Shulou(Shulou.com)11/24 Report--

CTOnews.com, October 6, according to the official website of the University of Science and Technology of China, recently, Pan Jianwei's team of the University of Science and Technology of China, in cooperation with the Shanghai Institute of Technology and the Xinjiang Observatory, has for the first time realized a 100-kilometer free-space and high-precision time and frequency transmission experiment in the world by developing high-power and low-noise optical comb, high-sensitivity and high-precision linear sampling, high-stability and high-efficiency optical transmission. The stability of time transfer reaches the order of femtosecond, and the stability of frequency transmission of ten thousand seconds is better than that of 4E-19. The experimental results effectively verify the feasibility of high-precision optical frequency standard comparison in satellite-to-ground link, and take an important step towards the establishment of wide-area optical frequency standard network. CTOnews.com learned that the results were published online in the famous international academic journal Nature on October 5, 2022.

In recent years, the stability of optical band atomic clock (optical clock) based on ultra-cold atomic lattice has reached the order of Emur19, and a new generation of time and frequency standard (optical frequency standard) will be formed. combined with wide-area and high-precision time-frequency transmission, a wide-area time-frequency network can be constructed, which will play an important role in precision navigation and positioning, global timing, wide-area quantum communication, testing of basic principles of physics and so on. For example, when the stability of global-scale time-frequency transmission reaches the order of Emur18, a new generation of "second" definition can be formed, and this redefinition of "second" will be discussed at the International Metrology Conference in 2026. Furthermore, the high orbit space has a lower gravitational field noise environment, and the stability of optical frequency standard and time-frequency transmission can theoretically enter the order of Emur21, which is expected to have important applications in the study of basic physical problems such as gravitational wave detection and dark matter search. However, the traditional microwave-based satellite time-frequency transmission stability is only Emur16, which can not meet the needs of high-precision time-frequency networks. The stability of free space time-frequency transmission technology based on optical frequency comb and coherent detection can reach the order of Emur19, which is the development trend of high-precision time-frequency transmission, but the previous international related work has low signal-to-noise ratio and short transmission distance. It is difficult to meet the needs of high-precision time-frequency transmission in satellite-to-ground links.

In this work, the research team developed the full polarization maintaining fiber femtosecond laser technology to achieve a high stable optical frequency comb with watt power output; based on low noise balance detection and integrated interference optical fiber optical path module, combined with high precision phase extraction post-processing algorithm, high sensitivity linear optical sampling detection is realized, and the single time measurement accuracy is better than 100 femtosecond. The stability and receiving efficiency of the optical transmission telescope are further improved. On the basis of the above technological breakthroughs, the research team successfully realized 113km free space time-frequency transmission in Urumqi, Xinjiang. The ten-second stability of time transmission reaches the order of femtosecond, the stability of frequency transmission is better than that of 4E-19, the relative deviation of the system is 6.3E-20 ±3.4E-19, and the maximum link loss of the system can be tolerated as high as 89dB, which is much higher than the typical expected value of medium-and high-orbit satellite-to-ground link loss (about 78dB). The feasibility of high-precision optical frequency standard comparison in satellite-to-ground link is fully verified.

Schematic diagram of ▲ 100 km high precision time-frequency transmission experiment | Source: official website of China University of Science and Technology

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