Microwave electrometry with Rydberg atoms in a vapor cell using microwave amplitude modulation

We have theoretically and experimentally studied the dispersive signal of the Rydberg atomic electromagnetically-induced transparency (EIT) Autler–Townes (AT) splitting spectra obtained using amplitude modulation of the microwave (MW) electric field. In addition to the two zero-crossing points interval Δf_zeros, the dispersion signal has two positive maxima with an interval defined as the shoulder interval Δf_sho, which is theoretically expected to be used to measure a much weaker MW electric field. The relationship of the MW field strength E_MW and Δf_sho is experimentally studied at the MW frequencies of 31.6 GHz and 9.2 GHz respectively. The results show that Δf_sho can be used to characterize the much weaker E_MW than that of Δf_zeros and the traditional EIT–AT splitting interval Δf_m; the minimum E_MW measured by Δf_sho is about 30 times smaller than that by Δf_m. As an example, the minimum E_MW at 9.2 GHz that can be characterized by Δf_sho is 0.056 mV/cm, which is the minimum value characterized by the frequency interval using a vapor cell without adding any auxiliary fields. The proposed method can improve the weak limit and sensitivity of E_MW measured by the spectral frequency interval, which is important in the direct measurement of weak E_MW.