Continuous drive heterodyne microwave sensing with spin qubits in hexagonal boron nitride

Quantum sensors that use solid state spin defects have emerged as effective probes of weak alternating magnetic signals. By recording the phase of a signal relative to an external clock, these devices can resolve signal frequencies to a precision orders of magnitude longer than the spin state lifetime. However, these quantum heterodyne protocols suffer from sub-optimal sensitivity, as they are currently limited to pulsed spin control techniques, which are susceptible to cumulative pulse-area errors, or single continuous drives which offer no protection of the spin coherence. Here, we present a control scheme based on a continuous microwave drive that extends spin coherence towards the effective $T_2 \approx \frac{1}{2}T_1$ limit and can resolve the frequency, amplitude and phase of GHz magnetic fields. The scheme is demonstrated using an ensemble of boron vacancies in hexagonal boron nitride, and achieves an amplitude sensitivity of $\eta \approx 3-5 \:\mathrm{\mu T \sqrt{Hz}}$ and phase sensitivity of $\eta_{\phi} \approx 0.076 \:\mathrm{rads \sqrt{Hz}}$. By repeatedly referencing the phase of a resonant signal against the coherent continuous microwave drive in a quantum heterodyne demonstration, we measure a GHz signal with a resolution $<$1 Hz over a 10 s measurement. Achieving this level of performance in a two-dimensional material platform could have broad applications, from probing nanoscale condensed matter systems to integration into heterostructures for quantum networking.