High precision optical clock and its absolute frequency measurement in China

Optical clocks can generate ultra-stable and ultra-precise optical frequencies, for potential use in time and frequency metrology, fundamental physics, relativistic geodesy, and so forth. Many groups in China began optical clock research in the early 21st century, and made significant progress in designing and establishing ultra-stable lasers, preparing quantum references, systematic shift evaluation, and absolute frequency measurement. Several groups have already evaluated the systematic frequency shifts of their optical clocks with uncertainties on the order of 10^-18. Some groups have made absolute frequency measurements, and the results of three types of optical clocks have been adopted by the Consultative Committee for Time and Frequency of the International Committee for Weights and Measures. This paper will review the development of high precision optical clocks and their absolute frequency measurements in China, and provide suggestions for future research.