International Optical Clock Comparison Using the European Optical Fiber Network

TL;DR

This paper reports on an international optical clock comparison using Europe's fiber network, achieving unprecedented agreement levels and supporting the redefinition of the SI second.

Contribution

It demonstrates the first international verification of optical clocks below 10^{-17} uncertainty using fiber links, advancing global timekeeping accuracy.

Findings

Optical frequency ratios with uncertainties from 7.7×10^{-18} to 6.1×10^{-17} were achieved.

First international verification of two independent optical clocks below 10^{-17}.

Improved frequency ratio uncertainties for several optical clocks, aiding SI second redefinition.

Abstract

Optical clocks have achieved remarkable estimated fractional frequency uncertainties reaching the $10^{-18}$ level and below, enabling applications in fundamental physics, general relativity, and geodesy. However, the challenge of verifying the international consistency of optical clocks remains critical as efforts intensify toward redefining the SI second based on an optical transition or transitions. We report on a two-month international clock comparison campaign involving seven optical clocks in four national metrology institutes (INRIM, LNE-OP, NPL, and PTB) connected via the optical fiber network established in Europe. The campaign resulted in optical frequency ratios with uncertainties ranging from $7.7\times10^{-18}$ to $6.1\times10^{-17}$. Among the results, the $^{171}$Yb$^+$(E3) clocks at NPL and PTB demonstrated agreement within an uncertainty of $7.7\times10^{-18}$, marking the first international verification of two independently developed optical clocks below one part in $10^{17}$. The operation of the $^{199}$Hg clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with $^{171}$Yb$^+$(E3), $^{171}$Yb, and $^{87}$Sr optical clocks. These results provide input for the redefinition of the second and underscore how fiber-linked clock networks can advance metrology and scientific applications.