Rydberg Atom-based Sensors: Two-photon and Three-photon readout schemes

We present intuitive explanations of atom-based RF electric-field sensors based on Rydberg states in hot vapors. There are two distinct strategies to detect the electric field strength of the RF wave, namely the Autler-Townes limit, where the splitting of the dressed states is proportional to the incident RF electric field strength and the amplitude regime, where we determine the electric field by measuring the change in transmission of a probe laser in the presence of the RF electromagnetic field. We present theoretical calculations for the amplitude regime, using a two-photon excitation scheme, extensible to other read-out schemes, that shows how the scattering of the probed transition changes in the presence of the RF electromagnetic field. We find an analytic expression in the thermal limit with finite wave vector mismatch that yields an accurate approximation and provides theoretical insight into the physics of the sensor. Furthermore, we present results on a three-photon excitation scheme, with which residual Doppler broadening is suppressed. The three-photon scheme enables a spectral resolution comparable to the Rydberg state decay rate, the spectral bandwith limitation, effectively eliminating the limitation of residual Doppler shifts. We present recent measurements on the sensitivity. The extension of the two-photon theory to the three-photon experiment is addressed.