Scaling quantum computers to large sizes requires the implementation of many parallel qubit readouts. Here we present an ultrastable superconducting-qubit readout method using the multi-tone self-phase-referenced Pound-Drever-Hall (PDH) technique, originally developed for use with optical cavities. In this work, we benchmark PDH readout of a single transmon qubit, using room-temperature heterodyne detection of all tones to reconstruct the PDH signal. We demonstrate that PDH qubit readout is insensitive to microwave phase drift, displaying
0.73∘ phase stability over 2 hours, and capable of single-shot readout in the presence of phase errors exceeding the phase shift induced by the qubit state. We show that the PDH sideband tones do not cause unwanted measurement-induced state transitions for a transmon qubit, leading to a potential signal enhancement of at least
14~dB.
We demonstrate continuous Pound-Drever-Hall (PDH) nondestructive monitoring of the electron spin polarization of an atomic vapor in a microfabricated vapor cell within an optical resonator. The two-chamber silicon and glass cell contains
87Rb and 1.3 amg of
N2 buffer gas, and is placed within a planar optical resonator formed by two mirrors with dichroic dielectric coatings to resonantly enhance the coupling to phase-modulated probe light near the
D2 line at 780 nm. We describe the theory of signal generation in this system, including the spin-dependent complex refractive index, cavity optical transfer functions, and PDH signal response to spin polarization. We observe cavity transmission and PDH signals across approximately 200 GHz of detuning around the atomic resonance line. By resonant optical pumping on the 795-nm
D1 line, we observe spin-dependent cavity line shifts, in good agreement with theory. We use the saturation of the line shift versus optical pumping power to calibrate the number density and efficiency of the optical pumping. In the unresolved sideband regime, we observe quantum noise limited PDH readout of the spin polarization density, with a flat noise floor of 9\ifmmode\times\else\texttimes\fi{}${10}^{9}\phantom{\rule{0.1em}{0ex}}{\text{spins cm}}^{\ensuremath{-}3}\phantom{\rule{0.2em}{0ex}}{\text{Hz}}^{\ensuremath{-}1/2}$ for frequencies above 700 Hz. We note possible extensions of the technique.