Transient Phase Sensing in a Three-Photon Rydberg Ladder Scheme

Although Rydberg atoms have shown promise for use in novel types of radio frequency (rf) receivers, they have generally not been considered phase sensitive without the use of closed-loop interferometry or auxiliary rf fields. Here, we show that the high coherency of a narrow-linewidth three-photon ladder excitation scheme unique to cesium atoms enables all-optical sensing of transient changes in rf phase within a room temperature vapor cell. The transient response on the probe laser's transmission originates from phase-to-amplitude conversion via a disturbance of the coherency of the system in response to the phase shift of the rf field. We show that the amplitude and frequency of the oscillatory response provide information on the magnitude and direction of any rf field detuning. We demonstrate that the detuning sensitivity can be used to identify Doppler shifts in radar applications, by applying phase shifts embedded in rf pulses. The phase modulation within the radar pulse acts as a form of compression that facilitates the simultaneous detection of both target position and velocity.