# Test of special relativity using a fiber network of optical clocks

## TL;DR

This paper uses a network of optical clocks connected by fiber links across Europe to test special relativity, achieving the most stringent constraints on time dilation violations to date.

## Contribution

It introduces a novel method of testing special relativity using fiber-linked optical clocks, significantly improving existing constraints on time dilation violations.

## Key findings

- Constraint on Robertson--Mansouri--Sexl parameter ||  1.1  10^{-8}
- Improved the best known constraint by a factor of two over previous Ives--Stilwell experiments
- Enhanced the constraint by two orders of magnitude compared to previous atomic clock comparisons

## Abstract

Phase compensated optical fiber links enable high accuracy atomic clocks separated by thousands of kilometers to be compared with unprecedented statistical resolution. By searching for a daily variation of the frequency difference between four strontium optical lattice clocks in different locations throughout Europe connected by such links, we improve upon previous tests of time dilation predicted by special relativity. We obtain a constraint on the Robertson--Mansouri--Sexl parameter $|\alpha|\lesssim 1.1 \times10^{-8}$ quantifying a violation of time dilation, thus improving by a factor of around two the best known constraint obtained with Ives--Stilwell type experiments, and by two orders of magnitude the best constraint obtained by comparing atomic clocks. This work is the first of a new generation of tests of fundamental physics using optical clocks and fiber links. As clocks improve, and as fiber links are routinely operated, we expect that the tests initiated in this paper will improve by orders of magnitude in the near future.