International atomic clock comparison achieving 5× 10−18 uncertainty
Abstract
Two single-ion optical clocks, located in the United Kingdom and in Germany, are compared remotely via a 2220\,km long optical fiber link. Both clocks are based on the 2S1/2 (F=0)↔ 2F7/2 (F=3) electric octupole (E3) transition in 171Yb+. Using more than 106 seconds of measurement data, we determine a fractional frequency difference of 1.0(4.8)× 10−18, demonstrating agreement of the independently developed systems. To our knowledge, this constitutes the first international clock comparison reaching a fractional uncertainty below 5× 10−18, an important milestone towards an anticipated optical redefinition of the SI second. The measurement uncertainty is dominated by that of the relativistic redshift correction between the two sites, which includes time-varying gravity potential differences. We exploit the large baseline between the two clocks to search for violations of Lorentz invariance. In the absence of statistically significant modulations, we improve constraints on the Robertson-Mansouri-Sexl parameter α by approximately a factor of five in the uncertainty, and provide limits on corresponding parameters within the Standard-Model Extension framework.
Related
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BibTeX
@misc{tofful2026international,
title = {{International atomic clock comparison achieving $5\times 10^-18$ uncertainty}},
author = {A. Tofful and M. Filzinger and T. Schmidt and A. Amy-Klein and E. Benkler and E. Cantin and C. Chardonnet and E. A. Curtis and H. Denker and B. Dutta and J. -O. Gaudron and W. Huang and J. Kronjäger and A. Kuhl and R. Le Targat and B. Lipphardt and O. Lopez and S. Mukherjee and A. L. Parke and B. Pointard and P. -E. Pottie and P. J. Regan and M. Risaro and M. Schioppo and M. Steinel and J. Tunesi and X. Zhang and R. M. Godun and N. Huntemann},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.29980},
eprint = {2610.29980},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.29980},
}
RIS
TY - UNPB
TI - International atomic clock comparison achieving 5× 10^−18 uncertainty
AU - A. Tofful
AU - M. Filzinger
AU - T. Schmidt
AU - A. Amy-Klein
AU - E. Benkler
AU - E. Cantin
AU - C. Chardonnet
AU - E. A. Curtis
AU - H. Denker
AU - B. Dutta
AU - J. -O. Gaudron
AU - W. Huang
AU - J. Kronjäger
AU - A. Kuhl
AU - R. Le Targat
AU - B. Lipphardt
AU - O. Lopez
AU - S. Mukherjee
AU - A. L. Parke
AU - B. Pointard
AU - P. -E. Pottie
AU - P. J. Regan
AU - M. Risaro
AU - M. Schioppo
AU - M. Steinel
AU - J. Tunesi
AU - X. Zhang
AU - R. M. Godun
AU - N. Huntemann
PY - 2026
DA - 2026/10/21
PB - arXiv
DO - 10.48550/arxiv.2610.29980
UR - https://arxiv.org/abs/2610.29980
AB - Two single-ion optical clocks, located in the United Kingdom and in Germany, are compared remotely via a 2220\,km long optical fiber link. Both clocks are based on the ^2S_1/2 (F=0)↔ ^2F_7/2 (F=3) electric octupole (E3) transition in ^171Yb^+. Using more than 10^6 seconds of measurement data, we determine a fractional frequency difference of 1.0(4.8)× 10^−18, demonstrating agreement of the independently developed systems. To our knowledge, this constitutes the first international clock comparison reaching a fractional uncertainty below 5× 10^−18, an important milestone towards an anticipated optical redefinition of the SI second. The measurement uncertainty is dominated by that of the relativistic redshift correction between the two sites, which includes time-varying gravity potential differences. We exploit the large baseline between the two clocks to search for violations of Lorentz invariance. In the absence of statistically significant modulations, we improve constraints on the Robertson-Mansouri-Sexl parameter α by approximately a factor of five in the uncertainty, and provide limits on corresponding parameters within the Standard-Model Extension framework.
ER -