Absolute frequency of $^{87}\mathrm{Sr}$ at $1.8 \times 10^{-16}$ uncertainty by reference to remote Primary Frequency Standards

TL;DR

This paper reports a highly precise measurement of the absolute frequency of the $^{87}$Sr optical lattice clock transition, achieving an uncertainty below 2 parts in 10^16 by referencing multiple primary frequency standards.

Contribution

The study provides the most accurate absolute frequency measurement of $^{87}$Sr to date, approaching the systematic limits of the SI second, and demonstrates consistency with other recent measurements.

Findings

Absolute frequency of $^{87}$Sr: 429,228,004,229,873.082(76) Hz

Fractional uncertainty below 1.8x10^{-16}

Results are consistent with other recent measurements

Abstract

The optical lattice clock NICT-Sr1 regularly reports calibration measurements of the international timescale TAI. By comparing measurement results to the reports of eight Primary Frequency Standards, we find the absolute frequency of the $^{87}\mathrm{Sr}$ clock transition to be f(Sr)=$429\,228\,004\,229\,873.082(76)$, with a fractional uncertainty of less than 1.8x10$^{-16}$ approaching the systematic limits of the best realization of SI second. Our result is consistent with other recent measurements and further supported by the loop closure over the absolute frequencies of $^{87}\mathrm{Sr}$, $^{171}\mathrm{Yb}$ and direct optical measurements of their ratio.