All-Optical Transient Phase Detection in a 3-Photon Rydberg-Atom Electrometer*

Radio frequency (RF) electrometry with Rydberg atoms offers multiple advantages over traditional antennas, including self-calibration, sub-wavelength resolution, all-dielectric construction, and a broad sensing bandwidth. Typical atom-based sensors rely on optical excitation of alkali atoms via a ladder scheme to Rydberg states, which are optically probed using electromagnetically induced transparency or absorption. These sensors are traditionally not considered phase sensitive without the use of closed-loop excitation schemes or auxiliary RF fields, as steady-state absorption is phase-independent. However, we demonstrate that a 3-photon ladder excitation scheme in a room temperature cesium vapour cell is capable of detecting transient changes in the RF field’s phase. Phase shifts disturb the coherence on the RF transition and are converted to a damped oscillatory amplitude response in the probe laser’s absorption that identifies RF detuning, amplitude, and phase shift magnitude and direction. The wavevector matching in the 3-photon system minimizes Doppler broadening and results in narrow <300 kHz linewidths that provide the necessary coherence for sensing transient phase shifts. Along with providing a high RF sensitivity, 45 nV/cm/√Hz, we show that the 3-photon system is useful for receiving digital communications, including differential quadrature amplitude modulation, and for detecting target velocity in pulse-Doppler radar using phase modulation.