A research team led by Prof. CHANG Hong from the National Time Service Center (NTSC) of the Chinese Academy of Sciences (CAS) has developed a strontium optical lattice clock with both frequency stability and systematic uncertainty surpassing 2×10⁻¹⁸. This achievement places China among the global leaders in the field of optical lattice clock development.

The breakthrough aligns with the roadmap set by the 27th General Conference on Weights and Measures (CGPM) in 2022, which proposed redefining the SI unit of time—the second—by 2030. The resolution outlined rigorous performance benchmarks for next-generation optical clocks. Strontium optical lattice clocks, known for their exceptional precision, have emerged as the most promising candidates for the redefinition, offering systematic uncertainties two orders of magnitude lower than those of the current cesium fountain clocks.

The results were published online on June 2 in a leading international academic journal in metrology, Metrologia.

The reviewers noted: "The newly developed Strontium optical clock at NTSC has achieved the world's second-smallest uncertainty among optical clocks."

"The Strontium optical lattice clock we developed fully meets the performance requirements for the redefinition of the second, China is now the second country after the United States to achieve both frequency stability and uncertainty below the 2×10⁻¹⁸ threshold," said Prof. CHANG Hong, leader of the research team.

To achieve this ultra-high precision and stability, the researchers integrated several cutting-edge technologies: moving optical lattice technology, Faraday cage technology, active temperature-controlled thermal shield technology, and shallow optical lattice technology based on the inclined lattice. These advancements tackled persistent measurement challenges for critical frequency shifts—including blackbody radiation and density shifts—in conventional strontium optical lattice clocks, suppressing them to the 10^-19 level while consistently maintaining DC Stark shifts at 10^-20. 

"The system also benefits from a highly efficient cold-atom quantum reference preparation process and ultra-narrow linewidth laser technology," said Associate Prof. LU Xiaotong, first author of the paper. Together, the system achieved a frequency stability of 3.6×10^-16 (τ/s)^-0.5 and 1.2×10^-18 over 57,000 seconds, with a total systematic uncertainty of 1.96×10^-18.

This development not only brings China a major step closer to contributing to the future redefinition of the second but also enhances the country's capabilities in precision metrology and fundamental physics research.

The setup of the NTSC SrII optical lattice clock (Imaged by NTSC)