Enabling cross-network synchronization of independent sources of indistinguishable photons

We demonstrate picosecond-level synchronization of two pulsed parametric down-conversion sources in the telecom C-band via the White Rabbit Precision Time Protocol (WR-PTP), tested for up to 25 km of separation by optical fiber. Each source is engineered to produce heralded single photons with about 35 ps pulse duration and highly single-mode character, ensuring high indistinguishability. To maximize available pump energy for generating indistinguishable photons, actively mode-locked Ti:Sapphire lasers are used, and they should be synchronized at remote locations throughout the network. Here, we use the White Rabbit network protocol for scalable network synchronization, and other methods will be tested for comparison. Using Allan deviation analysis, we demonstrate short- and long-term timing stability to within 10 ps between each synchronized pump pulse train, which meets the criteria for sufficient temporal overlap between the generated single photons.