Chinese scholars have made progress in the study of high-precision time-frequency transfer over long free space

Figure Schematic diagram of 100-kilometer high-precision time-frequency transmission experiment

With the support of the National Natural Science Foundation of China (approval number: T2125010, 61825505), Professor Pan Jianwei and Professor Zhang Qiang of the University of Science and Technology of China, in cooperation with the Shanghai Institute of Technology, the Xinjiang Astronomical Observatory, the National Timing Center of the Chinese Academy of Sciences, the Jinan Institute of Quantum Technology and Ningbo University, have realized the high-precision time-frequency transmission of 100 kilometers in free space for the first time in the world, effectively verified the feasibility of high-precision photofrequency benchmarking of the star-ground link, and taken an important step towards the establishment of a wide-area cursor network
。 The results are based on "Free-space dissemination of time and frequency with ^{10-19 in more than 113} kilometers of free space.
" instability over 113 km)" was published in the journal Nature with the link to the paper:

In recent years, the stability of the optical band atomic clock (optical clock) based on ultra-cold atomic crystal 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, and testing of basic principles of physics 。 Further, the high orbit space has a lower gravitational field noise environment, and the stability of the optical frequency scale and time-frequency transfer can theoretically enter the E-21 magnitude, which is expected to have major applications
in the study of basic problems in physics such as gravitational wave detection and dark matter search.

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

In this work, the research team developed the full-protection bias fiber femtosecond laser technology to achieve a highly stable optical frequency comb with watt-level power output; Based on low-noise balanced detection and integrated interferometric optical fiber optical path module, combined with high-precision phase extraction post-processing algorithm, high-sensitivity linear optical sampling detection in the nanowatt order is realized, and the accuracy of single time measurement is better than 100 femtoseconds; The stability and reception efficiency
of the light transmission telescope have been further improved.
On the basis of the above technological breakthroughs, the research team successfully realized 113 kilometers of free space time-frequency transfer in Urumqi, Xinjiang, with a time transfer stability of 10,000 seconds reaching the order of femtoseconds, a frequency transfer stability of 10,000 seconds better than 4E-19, a relative deviation of 6.
3E-20± 3.
4E-19, and the system can tolerate a maximum link loss of up to 89dB, which is much higher than the typical expected value of the mid-high orbit star-ground link loss (about 78dB) (Figure).

In summary, this work solves the problem of high-precision time-frequency transmission at the 100-kilometer level, fully verifies the feasibility of high-precision photofrequency standard comparison of the satellite-to-ground link, and lays a good foundation
for the construction of a wide-area time-frequency network to serve applications such as quantum communication, precision navigation, and global timing.