Precise measurement of a weak radio frequency electric field using a resonant atomic probe

We present a precise measurement of a weak radio frequency electric field with a frequency of less than or similar to 3 GHz employing a resonant atomic probe that is constituted with a Rydberg cascade three-level atom, including a cesium ground state |6S(1/2)⟩, an excited state |6P(3/2)⟩, and Rydberg state |nD(5/2)⟩. Two radio frequency (RF) electric fields, noted as local and signal fields, couple the nearby Rydberg transition. The two-photon resonant Rydberg electromagnetically induced transparency (Rydberg-EIT) is employed to directly read out the weak signal field having hundreds of kHz difference between the local and signal fields that is encoded in the resonant microwave-dressed Rydberg atoms. The minimum detectable signal fields of E-Smin = 1.36 +/- 0.04 mV/m for 2.18 GHz coupling |68D(5/2)⟩ -> |69P(3/2)⟩ transition and 1.33 +/- 0.02 mV/m for 1.32 GHz coupling |80D(5/2)⟩ -> |81P(3/2)⟩ transition are obtained, respectively. The bandwidth dependence is also investigated by varying the signal field frequency and corresponding -3 dB bandwidth of 3 MHz is attained. This method can be employed to perform a rapid and precise measurement of the weak electric field, which is important for the atom-based microwave metrology.