Improved absolute frequency measurement of ${}^{171}$Yb at NMIJ with uncertainty below $2\times 10^{-16}$

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 $1.9\times 10^{-16}$, 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.

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