Multiplexed optical link for ultra-stable frequency dissemination

We demonstrate the ultra stable transfer an optical frequency over 300 km of installed optical fibers. This ultrastable optical link uses an optical telecommunication network simultaneously carrying Internet data and goes through two Data Center Facilities using multiplexers and bidirectional erbium-doped fiber amplifiers. We have obtained an instability of 3×10^-15 at 1 s which scales down to around 7×10^-20 after about 20 hours. These results are promising and represent an intermediate step for the future development of continental-scale frequency transfer.