  We have experimentally demonstrated a clock comparison scheme utilizing
time-correlated photon pairs generated from the spontaneous parametric down
conversion process of a laser pumped beta-barium borate crystal. The
coincidence of two-photon events are analyzed by the cross correlation of the
two time stamp sequences. Combining the coarse and fine part of the time
differences at different resolutions, a 64 ps precision for clock
synchronization has been realized. We also investigate the effects of hardware
devices used in the system on the precision of clock comparison. The results
indicate that the detector's time jitter and the background noise will degrade
the system performance. With this method, comparison and synchronization of two
remote clocks could be implemented with a precision at the level of a few tens
of picoseconds.
