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Chinese Physics B

Optical state selection process with optical pumping in a cesium atomic fountain clock*

Lei Han, Fang Fang, Weiliang Chen, Kun Liu, Yani Zuo, Fasong Zheng, Shaoyang Dai, Tianchu Li

Published May 29, 2021• 3 citations
Publisher

Abstract

We propose and realize a new optical state selection method on a cesium atomic fountain clock by applying a two-laser 3–3′ optical pumping configuration to spin polarize atoms. The atoms are prepared in |F = 3, mF = 0〉 clock state with optical pumping directly after being launched up, followed by a pushing beam to push away the atoms remaining in the |F = 4〉 state. With a state selection efficiency exceeding 92%, this optical method can substitute the traditional microwave state selection, and helps to develop a more compact physical package. A Ramsey fringe has been achieved with this optical state selection method, and a contrast of 90% is obtained with a full width half maximum of 0.92 Hz. The short-term frequency stability of 6.8 × 10−14 (τ/s)−1/2 is acquired. In addition, the number of detected atoms is increased by a factor of 1.7 with the optical state selection.

DOI
10.1088/1674-1056/ac0698

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We propose and realize a new optical state selection method on a cesium atomic fountain clock by applying a two-laser 3–3′ optical pumping configuration to spin polarize atoms. The atoms are prepared in |F = 3, mF = 0〉 clock state with optical pumping directly after being launched up, followed by a pushing beam to push away the atoms remaining in the |F = 4〉 state. With a state selection efficiency exceeding 92%, this optical method can substitute the traditional microwave state selection, and helps to develop a more compact physical package. A Ramsey fringe has been achieved with this optical state selection method, and a contrast of 90% is obtained with a full width half maximum of 0.92 Hz. The short-term frequency stability of 6.8 × 10−14 (τ/s)−1/2 is acquired. In addition, the number of detected atoms is increased by a factor of 1.7 with the optical state selection.
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 | F = 4, mF = 0〉 clock state with a shortened cycle time. Two π-polarized laser beams overlapped in the horizontal plane have been applied after launching, one is resonant with | F = 4〉 → | F′ = 4〉 transition and the other is resonant with | F = 3〉 → | F′ = 4〉 transition. With optical pumping, the population accumulated in the | mF = 0〉 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.
We perform a new scheme of magnetic state selection in optically detected compact cesium beam clocks. Unlike the conventional method, we select atoms in the ground state |F = 4, mF ≠ -4⟩ by pointing the atomic collimator to the convex pole of the magnet realizing the two-wire magnetic field and detect atoms in |F = 3⟩ after interacting with the microwave field using a distributed feedback laser. The fluorescence background is greatly reduced as the inherent residual atoms |F = 4, mF = -4⟩ are avoided in this reversed scheme. The velocity distribution is narrowed, and the most probable velocity is decreased, since atomic trajectories are close to the weak-field region. We also investigate the relationship between the position of the atomic collimator and the distributions of the atomic beam, which is consistent with the Monte Carlo-based simulation model. By applying the reversed scheme and setting the deviated position of the collimator to 1.3 mm, the signal contrast is improved from 0.7 to 3, and the short-term frequency stability reaches 3.0 × 10-12 τ-1/2, nearly three times better than that of the high-performance version of Microsemi 5071A.
Herein, we report the scheme of an optically pumped atomic clock based on a cold cesium atomic beam source. We propose the laser system and physical mechanism of this atomic clock, wherein the atomic beam travels in an upper parabolic trajectory, thereby eliminating the light shift effect. In the experiments, when the length of the free evolution region was 167 mm, the line width of the Ramsey fringe was 37 Hz. When the expected signal-to-noise ratio of the Ramsey fringe that can be achieved is 36,000, the expected short-term frequency stability is about 3.6 × 10–14/√τ, which is significantly higher than that of a conventional optically pumped cesium clock of similar volume.
A portable cold87Rb atomic clock is under development at NIM. The clock uses87Rb atoms which are trapped and cooled in a small three-dimensional magneto-optical trap (3D-MOT) chamber. Within the 3D-MOT chamber, the free-falling atoms are prepared in∣F=2,mF=0⟩\vert F=2, m_{F}=0\rangle∣F=2,mF​=0⟩clock state with optical pumping (OP) directly after post cooling by2−2′2-2^{\prime}2−2′OP light and repumping. Following optical state selection, the free-falling atoms enter a microwave cavity where they undergo microwave interrogation. Eventually, the number of atoms in two clock states are detected individually by the fluorescence detection method in the detection chamber. The preliminary results show that, the selected atom number has been improved about 3 times with the optical state selection compared to the microwave state selection method. And a Ramsey fringe has been obtained experimentally, and the contrast of the central Ramsey fringe exceeds 88% with a full width half maximum (FWHM) of less than 9 Hz.