Implementation of quantum synchronization over a 20-km fiber distance based on frequency-correlated photon pairs and HOM interference

The quantum 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 timefrequency signals. Due to the difficulty of achieving stable HOM interference fringe after long-distance fiber transmission, this quantum synchronization is hampered from long-haul field application. Utilizing segmented fibers instead of a single long-length fiber, we successfully 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 auxiliary phase stabilization method. Referenced to this interference fringe, the balance of the two fiber arms is successfully 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.