Space cold atomic clocks, which are high-precision atomic clocks operating in space, have shown great potential for application in navigation positioning, deep space exploration, and fundamental physics research. Since the 1990s, with the development of laser cooling technology, atomic fountain clocks have been realized and have improved the frequency stability and accuracy from the 10^-14 level of cesium beam atomic clocks to the 10^-16 level. France, the United States, and China have all proposed plans to operate high-precision cold atomic clocks in microgravity environments. With the continuous development of space optical clock technologies, the United States, the European Union, and China have all presented different experimental project proposals based on space optical clocks.

With the implementation of atomic fountain clocks, French scientists proposed the concept of space cold atomic clocks, abbreviated as PHARAO, which utilize the advantages of microgravity to improve the accuracy of cold atomic clocks. On-ground results indicate that the frequency stability of PHARAO is 3.0×10^-13τ^-1/2, and the frequency accuracy is 2.3×10^-15. The frequency stability is expected to reach 1.1×10^-13τ^-1/2 when operating under microgravity. According to the latest report, PHARAO will be launched to the International Space Station in 2025. The United States almost simultaneously proposed the space cold atomic clock program with France, the cesium space atomic clock abbreviated as PARCS, and the rubidium space atomic clock abbreviated as RACE.

With the support of Chinas manned space program, the Shanghai Institute of Optics and Fine Mechanics (SIOM) began the engineering development of the space cold atomic clock in 2010, which achieved its first international in-orbit operation on the Tiangong-2 Space Lab in 2016. With appropriate parameter settings, an estimated short-term frequency stability of approximately 3.0×10^-13τ^-1/2 was attained. The demonstration of the long-term operation of cold atom clocks in orbit opens the possibility of applying space-based cold atom sensors. A comparison of the in-orbit and on-ground results indicates that a higher cooling efficiency exists under microgravity, including a smaller loss rate during the trapping and cooling process and a lower ultimate temperature of laser-cooled atoms. The China Space Station provides a better platform for a high-precision time-frequency experimental system. The cold atom microwave clock abbreviated as CAMiCS is one of three atomic clocks in the high-precision time-frequency experimental system of the China Space Station. CAMiCS provides a more stable and accurate frequency signal, evaluates strontium optical lattice clocks, and controls the hydrogen maser over the long term.

Currently, the frequency uncertainty and stability of optical clocks in the laboratory have reached the level of 10^-18. The European Unions space optical clock (SOC) project was funded by the European Space Agency in 2007, the goal of which was to install and operate an optical lattice clock on the International Space Station, and it is currently in the testing stage of a ground-based prototype. In 2018, Heinrich-Heine-Universität Düsseldorf, the University of Birmingham, and Physikalisch-Technische Bundesanstalt jointly reported the research results of a compact and high-performance ⁸⁸Sr optical lattice clock as an optical clock for space. A fractional uncertainty of 3×10^-17 was achieved. The volume of the device excluding the clock laser was 60 cm×163 cm×99 cm, which was significantly lower than the volume of the laboratory optical clock. Moreover, a stability of 4.1×10^-16τ^-1/2 was achieved under the condition of a spectral line width of 220 mHz.

The Mengtian experimental module of the China Space Station is equipped with a high precision time-frequency experimental system, which includes a space cold atomic optical lattice clock. The National Time Service Center of the Chinese Academy of Sciences conducted research on transportable optical clocks and the miniaturization of optical clocks in 2017 according to the optical clock in the laboratory. The volume of the optical clock vacuum system was reduced to 20 cm×42 cm×90 cm and a frequency stability of 3.6×10^-15τ^-1/2 and frequency uncertainty of 2.3×10^-16 were achieved. In the research on optical clocks for space station applications, the volume of the physical system was further reduced to 15 cm×20 cm×60 cm, which is equivalent to one thirtieth of the volume of the physical laboratory optical clock system. After parameter adjustment, the strontium atomic light clock of the China Space Station will achieve a stability of 1.5×10^-15τ^-1/2 and an uncertainty of 2×10^-17 under the joint efforts of the National Time Service Center of the Chinese Academy of Sciences, the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences, the Hangzhou Institute of Advanced Research of the National University of Science and Technology of China, and the National University of Defense Technology. The Mengtian experimental module was successfully launched in October 2022. The high precision time-frequency experimental system utilizes the combination of an optical lattice clock, active hydrogen maser, and cold atom microwave clock, as well as corresponding comparison links, to perform in-orbit tests and fundamental physics research.

Since the realization of cold atomic clocks in the 1990s and the subsequent technological development, ground-based cold atomic fountain clocks have been used to realize the definition of the international second and assist in timekeeping. Several space cold atomic clock projects have been realized or are under development. In 2016, the space cold atomic clock developed by SIOM was the first globally to operate in orbit in the Tiangong-2 Space Lab, marking an important milestone in the field of space quantum sensors. The successful operation of space cold atomic clocks in orbit has provided a technical foundation for establishing a high-precision space time-frequency standard, which is of great significance for improving the accuracy and stability of the global satellite navigation system. Cold atomic optical clocks, combined with technologies such as optical frequency combs, have also played an important role in experimentally verifying gravitational redshift and the drift of the fine structure constant over time. Several space optical clock projects are underway, and key technologies are continuously advancing. Despite the significant progress in space cold atomic clock-related technologies, several shortcomings remain, such as the relatively large volume, unsatisfactory reliability, and issues with environmental adaptability. There is still significant room for technical improvement in components with large volume and weight, such as physical vacuum and highly reliable optics, which have poor reliability. The development of these technologies, while applied to space cold atomic clocks, will also promote the wider application of ground-based cold atomic clocks.

The development of miniaturized space cold atomic microwave clocks will help improve the performance of the global satellite navigation system. With the continuous improvement in the requirements for accuracy and autonomous operation capabilities of satellite navigation systems, higher requirements have been put forward for the frequency stability and accuracy of on-board atomic clocks. Miniaturized space cold atomic microwave clocks can meet the future needs of on-board atomic clocks, and those carried by deep space craft can also be used for deep space autonomous navigation.

Compared with ground-based atomic clocks, space atomic clocks have the advantages of large gravitational potential differences, the ability to significantly modulate the gravitational potential at their location, and being unaffected by ground noise, providing more benefits for such research. The verification of general relativity, the detection of gravitational waves, and the exploration of dark matter are currently the most cutting-edge research directions in physics, and breakthroughs in these areas will very likely lead to the discovery of new physical laws.