Efficient readout for ensembles of nitrogen vacancy centers in diamond

Spin defects in solids have emerged as promising platforms for a broad array of applications in quantum
sensing, quantum information, and precision measurement. Defects in solids, such as the nitrogen-
vacancy color center in diamond, can be initialized into pure quantum states, can be coherently
controlled, and can have relatively long-lived quantum coherence at room temperature. Lack of high-
fidelity state readout, however, has so far limited the utility of solid-state quantum devices. Despite
extensive experimental effort (and significant progress in increasing fidelity in certain specialized
implementations), no universal, high-fidelity readout technique has been achieved for solid-state spin
ensembles. Here we demonstrate a novel, non-optical technique, which offers high-fidelity readout for any
solid-state spin defects with microwave-accessible transitions. By coupling ensembles of spins to a
microwave cavity, we show enhancement of the state-dependent dispersive shift produced by the
ensemble, allowing high-fidelity readout at room temperature. We demonstrate this technique by
employing an ensemble of nitrogen vacancy centers for magnetometry, achieving a sensitivity
unconstrained by optical photon shot noise and approaching the Johnson-Nyquist noise limit of the
system. These results pave a clear path to achieve unity readout fidelity of solid-state spin ensembles
through increased ensemble size, reduced spin-resonance linewidth, or improved cavity quality factor.