International optical clock comparison using the European optical fiber network
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 171Yb+(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 199Hg clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with 171Yb+(E3), 171Yb, and 87Sr 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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BibTeX
@article{pizzocaro2026international,
title = {{International optical clock comparison using the European optical fiber network}},
author = {Marco Pizzocaro and Clara Zyskind and Anne Amy-Klein and Erik Benkler and Sebastien Bize and Davide Calonico and Etienne Cantin and Christian Chardonnet and Cecilia Clivati and Stefano Condio and E. Anne Curtis and Simone Donadello and Sören Dörscher and Chen-Hao Feng and Melina Filzinger and Jacques-Olivier Gaudron and Rachel Godun and Irene Goti and Ian Hill and Wei Huang and Nils Huntemann and Matthew Johnson and Joshua Klose and Jochen Kronjäger and Alexander Kuhl and Rodolphe Le Targat and Filippo Levi and Burghard Lipphardt and Christian Lisdat and Jerome Lodewyck and Olivier Lopez and Helen Margolis and Maxime Mazouth-Laurol and Alberto Mura and Benjamin Pointard and Paul-Eric Pottie and Matias Risaro and Billy Robertson and Marco Schioppo and Kilian Stahl and Martin Steinel and Alexandra Tofful and Mads Tønnes and Jacob Tunesi},
journal = {Physical Review Research},
year = {2026},
month = sep,
doi = {10.1103/l4bh-ryxs},
eprint = {2604.27963},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05745100v1},
note = {CC BY},
}
RIS
TY - JOUR
TI - International optical clock comparison using the European optical fiber network
AU - Marco Pizzocaro
AU - Clara Zyskind
AU - Anne Amy-Klein
AU - Erik Benkler
AU - Sebastien Bize
AU - Davide Calonico
AU - Etienne Cantin
AU - Christian Chardonnet
AU - Cecilia Clivati
AU - Stefano Condio
AU - E. Anne Curtis
AU - Simone Donadello
AU - Sören Dörscher
AU - Chen-Hao Feng
AU - Melina Filzinger
AU - Jacques-Olivier Gaudron
AU - Rachel Godun
AU - Irene Goti
AU - Ian Hill
AU - Wei Huang
AU - Nils Huntemann
AU - Matthew Johnson
AU - Joshua Klose
AU - Jochen Kronjäger
AU - Alexander Kuhl
AU - Rodolphe Le Targat
AU - Filippo Levi
AU - Burghard Lipphardt
AU - Christian Lisdat
AU - Jerome Lodewyck
AU - Olivier Lopez
AU - Helen Margolis
AU - Maxime Mazouth-Laurol
AU - Alberto Mura
AU - Benjamin Pointard
AU - Paul-Eric Pottie
AU - Matias Risaro
AU - Billy Robertson
AU - Marco Schioppo
AU - Kilian Stahl
AU - Martin Steinel
AU - Alexandra Tofful
AU - Mads Tønnes
AU - Jacob Tunesi
PY - 2026
DA - 2026/09/01
JO - Physical Review Research
DO - 10.1103/l4bh-ryxs
UR - https://hal.science/hal-05745100v1
AB - 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 ^171Yb^+(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 10^17. The operation of the ^199Hg clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with ^171Yb^+(E3), ^171Yb, and ^87Sr 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.
SN - 2643-1564
SN - 2643-1564
ER -