We present and demonstrate a general three-step method for extracting the quantum efficiency of dispersive qubit readout in circuit QED. We use active depletion of post-measurement photons and optimal integration weight functions on two quadratures to maximize the signal-to-noise ratio of the non-steady-state homodyne measurement. We derive analytically and demonstrate experimentally that the method robustly extracts the quantum efficiency for arbitrary readout conditions in the linear regime. We use the proven method to optimally bias a Josephson traveling-wave parametric amplifier and to quantify different noise contributions in the readout amplification chain.

Quantum computing holds the promise of exponential speedup compared to classical computing for select algorithms and applications. Relatively small numbers of logical quantum bits or qubits could outperform the largest of supercomputers. Quantum dots in Si-based heterostructures and superconducting Josephson junctions are just two of the many approaches to construct the qubit. These, in particular, bear similarities to the transistors and interconnects used in advanced semiconductor manufacturing. While initial results on few-qubit systems are promising, advanced process control is expected to improve the qubit uniformity, coherence time, and gate fidelity needed for larger systems. This can be realized through the systematic characterization of film growth, interface control, and patterning.

We present a scalable scheme for executing the error-correction cycle of a monolithic surface-code fabric composed of fast-flux-tunable transmon qubits with nearest-neighbor coupling. An eight-qubit unit cell forms the basis for repeating both the quantum hardware and coherent control, enabling spatial multiplexing. This control uses three fixed frequencies for all single-qubit gates and a unique frequency-detuning pattern for each qubit in the cell. By pipelining the interaction and readout steps of ancilla-based X- and Z-type stabilizer measurements, we can engineer detuning patterns that avoid all second-order transmon-transmon interactions except those exploited in controlled-phase gates, regardless of fabric size. Our scheme is applicable to defect-based and planar logical qubits, including lattice surgery.