Picosecond synchronization of mode-locked lasers for metropolitan-scale quantum networks

We demonstrate picosecond-level synchronization of two actively mode-locked Ti:Sapphire lasers via the White Rabbit Precision Time Protocol (WR-PTP), tested over 120 km of deployed optical fiber. This synchronization capability, in combination with the highly single-mode, transform-limited pulses produced by each laser, enables their use as pump lasers for indistinguishable photon sources at remote locations in a quantum network. Here, the WR-PTP serves as a scalable network synchronization protocol, and its performance is compared to traditional methods of local synchronization. We demonstrate pulse-to-pulse synchronization better than 3 ps and time deviation (TDEV) values below 4 ps for all averaging times up to 10 s. With a designed coherence time of 35 ps for single photon sources utilizing these lasers, the achievable temporal overlap corresponds to 98 % Hong-Ou-Mandel (HOM) interference visibility between independent sources.