Chin. Phys. B, 2021, Vol. 30(5): 050602    DOI: 10.1088/1674-1056/abe22a
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# Improvement of the short-term stability of atomic fountain clock with state preparation by two-laser optical pumping

Lei Han(韩蕾)1, Fang Fang(房芳)2,†, Wei-Liang Chen(陈伟亮)2, Kun Liu(刘昆)2, Shao-Yang Dai(戴少阳)2, Ya-Ni Zuo(左娅妮)2, and Tian-Chu Li(李天初)1,2,‡
1 School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China;
2 Key Laboratory of Time and Frequency Standards, National Institute of Metrology(NIM), Beijing 100029, China
Abstract  To improve the signal to noise ratio (SNR) and the short-term stability of cesium atomic fountain clocks, the work of two-laser optical pumping is presented theoretically and experimentally. The short-term stability of the NIM6 fountain clock has been improved by preparing more cold atoms in the $\vert F=4, m_{F}=0\rangle$ clock state with a shortened cycle time. Two $\pi$-polarized laser beams overlapped in the horizontal plane have been applied after launching, one is resonant with $\vert F=4\rangle \to \vert F'=4\rangle$ transition and the other is resonant with $\vert F=3\rangle \to \vert F'=4\rangle$ transition. With optical pumping, the population accumulated in the $\vert m_{F}=0\rangle$ clock state is improved from 11% to 63%, and the detection signal is increased by a factor of 4.2, the SNR of the clock transition probability and the short-term stability are also improved accordingly.
Keywords:  optical pumping      atomic fountain clock      spin-polarization      short-term stability
Received:  21 December 2020      Revised:  11 January 2021      Accepted manuscript online:  02 February 2021
 PACS: 06.30.Ft (Time and frequency) 31.15.-p (Calculations and mathematical techniques in atomic and molecular physics) 32.10.Fn (Fine and hyperfine structure) 32.80.Xx (Level crossing and optical pumping)
Fund: Project supported by the National Natural Science Foundation of China (Grant No. 11873044).
Corresponding Authors:  Fang Fang, Tian-Chu Li     E-mail:  fangf@nim.ac.cn;litch@nim.ac.cn