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International optical clock comparison using the European optical fiber network

Marco Pizzocaro1,*, Clara Zyskind2, Anne Amy-Klein3, Erik Benkler4, Sebastien Bize2, Davide Calonico1, Etienne Cantin3, Christian Chardonnet3, Cecilia Clivati1 et al.

Stefano Condio1, E. Anne Curtis5, Simone Donadello1, Sören Dörscher4, Chen-Hao Feng5, Melina Filzinger4, Jacques-Olivier Gaudron5, Rachel M. Godun5, Irene Goti1, Ian R. Hill5, Wei Huang5, Nils Huntemann4, Matthew Johnson5, Joshua Klose4, Jochen Kronjäger4, Alexander Kuhl4, Rodolphe Le Targat2, Filippo Levi1, Burghard Lipphardt4, Christian Lisdat4, Jerome Lodewyck2, Olivier Lopez3, Helen S. Margolis5, Maxime Mazouth-Laurol2, Alberto Mura1, Benjamin Pointard2, Paul-Eric Pottie2, Matias Risaro1,5, Billy I. Robertson5, Marco Schioppo5, Kilian Stahl4, Martin Steinel4, Alexandra Tofful5, Mads Tønnes2, and Jacob Tunesi5

  • *Contact author: m.pizzocaro@inrim.it
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Phys. Rev. Research 8, 033250 – Published 1 September, 2026

DOI: https://doi.org/10.1103/l4bh-ryxs

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 7.7×10−18 to 6.1×10−17. Among the results, the Yb+171(E3) clocks at NPL and PTB demonstrated agreement within an uncertainty of 7.7×10−18, marking the first international verification of two independently developed optical clocks below one part in 1017. The operation of the Hg199 clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with Yb+171(E3), Yb171, and Sr87 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.

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Phys. Rev. Research 8, 033250– Published 1 September, 2026
  • Received 4 May 2026
  • Accepted 9 July 2026
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