Year: 2026
Type: preprint
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 ${}^2S_{1/2} (F=0)\leftrightarrow {}^2F_{7/2} (F=3)$ electric octupole (E3) transition in $^{171}\text{Yb}^+$. Using more than $10^6$ seconds of measurement data, we determine a fractional frequency difference of $1.0(4.8)\times 10^{-18}$, demonstrating agreement of the independently developed systems. To our knowledge, this constitutes the first international clock comparison reaching a fractional uncertainty below $5\times 10^{-18}$, 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.
Source: arXiv (Cornell University)
Authors: Tofful, A., Filzinger, M., Schmidt, T., Amy-Klein, A., Benkler, E., Cantin, E., Chardonnet, C., Curtis, E. A., Denker, H., Dutta, B., Gaudron, J. -O., Huang, W., Kronjäger, J., Kuhl, A., Targat, R. Le, Lipphardt, B., Lopez, O., Mukherjee, S., Parke, A. L., Pointard, B., Pottie, P. -E., Regan, P. J., Risaro, M., Schioppo, M., Steinel, M., Tunesi, J., Zhang, X., Godun, R. M., Huntemann, N.
Institutions: National Physical Laboratory, Physikalisch-Technische Bundesanstalt, Université Sorbonne Paris Nord, Laboratoire de physique des lasers, Leibniz University Hannover, Centre National de la Recherche Scientifique, Université de Lille, Université Paris Sciences et Lettres, Observatoire de Paris, Sorbonne Université, Laboratoire Temps Espace
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7223247093