The Pound-Drever-Hall Method for Superconducting-Qubit Readout

Scaling quantum computers to large sizes relies on the ability to implement many readout systems in parallel. Here we present a superconducting-qubit readout method using the self-phase referenced Pound-Drever-Hall (PDH) technique, originally developed for use with optical cavities. Our experimental apparatus benchmarks PDH readout of a single transmon qubit using room-temperature heterodyne detection of all three tones to reconstruct the PDH signal. We demonstrate that PDH qubit readout is insensitive to the clocking microwave signal generators, displaying single shot readout in the presence of phase errors exceeding the qubit-induced phase shift, as well as 1.4° phase stability over 2 hours. Furthermore, the discovery of the "scissors phase" allows stable, qubit-dependent phase readout even in the presence of timing offsets among the carrier generator, modulator, and digitizer.