Measurement of microwave electric field based on electromagnetically induced transparency by using cold Rydberg atoms

In this paper, we have theoretically and experimentally studied a quantum microwave electrometry in a cold atomic system using Rydberg electromagnetic induction transparency (EIT) and Autler-Townes splitting (EIT-AT splitting). We obtained spindle-shaped cold atomic clouds in a magneto-optical trap and then pumped cold atoms to quantum state 5S1/2，F=2，mF=2 by using the optical-pump laser. We obtained the Rydberg EIT spectrum peak with narrow linewidth by taking the advantages of the low temperature and small residual Doppler broadening. The results show that the typical EIT linewidth with 16 μK cold atoms is about 460 kHz which is 15 times narrowed than that of 7 MHz obtained in the thermal vapor cell. The microwave electric field amplitude is measured by EIT-AT splitting in the cold atoms for frequencies of 9.2 GHz, 14.2 GHz and 22.1 GHz , receptively. The results show that there is a good linear relationship between the EIT-AT splitting interval and the microwave electric field amplitude. The lower limit of the microwave electric field amplitude that can be measured in the linear region can reach as low as 222 μV/cm, which is enhanced about 22 times than that in the traditional thermal vapor cell about of 5 mV/cm. The improvement of the lower limit by EIT-AT splitting method is roughly scaled as the narrowing EIT line width by cold atom samples. This demonstrate that, benefiting from the smaller residual Doppler effect and the narrower EIT linewidth in cold atoms, the cold atom system is more advantageous in the experiment of measuring the weak microwave electric field amplitude by using the EIT-AT splitting method. This is of great benefit to the absolute calibration of very weak microwave electric fields. Furthermore, the lower limit of the microwave electric field amplitude that can be measured smaller than 1 μV/cm by using the change of transmittance of the prober laser at the EIT resonance, and the corresponding sensitivity can reach 1 μV/cm Hz-1/2. These results demonstrate the advantages of cold atomic sample in microwave electric field measurement and its absolute calibration.