The reporter learned from the Chinese Academy of Sciences that recently, the team of Pan Jianwei from the University of Science and Technology of China cooperated with the Shanghai Institute of Technology and Physics of the Chinese Academy of Sciences and other units to achieve the first 100 km level free space high-precision time frequency transmission experiment in the world. The stability of time transmission reached the femtosecond level, and the stability of frequency transmission per 10000 seconds was better than 4E-19 (equivalent to that the error of the clock in about 100 billion years was not more than 1 second), It can meet the time transmission requirements of the current highest precision optical clock. This achievement was published online in the international academic journal Nature on October 5. The second is one of the seven basic physical quantities. At present, the definition of "second" was determined in 1967 by the cesium atomic clock. The frequency of cesium atomic clock is in microwave band, and the error of 100 million years is only 1 second. In recent years, scientists have also developed new types of atomic clocks, such as strontium and ytterbium, which have higher frequencies and are in the optical band, so they are called "optical atomic clocks", or "light clocks" for short. This is expected to form a new generation of time and frequency standards - optical frequency standards, which will play an important role in precision navigation and positioning, global time service, wide area quantum communication, testing of basic principles of physics and other fields. Accurate timing should not be limited to laboratories, but also "fly into ordinary people's homes". Therefore, it is necessary not only to have the most accurate atomic clock, but also to have a time transfer technology that matches its accuracy. At present, microwave and optical fiber are commonly used time-frequency transmission methods, but both of them have their limitations. Scientists have found that there is a magic laser - optical frequency comb (optical comb), which can make it more accurate and easier for people to measure frequency and time interval. Free space time-frequency transfer technology based on optical comb and coherent detection is the development trend of high-precision time-frequency transfer. However, previously, optical frequency transmission technology in free space could only achieve transmission distance of the order of 10km. In this research, the research team realized a highly stable optical comb with watt level power output, realized a high sensitivity linear optical sampling detection at the level of nanowatts, and further improved the stability and reception efficiency of the optical transmission telescope. Based on the above technical breakthrough, the research team successfully realized 113 km free space time-frequency transmission in Urumqi, Xinjiang, fully verifying the feasibility of high-precision optical frequency standard comparison of satellite ground links. The reviewer of Nature magazine said that this research is a major breakthrough in the field of remote optical time and frequency transmission in satellite earth free space, and will have an important impact on basic physics research such as dark matter detection, the test of basic constants of physics, and the test of relativity. (Liu Xinshe)