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

Phase compensated optical fiber links enable high accuracy atomic clocksseparated 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 latticeclocks in different locations throughout Europe connected by such links, we improve upon previous tests of time dilationpredicted by special relativity. We obtain a constraint on the Robertson–Mansouri–Sexl parameter |α| 1.1 × 10 −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 ordersof magnitudethe 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 ordersof magnitudein the near future.