Fig. 1: Microwave heterodyne detection principle and its realization with a quantum sensor.

a Heterodyne detection of a multimode microwave frequency field relative to an external frequency reference using the NV center sensor as a mixer. The interaction with the signal is controlled by application of dressing fields. We benchmark two techniques: (1) pulsed Mollow absorption using dynamical decoupling sequence and (2) Floquet dynamics under strong RF-driving. The demodulated signal leads to high spectral resolution. b Heterodyne detection by creating the same initial state \(\left|{\psi }_{{\rm{init}}}\right\rangle\) (blue) from a coherent external MW-source between sequential measurements. A long coherent signal Ω(t) (orange) is stroboscopically observed in the rotating frames. Finally, the z-projection (green) of \(\left|{\psi }^{\prime}\right\rangle\), the final state after evolution under Ω, is measured, allowing to reconstruct the relative microwave phase.