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 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.