Approaching the standard quantum limit of a Rydberg-atom microwave electrometer

The development of a microwave electrometer with inherent uncertainty approaching its ultimate limit carries both fundamental and technological significance. However, due to the thermal motion of atoms, the state-of-the-art Rydberg electrometer falls considerably short of the standard quantum limit by about three orders of magnitude. Here, we utilize an optically thin medium with approximately 5.2×105 laser-cooled atoms to implement the microwave heterodyne detection. By mitigating various noises and strategically optimizing the electrometer parameters, our study reduces the equivalent noise temperature by a factor of 20 and achieves an electric-field sensitivity of 10.0nVcm-1Hz-1/2, finally reaching a factor of 2.6 above the standard quantum limit. Our work also provides valuable insights into the inherent capabilities and limitations of Rydberg electrometers, offering superior sensitivity in detecting weak microwave signals for numerous applications.

Methods
This is the main experimental data of this experimental work, including atomic spectral line diagrams, time-domain waveform diagrams captured by an oscilloscope, and spectral density diagrams obtained after digital Fourier transformation. It also includes indirect results such as sensitivity obtained from spectral density, with specific details detailed in the relevant papers. The theoretical simulation results provided in the paper can be calculated based on the calculation conditions and equations given in the article.