Work package leader: Marco Pizzocaro, INRIM

Other participants: LNE, NPL, OBSPARIS, PTB, SRC PAS, UMK

The aim of this work package is to incorporate optical clocks into International Atomic Time (TAI) as secondary representations of the second. This integration is an essential prerequisite for a redefinition of the second based on an optical transition.

With the current definition of the second being based on the caesium ground state hyperfine transition, optical clocks may only contribute to TAI steering with the uncertainty of the recommended value for the secondary representation of the second. These recommended frequency values and uncertainties are decided by the CCL-CCTF Frequency Standards Working Group (WGFS). Within this work package we will perform up-to-date evaluations of the expanding world-wide body of clock comparison data, using methods developed within the SIB55 ITOC project, and taking full account of correlations for the first time. In this way, optimal values for the frequencies and uncertainties of optical secondary representations of the second will be derived and shared with the WGFS when they are considering updates to the list of recommended frequency values.

In order to contribute to TAI steering, optical clocks will be compared with hydrogen masers that are used in the computation of TAI, and the frequency comparison data submitted to the International Bureau of Weights and Measures (BIPM) for inclusion in the monthly bulletin Circular T. Our target is for at least one optical clock per participating laboratory to be submitting data to the BIPM on a regular basis by the end of the project.

The incorporation of optical clocks into TAI will enable the metrology community to prepare for a future redefinition of the SI second, enabling the long-term performance and reliability of the clocks to be assessed.