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 Δ f zeros , the dispersion signal has two positive maxima with an interval defined as the shoulder interval Δ f 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 Δ f sho is experimentally studied at the MW frequencies of 31.6 GHz and 9.2 GHz respectively. The results show that Δ f sho can be used to characterize the much weaker E MW than that of Δ f zeros and the traditional EIT–AT splitting interval Δ f m ; the minimum E MW measured by Δ f sho is about 30 times smaller than that by Δ f m . As an example, the minimum E MW at 9.2 GHz that can be characterized by Δ f 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 .