High-sensitivity and low-harmonic microwave detection using Rydberg atoms based on a loop-level structure

We propose and investigate a Rydberg atom sensor based on a loop-level structure, which utilizes the eigenfrequencies of the energy levels as a reference. This design eliminates the need for the local oscillator (LO) electric field required in the traditional superheterodyne structure while maintaining a linear relationship between the readout signal and the microwave (MW) electric field to be measured, along with exhibiting significantly lower harmonic distortion. For experimental validation, we implemented a cesium atomic system based on the loop five-level structure. The system achieved a sensitivity of 62 nVcm^-1Hz^-1/2 and maintained 30 dB of harmonic suppression even near the nonlinear operational region under the strong electric field, which is approximately 20 dB higher than that of the superheterodyne structure. This work demonstrates that loop-level-based atomic sensors enable high-sensitivity and wide-dynamic-range (including nonlinear region) MW detection, making them particularly suitable for receiving frequency- and phase-modulated signals that are insensitive to power variations.