Improved absolute frequency measurement of $^{171}$Yb at NMIJ with uncertainty below $2\times10^{-16}$

TL;DR

This paper reports a highly precise absolute frequency measurement of the $^{171}$Yb optical transition, achieving an uncertainty below $2 imes 10^{-16}$, surpassing previous measurements and aligning well with the recommended value.

Contribution

The study presents an improved measurement of the $^{171}$Yb transition frequency with lower uncertainty, using comparison with primary standards over an extended period.

Findings

Absolute frequency measured as 518 295 836 590 863.62(10) Hz

Fractional uncertainty of 1.9×10⁻¹⁶ achieved

Measurement uncertainty reduced by a factor of 2.6 compared to previous results

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 $1.9\times10^{-16}$, 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.