Sub-femtosecond precision timing synchronization systems

We present a timing synchronization system that can synchronize optical and microwave signals with attosecond-level precision across kilometer distances. With this technique, the next-generation photon science facilities like X-ray free-electron lasers and intense laser beamlines can be enabled to observe ultra-fast dynamics in atoms, molecules and condensed matter taking place on an attosecond time scale. We discuss some key technologies including master-laser jitter characterization, local one-color laser synchronization, remote two-color laser synchronization, and analyze technical noise contributions in the system. A 4.7-km laser-microwave network with 950-attosecond timing jitter is realized over tens of hours of continuous operation. Finally, an integrated balanced optical cross correlator is introduced. With the same input power level, the required operational power for each timing link is significantly reduced.