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Chinese Physics B

Improvement of the short-term stability of atomic fountain clock with state preparation by two-laser optical pumping*

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

Published Feb 2, 2021• 4 citations
Publisher

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 | 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.

DOI
10.1088/1674-1056/abe22a

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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 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.
We have implemented a technique of optical pumping (OP) to increase the clock state populations in a caesium fountain frequency standard. Our initial experiments have demonstrated accumulation of 75% of the atoms in the mF= 0 sublevel (clock state), which can be compared to a typical 11% population of this sublevel with no additional OP applied. As the (OP) process requires scattering of some 20 photons per atom, the atomic cloud is heated by nearly 1 μK and the final number of atoms reaching the detector is reduced by up to a factor of two, depending on the molasses temperature. Using the OP technique in the NPL-CsF2 fountain we have observed a four-fold net increase of the clock state population.
We report the design of a new cesium fountain clock NIM6, which is under construction in NIM. Besides some improvements on the vacuum system, Ramsey cavity and microwave synthesizer to reduce the Type B uncertainty. Another major improvement on NIM6 is to collect more atoms from a MOT loading optical molasses and optical pumping to get a better signal to noise ratio at the detection. The atom distribution will be more uniform compared with a 2D MOT loading optical molasses, and the diameter of the cloud can be adjusted by the intensity and detuning of lights during the post cooling to keep the collisional-induced frequency shift low. The atom numbers can be further increased by a new de-pumping - optical pumping procedure to pump atoms to the |F=3, mF=0> clock state directly. With a new cryogenic sapphire oscillator (CSO) based frequency synthesizer, NIM6 is aiming to reach the quantum projection noise, thus leading to a reduced Type A uncertainty compared with NIM5.
In this paper we discuss how to improve both short and medium-long term frequency stability of a vapor cell frequency standard working in pulsed regime. We propose in particular a multi-step pumping technique that is able to put almost all the atoms in one of the two clock levels, increasing in this way the signal-to-noise ratio with respect to the usual optical pumping made with a single laser pulse. Moreover, we discuss the phenomena affecting the medium-long term frequency stability and how it is possible to control them, making the overall clock stability compatible with sigmay(tau) =3x10-13tau-frac12for integration times up to tau = 105s.