Test of Special Relativity Using a Fiber Network of Optical Clocks

TLDR

Phase‑compensated optical fiber links enable high‑accuracy atomic clocks separated by thousands of kilometers to be compared with unprecedented statistical resolution, and as clocks improve and fiber links are routinely operated, we expect that the tests initiated in this paper will improve by orders of magnitude in the near future. The authors searched for a daily variation of the frequency difference between four strontium optical lattice clocks in different European locations connected by such links to improve upon previous tests of time dilation predicted by special relativity. They compared the frequency differences of the four strontium optical lattice clocks using phase‑compensated optical fiber links across Europe. They obtained a constraint on the Robertson–Mansouri–Sexl parameter |α|≲1.1×10⁻⁸, improving by a factor of about two the best known Ives–Stilwell constraint and by two orders of magnitude the best atomic‑clock comparison, and demonstrated that this is the first of a new generation of fundamental‑physics tests using optical clocks and fiber links.

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.