respective first and second control ports. The control lines are capacitively coupled to the two qubit electrodes. The method includes obtaining simulation data and experimental data from measurements of the quantum processing system, and modifying the design based on the simulation data and the experimental data.

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: 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, 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 a general aspect, a superconducting via for routing electrical signals through a substrate includes the substrate and a layer formed of superconducting material. The substrate has a first orifice disposed on a first surface and a second orifice disposed on a second surface. A cavity extends through the substrate from the first orifice to the second orifice. The layer of superconducting material includes a first portion occluding the first orifice and having an exterior surface facing outward from the substrate. The layer also includes a second portion in contact with a side wall of the cavity an extending to the second orifice. A quantum circuit element may optionally be disposed on the first surface and electrically coupled to the exterior surface of the first portion of the layer.

Abstract: In some aspects, a cloud-based computer system includes: a quantum computing system comprising a quantum processing unit; a container management and execution system configured to receive a container and execute a program within the container; and a communication channel between the container management and execution system and the quantum computing system for providing program instructions to the quantum computing system. The container management and execution system and the quantum computer system may be co-located in a data center or located in different data centers. The latency of the communication channel may be selected to optimize cost for a required computer performance.

Abstract: In some implementations, 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, a superconducting quantum processing unit includes a first qubit device, a second qubit device, and a tunable floating coupler device coupled between the first and second qubit devices. Values of a coupling strength of the first and second qubit devices at a plurality of operating points of the tunable floating coupler device are measured. The operating points correspond to respective values of a magnetic flux applied to the tunable floating coupler device. Based on the measured values of the coupling strength, a parking value of the magnetic flux is identified. The parking value of the magnetic flux corresponds to a magnitude of the coupling strength being less than or equal to a threshold value; the threshold value is associated with a target gate fidelity for the superconducting quantum processing unit.

Abstract: In a general aspect, junction properties of tunnel junctions are altered by voltage-assisted annealing processes. In some cases, a method includes obtaining a circuit comprising a tunnel junction, modifying a junction resistance of the tunnel junction by applying a voltage across the tunnel junction, and obtaining a measured value of the junction resistance. The tunnel junction may include a metal and a metal oxide.

Abstract: In a general aspect, a multi-qubit quantum logic gate for a multi-qubit hardware-efficient stabilizer measurement is performed. In some implementations, a superconducting quantum processing unit includes a stabilizer check qubit device and two or more data qubit devices operably coupled to the stabilizer check qubit device through respective tunable-frequency coupler devices. A method includes applying a multi-qubit quantum logic gate on the stabilizer check qubit device and the two or more data qubit devices. Applying the multi-qubit quantum logic gate includes evolving the stabilizer check qubit device and the two or more data qubit devices under an interaction Hamiltonian with a plurality of terms.

Abstract: In a general aspect, an augmented group of operations is calibrated for execution on a quantum processing unit. In some cases, a quantum program to be executed by a quantum processing unit is obtained. The quantum program includes a sequence of native quantum logic gates. A group of operations corresponding to a time step in the sequence is identified. The group of operations includes the native quantum logic gates that are applied to a first subset of qubits in parallel during the time step. An augmented group of operations is defined by adding single-qubit identity gates to the group. The single-qubit identity gates are applied to a second subset of qubits in parallel during the time step. A calibration process is performed over the first and second subsets of qubits to determine control parameters to execute the augmented group of operations on the quantum processing unit.

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 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, a quantum state transferring process is performed in a computing network. In some implementations, a method of transferring a quantum state between nodes in a computing network includes receiving a signal at a first node transmitted on a transmission line from a second node. The first node includes a superconducting quantum processing circuit including a tunable-frequency coupler device with a superconducting circuit loop and a first resonator device having a tunable linewidth. The first resonator device is capacitively coupled to the tunable-frequency coupler coupled to the transmission line. The second node includes a second resonator device having a fixed linewidth coupled to the transmission line. The method includes modifying the tunable linewidth of the first resonator device over time while the signal transfers a quantum state to the first resonator device from the second resonator device, by varying a magnetic flux pulse applied to the superconducting circuit loop.

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.