Breaking the Femtosecond Barrier in Multi-Kilometer Timing Synchronization Systems

To observe electronic dynamics in atoms, molecules, and condensed matter taking place on an attosecond time scale, next-generation photon science facilities like X-ray free-electron lasers and intense laser beamlines require system-wide attosecond-level synchronization of dozens of optical and microwave signals up to kilometer distances. Here, we present for the first time a timing synchronization...