Pan Jianwei et al. realized high-precision time-frequency transmission in 100 kilometers of free space

At 23 o’clock on October 5, the international academic journal “Nature” published online the new results of cooperation between Professor Pan Jianwei of the University of Science and Technology of China and his colleagues Zhang Qiang, Jiang Haifeng, Peng Chengzhi and other units such as Shanghai Institute of Technology, Xinjiang Astronomical Observatory, National Timing Center of the Chinese Academy of Sciences, Jinan Institute of Quantum Technology and Ningbo University. Through the development of high-power and low-noise optical comb, high-sensitivity and high-precision linear sampling, high-stability and high-efficiency optical transmission, etc., this achievement realizes the 100-kilometer-level free space high-precision time frequency transmission for the first time in the world, effectively verifies the feasibility of high-precision optical frequency scale comparison of satellite-ground links, and takes an important step towards the establishment of wide-area optical frequency standard network.

The reviewer commented: “This work is a major breakthrough in the field of long-distance optical time-frequency transmission in satellite-ground free space, and will have an important impact on basic physics research such as dark matter detection, physics fundamental constant test, and relativity test.” ”

In recent years, the stability of the optical band atomic clock (light clock) based on the ultracold atomic optical lattice has entered the E-19 level, which will form a new generation of time frequency standards (optical frequency scale), combined with wide-area, high-precision time-frequency transmission can build a wide-area time-frequency network, which will play an important role in precision navigation and positioning, global timing, wide-area quantum communication, physics basic principle testing and other fields. For example, when the stability of time-frequency transmission at the global scale reaches the E-18 level, a new generation of “second” definition can be formed, and this “second” redefinition will be discussed at the 2026 International Conference on Weights and Measures. Further, the high-orbit space has a lower gravitational field noise environment, and the stability of optical frequency scale and time-frequency transmission can theoretically enter the E-21 level, which is expected to have major applications in the research of basic problems in physics such as gravitational wave detection and dark matter search.

However, the time-frequency transmission stability of traditional microwave-based satellites is only on the order of E-16, which cannot meet the needs of high-precision time-frequency networks. Based on the free space time-frequency transmission technology of optical frequency comb and coherent detection, the stability can reach the E-19 level, which is the development trend of high-precision time-frequency transmission, but the previous international related work has low signal-to-noise ratio and close transmission distance, which is difficult to meet the needs of high-precision time-frequency transmission of satellite-ground links.

In the results published this time, the research team developed the full-fidelity fiber femtosecond laser technology to achieve a highly stable optical frequency comb with watt-level power output; Based on low-noise balance detection and integrated interferometric fiber optical path module, combined with high-precision phase extraction post-processing algorithm, high-sensitivity linear optical sampling detection of the nano-level is realized, and the accuracy of a single time measurement is better than 100 femtoseconds. The stability and reception efficiency of the optical transmission telescope are further improved.

On the basis of the above technological breakthroughs, the research team successfully realized 113 kilometers of free space time-frequency transmission in Urumqi, Xinjiang, the stability of time transmission in 10,000 seconds reached the femtosecond level, the stability of frequency transmission in 10,000 seconds was better than 4E-19, and the system could tolerate the maximum link loss of up to 89dB, which was much higher than the typical expected value of the medium-high orbit satellite-ground link loss (about 78dB), which fully verified the feasibility of high-precision optical frequency scale comparison of satellite-ground links.

Schematic diagram of 100 km high-precision time-frequency transmission experiment (Courtesy of China University of Science and Technology)

Shen Qi, Guan Jianyu, and researcher Ren Jigang of the University of Science and Technology of China are the co-first authors of this paper. The work was funded and supported by the Chinese Academy of Sciences, the Ministry of Science and Technology, the Foundation Committee, Anhui Province, Shanghai Municipality and Shandong Province. (Source: Ni Sijie, China Science Daily)

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