China News Service, Beijing, October 5th (Reporter Sun Zifa) In the field of long-distance optical time-frequency transmission in star-earth free space, Chinese scientists have recently made an important breakthrough for the first time in the world - achieving high-precision time-frequency transmission in free space of 100 kilometers.

  This major achievement paper in the field of basic scientific research, which is expected to have major applications in the study of basic problems in physics from navigation to gravitational wave detection and dark matter search, was published online in the internationally renowned academic journal "Nature" on the night of October 5th, Beijing time.

The reviewers of "Nature" highly praised that this research work is a major breakthrough in the field of long-distance optical time-frequency transfer in free space between stars and the earth, and will conduct research on basic physics such as dark matter detection, basic constant testing of physics, and relativity testing. have an important impact.

  The Chinese Academy of Sciences (CAS) released information to the media that this research work was carried out by the team of Academician Pan Jianwei of the University of Science and Technology of China, the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences, the Xinjiang Observatory of the Chinese Academy of Sciences, the National Time Service Center of the Chinese Academy of Sciences, Jinan Institute of Quantum Technology, Ningbo University, etc. , through the development of high-power and low-noise optical combs, high-sensitivity and high-precision linear sampling, high-stability and high-efficiency optical transmission and other technologies, it is the first time in the world to achieve a hundred-kilometer-level free space high-precision time-frequency transfer experiment. On the order of seconds, the stability of frequency transfer in ten thousand seconds is better than that of 4E-19 (on the order of E-19, which is equivalent to the error of the clock for about 100 billion years of no more than one second).

This experimental result effectively verifies the feasibility of high-precision optical frequency standard comparison of satellite-to-ground links, and takes an important step towards establishing a wide-area optical frequency standard network.

  According to the research team, in recent years, the stability of optical-band atomic clocks (optical clocks) based on ultra-cold atomic optical lattices has reached the E-19 level, and will form a new generation of time-frequency standards (optical frequency standards) that combine wide-ranging Domain, high-precision time-frequency transfer 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.

For example, when the stability of global-scale time-frequency transmission reaches the magnitude of E-18 (equivalent to the clock's error of no more than one second for about ten billion years), a new generation of "second" definition can be formed. The conference will discuss this redefinition of the "second".

Further development on this basis, the high-orbit space has a lower gravitational field noise environment, and the stability of the optical frequency standard and time-frequency transfer can theoretically enter the E-21 (equivalent to about ten trillion years of the clock error of no more than one It is expected to have major applications in the study of fundamental problems in physics such as gravitational wave detection and dark matter search.

  However, the traditional microwave-based satellite time-frequency transfer stability is only on the order of E-16 (equivalent to a clock with an error of less than one second for about 100 million years), which cannot meet the needs of high-precision time-frequency networks.

The free space time-frequency transfer technology based on optical frequency comb and coherent detection, the stability can reach E-19 level, which is the development trend of high-precision time-frequency transfer, but the related work in the world has low signal-to-noise ratio and short transmission distance. , it is difficult to meet the requirements of high-precision time-frequency transmission of satellite-to-ground links.

  In view of the fact that the traditional time-frequency transfer stability cannot meet the needs of high-precision time-frequency networks, the scientific research team has mainly overcome three problems in this cooperative research: First, by developing the all-polarization-maintaining fiber femtosecond laser technology, the watt-level power output can be achieved. High-stability optical frequency comb; second, based on low-noise balanced detection and integrated interference fiber optical path module, combined with high-precision phase extraction post-processing algorithm, to achieve nanowatt-level high-sensitivity linear optical sampling detection, and the single-time measurement accuracy is better than 100 femtosecond; third, the stability and receiving efficiency of the optical transmission telescope are further improved.

  Based on the above-mentioned three major technological breakthroughs, a team of Chinese scientists has successfully achieved 113 kilometers of free space time-frequency transmission in Urumqi, Xinjiang. The relative deviation is 6.3E-20±3.4E-19, and the system can tolerate a maximum link loss of up to 89dB (ie, signal loss to about one part in a billion), which is much higher than the typical expected value of medium and high orbit satellite-ground link loss ( About 78dB, that is, the signal loss is about 1/100,000,000), which fully verifies the feasibility of the high-precision optical frequency standard comparison of the satellite-ground link.

  The scientific research team revealed that after the first 100-kilometer-level free space high-precision time-frequency transfer experiment in the world, the team will also combine the development of medium and high orbit quantum satellites in the next step, and strive to be the first in the world to achieve high-precision time-frequency transfer from satellite to ground. .

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