  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.
