A Narrow Linewidth 3-Photon Scheme for Rydberg-Atom Electrometry

Rydberg states are utilized in quantum sensors that detect radio frequency (RF) electric fields with high sensitivity. In the Autler-Townes regime, Rydberg atom-based sensors are self-calibrated to well-known atomic properties. The lower limit for self-calibrated RF electric field strength measurements is set by the spectral linewidth of the probe laser’s absorption feature. In a standard 2-photon setup linewidths of several MHz are possible, but are dominated by residual Doppler broadening due to wavevector mismatch of the lasers. In this work, we demonstrate a co-linear 3-photon excitation scheme for cesium atoms that minimizes wavevector mismatch and achieves sub-200 kHz linewidths at room temperature. In the self-calibrated regime, we resolve continuous wave RF electric fields down to 300 μV/cm amplitudes at 10.7 and 109 GHz. We show that the linewidth is no longer limited by wavevector mismatch, but other effects including transit time broadening, laser linewidths, and power broadening. We also discuss sensitivity limits beyond the self-calibrated regime in response to microsecond RF pulses. This work demonstrates the benefits of a narrow linewidth 3-photon setup in extending the self-calibrated regime to weaker RF field amplitudes, while obtaining a high sensitivity to pulses in the amplitude regime.