All-Optical RF Phase Detection in Rydberg Atom-Based Sensors using Closed-Loop Dynamics

Rydberg atom-based radio frequency (RF) sensors offer multiple advantages over traditional RF receivers, including self-calibration and electromagnetic transparency, but to date have focused on amplitude sensing as a square law detector. Detection of phase is an important receiver feature especially for communications applications, but requires an external heterodyning field or an atomic closed-loop interferometer. Here we demonstrate all-optical RF phase detection using a closed-loop excitation scheme in a room temperature cesium vapour cell that avoids the disadvantages of heterodyning. We show that the probe laser’s transmission oscillates at a frequency determined by the detuning of the RF field and lasers in the loop, with a phase directly related to that of the RF field. Approximate closed form expressions for the probe transmission provide insight into the multi-photon interference processes in the loop as well as the experimental considerations for achieving accurate phase measurements [1]. Experimental results are consistent with theoretical predictions and demonstrate practical loop coherence times on the order of a millisecond. We demonstrate that measurements of RF amplitude and phase can be extracted from the oscillating dynamical response to an in-phase and quadrature (IQ) modulation on the RF. This expands the capability of Rydberg atom-based quantum sensors as receivers for digital communications.

[1]  M. Schmidt et. al., Phys. Rev. Lett. 135, 093602 (2025).