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Currently, metrology laboratories worldwide suffer from a lack of information to characterize their clocks. Until recently, satellite-based comparison techniques using GPS atomic clocks made it possible to compare clocks with each other (and thus to calibrate the stability of one's own clock), but these frequency comparisons are hampered by the limitations of GPS itself.

Despite their high level of maturity, the performance of such comparisons is currently limited to a level of the order of 10-14 - 10-15 in relative terms. These performances are insufficient to compare the best new atomic clocks, and they are also difficult to improve because they are limited by the fluctuations introduced by the signals passing through the atmosphere. It is therefore impossible to use this technique for today's best systems.

Another option, although not very flexible, time-consuming and complicated, is possible if one of the clocks to be compared can be transported to the remote laboratory. However, this solution is far from permanent and, given the complexity of the systems and the number of people required for this operation, these possibilities are limited.

To tackle this problem, the French teams at LPL and SYRTE have been working on the possibility of transferring this frequency stability directly via optical fibers, using a noise correction system introduced during signal propagation in the optical fiber to overcome these limitations. They have demonstrated on the Villetaneuse-Reims-Villetaneuse section of the RENATER academic Internet network (a round trip distance of 540km), that the clock signal carried by an ultra-stable laser with a wavelength of 1542.14 nm, can be transmitted in the optical fiber of the Internet network with an exceptional recopy quality of 2x10-19, i.e. an uncertainty equivalent to 0.1s on the age of the universe!

This alternative method paves the way for a continental-scale comparison of the best terrestrial clocks, whose relative accuracy today stands at some 10-16 and soon 10-17.

The REFIMEVE+ project aims to generalize this concept on a national scale, thanks to close collaboration with RENATER and innovative French SMEs (MuQuans, Keopsys, Syrlinks and Kylia), who will produce the equipment in industrial format, to distribute throughout France a metrological frequency signal generated at Paris Observatory. In addition to LPL, SYRTE and RENATER, which will design, build and supervise the network, 17 other laboratories will receive the metrological signal.

REFIMEVE+ is the first step towards building a Europe-wide network, with an initial extension to Germany envisaged with the support of DFN[M1] (the German academic network). Many other countries are already interested in this possibility. Efficient equipment management on a European scale could be ensured by the DANTE[M2] consortium and its GEANT2[M3] network, which interconnects national academic networks such as RENATER and DFN.

Numerous disciplinary fields will benefit from the dissemination of this frequency: time/frequency metrology, fundamental physics on earth and in space, precision spectroscopy applied to the environment and atmospheric physics. In addition, this instrument can be used as a giant gyrometer and also as a seismic sensor, thanks to an extensive network of optical fibers worldwide.