  We demonstrate a point-to-point clock synchronization protocol based on
bidirectionally propagating photons generated in a single spontaneous
parametric down-conversion (SPDC) source. Tight timing correlations between
photon pairs are used to determine the single and round-trip times measured by
two separate clocks, providing sufficient information for distance-independent
absolute synchronization secure against symmetric delay attacks. We show that
the coincidence signature useful for determining the round-trip time of a
synchronization channel, established using a 10\,km telecommunications fiber,
can be derived from photons reflected off the end face of the fiber without
additional optics. Our technique allows the synchronization of multiple clocks
with a single reference clock co-located with the source, without requiring
additional pair sources, in a client-server configuration suitable for
synchronizing a network of clocks.
