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Quantum Optics and Photon Counting

Preliminary progress of the state preparation with a new optical pumping method in a cesium atomic fountain clock

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

Published Apr 14, 2021• 0 citations
Publisher

Abstract

In this paper, a new optical pumping method, redistributing the atom populations among the Zeeman magnetic sublevels of the ground state |F=3> is proposed as the state preparation process to improve the signal to noise ratio (SNR) of the atomic fountain clock with a larger atom population on the |F=3, mF=0>clock state. A preliminary experimental result of state preparation efficiency exceeding 60% is obtained on the NIM6 cesium (Cs) fountain clock.

DOI
10.1117/12.2592287

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In this paper, a new optical pumping method, redistributing the atom populations among the Zeeman magnetic sublevels of the ground state |F=3> is proposed as the state preparation process to improve the signal to noise ratio (SNR) of the atomic fountain clock with a larger atom population on the |F=3, mF=0>clock state. A preliminary experimental result of state preparation efficiency exceeding 60% is obtained on the NIM6 cesium (Cs) fountain clock.
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 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.
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.
In this paper, we introduce the design of our new fountain clock NIM6. Besides some improvements on the vacuum system, Ramsey cavity to reduce the Type B uncertainty, NIM6 collects atoms from a MOT loading optical molasses to get a better signal to noise ratio. The atom density will be more uniform compared to 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 after MOT to keep the collisional shift low. Cs atoms are collected in the lower MOT chamber for 0.5 second, and then the MOT magnetic field is turned down. Atoms are accelerated for 1 ms in a moving molasses, and further cooled for another 1.5 ms by adiabatically reducing the cooling beam intensities and red offsetting the frequencies.
We propose a simple pumping method to increase the effective population of cold atoms in the clock state and investigate the factors which affect the pumping efficiency in cold atom systems. We report the theory and demonstrate the corresponding experiment in an 87Rb integrating sphere cold atom clock. The experimental results show that the population of cold atoms in the Zeeman sublevel |F = 2, mF = 0〉 is approximately 1.62 times that of the result using optical pumping alone. This method can also be applied to increase the effective population in any one of the target Zeeman sublevels in other cold atom systems.