Fast dispersive readout of superconducting qubits for fault-tolerant quantum computing*

Fault-tolerant quantum computing requires repetitive high-fidelity quantum parity measurements. In quantum processors based on circuit QED, the fidelity of indirect quantum parity measurements using an ancillary qubit is compromised by errors induced in the coherent interaction step and in the ancilla measurement. Here, we improve upon the state of the art by combining two techniques to reduce ancilla measurement time and measurement-induced cross-dephasing of data qubits: dedicated Purcell filtering for each qubit and active photon depletion. We find the parity measurement speed limit by minimizing the parity measurement error rate as a function of the cycle time. We individually quantify the error contributions of one- and two-qubit gates, residual interactions, cross-talk, parasitic measurement-induced dephasing, and quantum demolition.