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Chinese Physics Letters

Laser Cooling Alkaline-earth Atoms for Optical Clock in Chinese Space Station

Guodong Zhao, Jian Guo Xia, Yun Liu, Yongzhuang Zhou, Chihua Zhou, Feng Guo, Wenhai Wang, Dejing He, Min Feng, Ting Liang, Jie Ren, Qinfang Xu Qinfang Xu, Junwei Meng, Feng Gao, Yong Shen, Xiaotong Lu, Benquan Lu, Yebing Wang, Xiaohua Hu, Wei Tan, Hongxin Zou, Hong Chang

Published Apr 22, 2025• 12 citations
Publisher

Abstract

Abstract This study presents an achievement of laser cooling of alkaline-earth atoms in the Chinese Space Station’s strontium (Sr) atomic space optical clock. The system’s core components, physical unit, optical unit, and electrical unit, have a total volume of 306 L and a total mass of 163.8 kg. These compact and robust units can overcome mechanical vibrations and temperature fluctuations during space launch. The laser sources of the optical unit are composed of diode lasers, and the injection locking of slave lasers is automatically performed by a program. In the experiment, a blue magneto-optical trap of cold atoms was achieved, with the atom numbers estimated to be approximately (1.50 ± 0.13) × 10 6 for 87 Sr and (8.00 ± 0.56) × 10 6 for 88 Sr. This work establishes a foundation for atomic confinement and high-precision interrogation in space-based optical clocks and expands the frontiers of cold atom physics in microgravity.

DOI
10.1088/0256-307x/42/6/063701

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Obtaining ultracold alkaline earth(-like) atoms in space encounters the problem of performing narrow-line laser cooling in microgravity environments ( μ -gEs). This paper reports an analysis of the magneto-optical trap (MOT) based on the narrow-line transition in 88 Sr, while paying special attention to the role of the gravity. This analysis suggests the MOTs based on narrow-line transitions cannot be cold and dense enough in a μ -gE. We thus propose a strategy: that one can use a dual-frequency MOT to realize a low-temperature, high density, and high transfer efficiency, narrow-line red MOT in μ -gEs.
Optical atomic clocks provide exceptionally accurate and precise signals for timekeeping and precision measurements, but they require high-power, free-space laser configurations that limit scalability. We introduce and explore a scalable infrastructure for strontium (Sr) optical-lattice clocks that incorporates co-design of atomic-beam slowing and a magneto-optical trap (MOT) from an effusion source, generation of complex, three-dimensional free-space laser configurations with a photonic integrated circuit (PIC) and metasurface (MS) optics, and laser stabilization to a frequency-comb supercontinuum generated with integrated nonlinear photonics. With these elements, we realize MOTs of all stable strontium isotopes (84^{84}84Sr, 86^{86}86Sr, 87^{87}87Sr, 88^{88}88Sr) with populations commensurate with natural abundances, demonstrating precise beam control and robustness. Access to laser-cooled alkaline-earth atoms with scalable integrated photonics enables system engineering for optical clocks, quantum sensing, and quantum information, and our experiments demonstrate extensible technologies that advance toward a Sr optical clock largely free of bulk optics.
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Abstract We report on a transportable optical clock, based on laser-cooled strontium atoms trapped in an optical lattice. The experimental apparatus is composed of a compact source of ultra-cold strontium atoms including a compact cooling laser setup and a transportable ultra-stable laser for interrogating the optical clock transition. The whole setup (excluding electronics) fits within a volume of \\2 m3. The high degree of operation reliability of both systems allowed the spectroscopy of the clock transition to be performed with 10 Hz resolution. We estimate a relative uncertainty of the clock of 7 9 10-15. 1