精密时频基准是国际时间频率研究的热点与难点。国际时间频率咨询委员会（CCTF）建议在2025-2028年重新定义“秒”，光钟将替代铯原子喷泉钟成为下一代秒定义复现装置。利用铯喷泉钟直接比对评估光钟绝对频率存在两点局限：1）受限于本地铯喷泉钟性能；2）单一评估方法缺乏验证、并不可靠。为了解决上述问题，本项目利用国际原子时TAI链路开展光钟绝对频率远程溯源与比对研究：1）建立测量模型，从理论上证明溯源可行性；2）评估本地飞轮（氢钟）噪声和测量死区引起的不确定度，提高溯源可靠性；3）研究提升测量能力的关键技术，评估不确定度进入E-16量级；4）NIM锶光钟绝对频率测量，实验验证溯源方法。项目成果将推动光钟技术发展、奠定秒定义修改的国际话语权。

Precision time and frequency reference is a hotspot and difficulty in international time and frequency research. The International Consultative Committee on Time and Frequency (CCTF) recommends that ‘second’ will be redefined in 2025-2028, and then optical clock will replace cesium atomic fountain clock as the next generation of second definition reproduction device. There are two limitations in evaluating the absolute frequency of optical clock by direct comparison with local cesium fountain clock: 1) the performance of local cesium fountain clock may be low; 2) the single evaluation method is not validated and reliable. In order to solve the above problems, the project performs remote traceability and comparison of optical clock absolute frequency by international Atomic Time (TAI) link: 1) establish a measurement model to prove the feasibility of absolute frequency traceability in theory; 2) evaluate the uncertainty caused by local flywheel (hydrogen clock) noise and measurement dead zone to improve the traceability reliability; 3) study the key technologies to improve the measurement capability and to enter E-16 magnitude of uncertainty; 4) Measure the absolute frequency of NIM strontium optical clock to verify the traceability method by experiments. The project will promote the development of optical clock technologies and support Chinese voice about second definition modification in the world.