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

[https://doi.org/10.5061/dryad.sbcc2frgx](https://doi.org/10.5061/dryad.sbcc2frgx)

## Description of the data and file structure

The photovoltage is recorded by a digital acquisition oscilloscope (R&S RTE1024), and the amplitude spectral density for MW sensitivity is analysed using a desktop computer. For measurements within the 3-dB bandwidth, an avalanche photodetector (Thorlabs APD130A) with a bandwidth replaces the photodetector. We conduct a discrete Fourier transform (DFT) on the photovoltage to extract the frequency and amplitude data, where a Blackman window is applied in the heterodyne time. The extracted frequency represents the intermediate frequency of the cold Rydberg-atom receiver, whilst the extracted amplitude signifies the strength of the heterodyne signal. In order to correct the amplitude of the windowed signal during the DFT, we utilize the calibration information from the balan...