  We present an algorithm for synchronizing two clocks based on second-order
quantum interference between entangled photons generated by parametric
down-conversion. The procedure is distinct from the standard Einstein two-way
clock synchronization method in that photon correlations are used to define
simultaneous events in the frame of reference of a Hong-Ou-Mandel (HOM)
interferometer. Once the HOM interferometer is balanced, by use of an
adjustable optical delay in one arm, arrival times of simultaneously generated
photons are recorded by each clock. Classical information on the arrival times
is sent from one clock to the other, and a correlation of arrival times is done
to determine the clock offset.
