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Frontiers in Physics

An optically pumped atomic clock based on a continuous slow cesium beam

Weibin Xie, Qing Wang, Xuan He, Zhichao Yuan, Shengwei Fang, Nan Chen, Zezheng Xiong, Junan Zhang, Xianghui Qi, Shaoguang Li, Xuzong Chen

Published Dec 13, 2022• 3 citations
Publisher

Abstract

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.

DOI
10.3389/fphy.2022.1073854

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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.
We reported on the progress toward an atomic clock based on a continuous cold 87Rb atomic beam. The atomic beam source is generated directly from a 2D+Magneto-Optical Trapping (MOT) with the flux of about 3×109atoms/s and mean longitudinal velocity of 10 m/s. The beam had a free evolution time of 9 ms over the separated distance of 90 mm, producing a 54-Hz-wide Raman-Ramsey fringe. The Signal-to-Noise ratio (SNR) of Raman-Ramsey fringes is effectively increased by modulation and demodulation methods, which offer a promising approach to improving the frequency stability of the atomic clock.
We present a novel cesium beam physics package, which uses optical techniques to combine the high signal-to-noise ratios achievable by optical pumping and beam brightening with the narrow linewidth allowed by a slow atomic beam. Our design can be realized in a compact, robust package suitable for space applications. Our approach differs from other laser-cooled clocks in that we use a cold-atom source that provides a continuous beam. We estimate the shot noise-limited stability that can be achieved with this physics package to be two to three orders of magnitude better than that of current compact cesium beam atomic clocks.
We present a compact cold-atom clock configuration where isotropic laser cooling, microwave interrogation, and clock signal detection are successively performed inside a spherical microwave cavity. For ground operation, a typical Ramsey fringe width of 20 Hz has been demonstrated, limited by the atom cloud's free fall in the cavity. The isotropic cooling light's disordered properties provide a large and stable number of cold atoms, leading to a high signal-to-noise ratio limited by atomic shot noise. A relative frequency stability of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ has been achieved, averaged down to $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ after $5\ifmmode\times\else\texttimes\fi{}{10}^{3}$ s of integration. Development of such a high-performance compact clock is of major relevance for on-board applications, such as satellite-positioning systems. As a cesium clock, it opens the door to a new generation of compact primary standards and timekeeping devices.
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.