Optically-biased Rydberg microwave receiver enabled by hybrid nonlinear interferometry

The coupling of Rydberg vapour medium to both microwave and optical fields allows harnessing the merits of all-optical detection, e.g. weak disruption of the measured field and invulnerability to extremely strong fields, owing to the lack of a conventional antenna in the detector. However, the highest sensitivity in this approach is typically achieved by introducing an additional microwave field acting as a local oscillator, thereby compromising the all-optical nature of the measurement. Here we propose an alternative method, optical-bias detection, that allows truly all-optical operation, while retaining exceptional sensitivity. We tackle the issue of laser phase noise, emerging in this type of detection, via a simultaneous measurement of the laser phase noise in a nonlinear process and real-time data processing, which overall yields an improvement of $35\ \mathrm{dB}$ in terms of signal-to-noise ratio compared with the basic approach. We report the sensitivity of $176\ \mathrm{nV/cm/\sqrt{Hz}}$ and reliable operation up to $3.5\ \mathrm{mV/cm}$ of $13.9\ \mathrm{GHz}$ electric field. We also demonstrate a quadrature-amplitude modulated data transmission, underlining the capability of the system to detect quadratures of the microwave field. This approach is thus directly comparable to the state-of-the-art superheterodyne, while retaining the merits of all-optical detection.