Ancilla qubit devices in a superconducting quantum processor

A quantum computing method comprising:

• storing information in a quantum processor cell comprising a superconducting quantum circuit system, the superconducting quantum circuit system comprising:
• 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 connected only to the tunable-frequency primary qubit device;
• and
• by operation of a control system communicably coupled to the quantum processor cell, applying a parametrically-activated two-qubit quantum logic gate to the tunable-frequency primary qubit device and the fixed-frequency ancilla qubit device, wherein applying the parametrically-activated two-qubit quantum logic gate comprises sending, to the flux-bias device, a radio-frequency control signal that modulates the tunable-frequency primary qubit device.

A quantum computing system comprising:

• a quantum processor cell comprising a superconducting quantum circuit system, the superconducting quantum circuit system comprising:
• tunable-frequency primary qubit devices;
• flux-bias devices coupled to the respective tunable-frequency primary qubit devices;
• fixed-frequency ancilla qubit devices, each fixed-frequency ancilla qubit device connected only to a respective one of the tunable-frequency primary qubit devices;
• and
• connections between neighboring pairs of primary and ancilla qubit devices, each neighboring pair comprising one of the tunable-frequency primary qubit devices and one of the fixed-frequency ancilla qubit devices;
• and
• a control system communicably coupled to the quantum processor cell and configured to apply parametrically-activated two-qubit quantum logic gates to the neighboring pairs, wherein the control system is configured to apply the parametrically-activated two-qubit quantum logic gates by sending, to the flux-bias device coupled to the tunable-frequency primary qubit device in a respective neighboring pair, a radio-frequency control signal that modulates the tunable-frequency primary qubit device in the respective neighboring pair.