Quantum clock synchronization over 20-km multiple segmented fibers with frequency-correlated photon pairs and HOM interference

The quantum clock synchronization based on frequency-correlated photon pairs and HOM interference has shown femtosecond-level precision and great application prospect in numerous fields depending on high-precision time-frequency signals. Due to the difficulty of achieving stable HOM interference fringe after long-distance fiber transmission, this scheme of synchronization is hampered from long-haul field application. Utilizing segmented fibers instead of a single long-length fiber, we achieved the stable observation of the two-photon interference of the lab-developed broadband frequency-correlated photon pairs after 20 km-long fiber transmission, without employing the auxiliary phase stabilization method. Referenced to this interference fringe, the balance of the two fiber arms is achieved with a long-term stability of 20 fs. The HOM-interference-based synchronization over a 20-km fiber link is thus demonstrated, and a minimum stability of 74 fs has been reached at 48 000 s. This result not only provides a simple way to stabilize the fiber-optic two-photon interferometer for long-distance quantum communication systems but also makes a great stride forward in extending the quantum-interference-based synchronization scheme to the long-haul field applications.