Coherent Optical-Fiber Link Across Italy and France

The dissemination of atomic clocks with fiber-based techniques finds application in the fields metrology, fundamental physics, navigation, and spectroscopy but is a challenge terms reliability, maintenance, performance. Here, we describe realization 1023-km-long fiber link between metrological institutes Italy France that shares infrastructure Internet traffic exploits segmentation shorter, cascaded spans to fight optical losses exceeding 280 dB. With four months quasicontinuous operation this link, compared $\mathrm{Cs}$, $\mathrm{Rb}$, $\mathrm{Yb}$ at our laboratories, highlighting potential tool assess clock uncertainty budgets, characterize advanced satellite techniques, develop timescales. integration metrological, infrastructures two countries, connecting photonics laboratories as well telescope facilities, provides research community physical layer over which applications can be built on.