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.