Time measurement plays an irreplaceable role in modern society, not only in frontier basic research and high-tech fields but also in positioning and navigation, electricity, finance, geodesy, and other fields where it is the core of the technical foundation. The basis of time measurement—the time unit second is defined by atomic time and generated by atomic frequency standards. In recent years, atomic frequency standards have undergone rapid development, and their performance indicators have continuously improved, driving great progress in related fields of precision measurement. Typical examples include the global satellite navigation system based on time measurement achieving sub-meter level positioning accuracy, the time unit second becoming the most accurate and fundamental standard unit among the seven basic units of the International System of Units, and playing an important role in the quantum transformation of the International System of Units. These advances have directly affected all aspects of life.

Atomic frequency standards have continuously improved the uncertainty by one order of magnitude every 20 years since they were established in the 1950s. The earliest highest standard was the cesium atomic beam, which had a level of 10^-14. In the 1980s, laser cooling enabled atomic frequency standards to achieve a technological leap, giving birth to cold atom frequency standards represented by atomic fountains. This technology effectively reduced the atomic loss caused by thermal motion, and parabolic motion significantly increased the coherence time. Combined with high-sensitivity optical detection and other technologies, the uncertainty of fountain frequency standards was improved to the level of 10^-16. At the end of the last century, optical frequency standards emerged, and by raising the working spectral lines from the microwave band to the optical band, frequency measurement achieved another technological leap. The uncertainty of frequency standards, which decreased by two orders of magnitude, reached 10^-18. However, time-frequency signals are primarily based on microwaves, and microwave frequency standards are more mature. The time measurement in the world is mainly undertaken by microwave frequency standards, among which atomic fountain is the highest standard for reproducing second and it plays an important role in generating time scales of various countries and establishing international atomic time.

With the development of science and technology, time-frequency measurements extend from the ground to outer space, and the space microgravity environment is conducive to achieving frequency standards with narrower spectral lines and better performance. Space cold atom frequency standards have become a current research hotspot, which can not only calibrate the time-frequency signals of navigation satellites in all weather but also greatly help to improve the accuracy of the global space-time system and provide a unique platform for basic physical experiments, which can play an important role in the research of relativity verification, basic constant measurement, etc.

In the past few years, with the emergence of the space clock-time system, time reference has not been generated only by the ground laboratory, which will build up an accurate and sensitive space-time system, making time and space more closely integrated. Therefore, it is necessary to summarize the existing research to guide the future development of time and frequency fields more reasonably.

A frequency standard is a device that outputs a standard frequency by locking a microwave or optical frequency signal to the resonance frequency of the atomic (molecular, ion) transition energy level and realizes quantum interrogation by Rabi oscillation or Ramsey effect. The interrogation spectral linewidth is the most important indicator of the frequency standard. Cold atom frequency standards can effectively reduce the influence of thermal motion and often yield excellent performance indicators. Fountain frequency standards are typical cold atom frequency standards. They use cold atoms to interact with microwaves twice in the parabolic motion of throwing up and falling down through the microwave cavity and realize Ramsey interference. Fountain frequency standards can achieve a 1 Hz spectral line, currently the best-performing microwave frequency standard. The short-term stability evaluated by Allan deviation is 1×10^-13τ^-1/2, and the long-term stability is 10^-17. The type-B uncertainty of the fountain frequency standards was obtained by evaluating the physical effects that affect the frequency, which was 10^-16. Advanced time-frequency laboratories worldwide are developing fountain frequency standards for frequency traceability and time-frequency signal generation. The stability of fountain frequency standards is often limited by the microwave oscillator, which can be overcome by upgrading the oscillator. A photonic microwave generator is a commonly used scheme (Fig. 6) that transfers the excellent performance of an ultra-stable laser to the microwave band through an optical frequency comb and can achieve 10^-15 or even better second stability. Using a photonic microwave generator as the local oscillator, the stability of the fountain clock can reach the quantum projection noise limit. To meet the requirements of space-time frequency measurement, cold atom microwave frequency standards extend from the ground to space. In 2016, the Shanghai Institute of Optics and Mechanics (Chinese Academy of Sciences) realized the worlds first cold atom clock in orbit. The improved in-situ measurement cold atom clock (Fig.11) was launched into space and will play an important role in integrated space-ground time-frequency systems in the future.

In recent years, cold atom microwave frequency standards have rapidly developed, and these frequency standards have played an important role in the generation and unification of time-frequency signals in various countries worldwide. The Shanghai Institute of Optics and Mechanics (Chinese Academy of Sciences) has conducted systematic research in related fields, such as fountain frequency standards and optically generated microwaves, and realizes the worlds first space cold atom clock in orbit. The related technical indicators have reached or approached the advanced international level. In the future, cold atom microwave frequency standards will play a more important role in many fields related to the national economy, peoples livelihoods, and national defense security, especially in measurement, geodesy, basic research, positioning, navigation timing, and other fields.