In the past few years,optical atomic clocks have made spectacular progress, becoming 100 times more precise than the best caesium clocks. So far, or their precision has been available only locally,since frequency transfer via satellite cannot provide sufficient resolution. This has recently changed thanks to a fresh direct optical connection between France and Germany, established by joint work of Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, and Systèmes de Référence Temps-Espace (LNE-SYRTE) in Paris,and Laboratoire de Physique des Lasers (LPL) in Villetaneuse. tall-precision optical frequencies can now "travel" through a 1400 km optical fibre link between LNE-SYRTE and PTB, where the most precise optical clocks in Europe are operated.
Source: phys.org