Supported by the National Natural Science Foundation of China (grant No. T2125010, 61825505) and other projects, the research team led by Prof. Jianwei Pan and Prof. Qiang Zhang from University of Science and Technology of China, in collaboration with Shanghai Institute of Technical Physics, Xinjiang Astronomical Observatory and National Time Service Center of Chinese Academy of Sciences, Jinan Institute of Quantum Technology and Ningbo University, have realized the experiment of high-precision time and frequency dissemination over a 113 km free-space path for the first time in the world. The work effectively verifies the feasibility of high-precision optical frequency standard comparison of satellite-ground links, taking a significant step towards establishing a wide-area optical frequency standard network. The research results were published in Nature on October 5, 2022, entitled of “Free-space dissemination of time and frequency with 10^-19 instability over 113 km”. The article can be accessed via the link: https://www.nature.com/articles/s41586-022-04942-4.

The frequency stability of state-of-the-art optical clocks based on ultracold atomic optical lattices has recently reached the E-19 level, which can be utilized for the next-generation definition of time and frequency standard (namely, optical frequency standard). Combined with large range and high-precision time and frequency dissemination, a wide-area time and frequency network can be constructed, which will play an indispensable role in many application fields such as precision navigation and positioning, wide-area quantum communication, global positioning, Navigation and Timing (PNT), and testing of fundamental physics. Furthermore, since the high earth orbit has a lower gravitational noise environment, the stability of optical frequency standard and time-frequency dissemination can reach the E-21 level theoretically, which is expected to have major applications in the fields of gravitational wave detection, dark matter studies, and other fundamental researches in physics.

However, the stability of time-frequency dissemination of traditional microwave-based satellite has only reached the E-16 level so far, unable to meet the requirements of high-precision time-frequency network. Although the free-space time-frequency dissemination technology based on optical frequency comb and coherent optical detection, as the trend of high-precision time and frequency dissemination, can obtain a stability of E-19 level, several studies only demonstrated the experiment with lower attenuation limits over short dissemination links, which are difficult to directly apply to high-precision time-frequency dissemination in satellite-ground links.

In this work, the research team developed an all polarization-maintaining (PM) fiber femtosecond laser technology to achieve a stable optical frequency comb with watt level output power. On the basis of low-noise balanced detection and integrated interferometry fiber module, combined with high-precision phase extraction post-processing algorithm, the high-sensitivity linear optical sampling detection was achieved with nano-watt level power. The accuracy of single time measurement was better than 100 femtoseconds, as well as the stability and receiving efficiency of the optical dissemination telescope were further improved in this research. Based on the above technological breakthroughs, the research team successfully realized the 113 km free-space time-frequency dissemination in Urumqi, Xinjiang. The stability of time dissemination in 10,000 seconds reached the femtosecond level, as well as the stability of frequency dissemination in 10,000 seconds was better than 4E-19. The relative deviation of this system was 6.3E-20 ± 3.4E-19, and the maximum link loss could be up to 89dB, which is much higher than the typical expected value of satellite-ground link loss (about 78dB) in medium and high earth orbit.

In summary, this research solves the key problem of high-precision time-frequency dissemination over 100 kilometers free-space links, which fully verifies the feasibility of high-precision optical frequency standard comparison of satellite-ground links to lay a good foundation for the construction of wide-area time-frequency network, quantum communication, precision navigation, global timing and other applications.

Figure. Schematic of the high-precision time-frequency dissemination experiments over the 113 km free-space link