Improved absolute frequency measurement of 171Yb at NMIJ with uncertainty below 2x10-16

We report improved absolute frequency measurement of the S-1(0)-P-3(0) transition of Yb-171 at National Metrology Institute of Japan (NMIJ) by comparing the Yb-171 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 1.9x10(-16), in good agreement with the recommended frequency of Yb-171 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 Yb-171 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.