Improved absolute frequency measurement of 171Yb at NMIJ with uncertainty below 2 × 10-16

Abstract We report improved absolute frequency measurement of the 1 S 0 - 3 P 0 transition of 171 Yb at National Metrology Institute of Japan (NMIJ) by comparing the 171 Yb optical lattice clock NMIJ-Yb1 with 13 Cs primary frequency standards via International Atomic Time from August 2021 to May 2023. The measured absolute frequency is 518 295 836 590 863.62(10) Hz with a fractional uncertainty of 1.9 × 10 -16 , in good agreement with the recommended frequency of 171 Yb as a secondary representation of the second. This uncertainty is 2.6 times lower than our previous measurement uncertainty, and slightly lower than any uncertainties of the absolute frequency measurements of 171 Yb that have so far been reported by other institutes. We also estimate correlation coefficients between our present and previous measurements, which is important for updating the recommended frequency.