China has used an optical clock to calibrate International Atomic Time (TAI) for the first time, marking a milestone in its contribution to TAI calibration.

According to Circular T 457, the latest issue released on February 11 by the International Bureau of Weights and Measures (BIPM), the strontium optical lattice clock NTSC-Sr2, developed by the National Time Service Center (NTSC) of the Chinese Academy of Sciences, was used as a secondary representation of the second to calibrate TAI.

BIPM computes TAI monthly as a reference for calibrating national or regional standard time scales to achieve worldwide time uniformity. TAI is generated by daily, satellite-based comparisons of over 400 continuously operating clocks from various countries and regions. The clocks' data are combined with appropriate weighting to produce a stable, free atomic time scale. This timescale is then calibrated using current caesium fountain primary standards, as well as secondary standards, such as rubidium fountain clocks and optical clocks, to obtain an atomic timescale that is both accurate and stable.

Under the current definition of the base unit of time, the "second," the primary standards in use today are cesium fountain clocks. Cesium fountain clocks developed by the NTSC and the NIM both serve as primary standards for calibrating TAI.

However, optical clocks outperform cesium fountain clocks by more than two orders of magnitude. BIPM has established a roadmap for redefining the second based on optical clocks, with implementation planned after 2030, and has set stringent performance requirements for optical clocks. After more than a decade of dedicated research and development, the NTSC strontium optical clock NTSC-Sr2 has achieved a frequency stability and uncertainty both better than 2×10^-18, reaching an internationally advanced level. It is China's first optical clock whose performance meets the requirements for a future redefinition of the second.

BIPM has a dedicated expert group, WG-SPFS, which is responsible for evaluating and endorsing primary and secondary standards. Data from strontium optical clocks submitted by both NTSC and NIM have passed this review process. In January 2026, the NTSC formally began steering TAI, an accomplishment previously achieved by only a few countries, including the United States, the United Kingdom, France, and Japan.

This milestone signifies that China has mastered the full technical chain of capabilities, from independently developing high-performance optical clocks to using them to steer TAI. 

NTSC generates, maintains, and disseminates China's national standard time, commonly known as Beijing Time, through distribution methods that support economic and social infrastructure. At present, Beijing Time is independently controlled, and its accuracy ranks first internationally.