  As a new approach to realizing high-precision time synchronization between
remote time scales, quantum two-way time transfer via laboratory fiber link has
shown significant enhancement of the transfer stability to several tens of
femtoseconds. To verify its great potential in practical systems, the field
test in long-haul installed fiber optic infrastructure is required to be
demonstrated. In this paper, we implement the two-way quantum time transfer
over a 103 km urban fiber link. A time transfer stability of 3.67 ps at 10 s
and 0.28 ps at 40000 s has been achieved, despite the large attenuation of 38
dB leading to fewer than 40 correlated events per second. This achievement
marks the first successful step of quantum two-way time transfer in the task of
high-precision long-distance field transfer systems.
