Quantum Metrology IV

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.

Funding acknowledgement

This work has been supported by the Defense Advanced Research Projects Agency (DARPA) Science of Atomic Vapors for New Technologies (SAVaNT) program under Contract No. HR00112190080, and by the National Research Council Internet of Things: Quantum Sensors Challenge program through Contract No. QSP-105-1.