Synchronous mode-locked laser network with sub-fs drift and multi-km distance

We report recent progress made in a synchronous multi-color mode-locked laser network over 4.7-km distance. Output of two remotely synchronized lasers shows only 0.6-fs RMS drift over 40 hours reaching 20th decimal uncertainty in less than 10000-s averaging time. This work will enable sub-fs synchronization of new generation photon science facilities and allow km-scale optical clock comparison with unprecedented precision.