Precise measurement of a weak radio frequency electric field using a resonant atomic probe<xref rid="cpb_29_3_033201fn1" ref-type="fn">*</xref><fn id="cpb_29_3_033201fn1"><label>*</label><p>Project supported by the National Key R&D Program of China (Grant No. 2017YFA0304203), the National Natural Science Foundation of China (Grant Nos. 61475090, 61675123, 61775124, and 11804202), the State Key Program of National Natural Science of China (Grant Nos. 11434007 and 61835007), and Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (Grant No. IRT 17R70).</p></fn>

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

Project supported by the National Key R&D Program of China (Grant No. 2017YFA0304203), the National Natural Science Foundation of China (Grant Nos. 61475090, 61675123, 61775124, and 11804202), the State Key Program of National Natural Science of China (Grant Nos. 11434007 and 61835007), and Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (Grant No. IRT 17R70).

Hao Liping¹, Xue Yongmei¹, Fan Jiabei¹, Bai Jingxu¹, Jiao Yuechun^{1, 2}, Zhao Jianming^{1, 2, †}

(a) Schematic diagram of experiments. The coupling and probe beams, forming a Rydberg three-level system, counter-propagate through a cesium vapor cell along the x axis. Two radio frequency fields with hundreds of kHz difference, denoted with solid and dashed arcs, interact with the three-level system. The one radio frequency field is denoted as a local field (E_L) and the other one as a weak signal field (E_S). The transmission of the probe is used to directly read out the signal field by a photodiode (PD) after a dichroic mirror (DM). (b) Energy level diagram. The probe (coupling) laser is resonant with the lower |6S_1/2 F = 4⟩ → |6P_3/2 F′ = 5⟩ (up |6P_3/2 F′ = 5 ⟩ → |68D_5/2⟩) transition. The local (signal) field with the frequency ∼ 2.18 GHz couples Rydberg transition |68D_5/2⟩ → |69P_3/2⟩. Small frequency difference between the two radio frequency fields results in the modulation to the local field, see text. (c) Measurements of Rydberg EIT-AT spectra as a function of the coupling laser detuning Δ_c with a local field E_L = 0.49 V/m. The solid lines correspond to Lorentz fittings to the EIT-AT spectra. The extracted EIT linewidth is Γ_EIT = 2π × (12.54 ± 0.13) MHz. The coupling frequency is continuously scanned by the SC110 module of a commercial laser Toptica SHG110 and the probe transmission spectrum is detected by a PD and recorded with an oscilloscope.