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Optimization of Operation Parameters in a Cesium Atomic Fountain Clock Using Monte Carlo Method

Hui Li, Yuanbo Du, Xian Yang, Yunyi Guo, Mingming Liu, Wenbing Li, Hongli Liu, Z. H. Lu

Published Jan 1, 2021• 3 citations

Abstract

The number of atoms detected and the contrast of Ramsey fringe are critical operating parameters in an atomic fountain clock. To optimize these operating parameters, a Monte Carlo method is used to simulate the evolution of the atomic cloud. It is found that the physical variables, including atomic temperature, the launching direction of the atomic cloud, and the initial position of the atomic cloud, have a large influence on the ratio of the falling-back atoms to the launching atoms, and consequently on the detected atomic number, and that the atomic temperature affects the pattern of the Ramsey fringe. In the experiment, these relevant physical variables are optimized and a falling-back ratio as high as 8.6%±0.4% is obtained for a cesium atomic fountain with a launch height of 1 m. Moreover, the Ramsey fringe is experimentally optimized and the contrast of the central Ramsey fringe reaches 90%. This optimization method is also applicable to similar experiments with cold atoms.

DOI: 10.1109/access.2021.3113161

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The number of atoms detected and the contrast of Ramsey fringe are critical operating parameters in an atomic fountain clock. To optimize these operating parameters, a Monte Carlo method is used to simulate the evolution of the atomic cloud. It is found that the physical variables, including atomic temperature, the launching direction of the atomic cloud, and the initial position of the atomic cloud, have a large influence on the ratio of the falling-back atoms to the launching atoms, and consequently on the detected atomic number, and that the atomic temperature affects the pattern of the Ramsey fringe. In the experiment, these relevant physical variables are optimized and a falling-back ratio as high as 8.6%±0.4% is obtained for a cesium atomic fountain with a launch height of 1 m. Moreover, the Ramsey fringe is experimentally optimized and the contrast of the central Ramsey fringe reaches 90%. This optimization method is also applicable to similar experiments with cold atoms.

We have employed an intuitive model and 3D Monte Carlo simulations to study the effect of ballistic expansion of the atomic cloud on the collision energies and the resulting frequency shift in a fountain clock. In particular, we show that the energies relevant for collisions contributing to the clock shift correspond to temperatures which may be significantly smaller than the temperature of the cloud at launch. Both the assumptions and predictions of our model are related to realistic parameters of operating caesium fountain clocks.

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 a novel method for temperature measurement in a cesium fountain clock and experimental investigation about its dependence respect to diverse operational parameters. The temperature of the Cs atomic cloud is inferred from the width of the velocity distribution, which is obtained from two measurements in different regions of the spatial distribution of the atomic cloud using the time of flight (TOF) method. We use a simple process to determine the velocity distribution by calculating the square speed point by point from these two TOF signals. This new method is demonstrated to characterize the temperature of the Cs atoms on the CENAM Cs fountain clock. The minimum temperature reached in the CENAM Cs fountain clock is 612(193) nK.

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.