Femtosecond synchronization of optical clocks over free-space links
Jean-Daniel Deschenes, Laura C. Sinclair, Fabrizio R. Giorgetta, William C. Swann, Esther Baumann, Hugo Bergeron, Michael A. Cermak, Nathan R. Newbury
The use of optical clocks/oscillators in future ultra-precise navigation, gravitational sensing, and relativity experiments will require time comparison and synchronization over terrestrial or satellite free-space links. Here we demonstrate full synchronization of two optical oscillators across a free-space link. The time deviation between synchronized oscillators is below 1 fs over timescales from 0.1 s to 6500 s, despite atmospheric turbulence, beam interruptions, and kilometer-scale path length variations. Over 50 hours, the time offset is below ±20 fs. Our approach relies on the two-way reciprocity of a single spatial-mode optical link, valid to below 225 attoseconds across a turbulent 4-km path. This fs-level of time-frequency transfer should enable optical networks using state-of-the-art optical clocks/oscillators.