  Based on the second-order quantum interference between frequency entangled
photons that are generated by parametric down conversion, a quantum strategic
algorithm for synchronizing two spatially separated clocks has been recently
presented. In the reference frame of a Hong-Ou-Mandel (HOM) interferometer,
photon correlations are used to define simultaneous events. 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.
The clock offset is determined by correlation measurement of the recorded
arrival times. Utilizing this algorithm, we demonstrate a proof-of-principle
experiment for synchronizing two clocks separated by 4km fiber link. A minimum
timing stability of 0.4 ps at averaging time of 16000 s is achieved with an
absolute time accuracy of 59.4 ps. The timing stability is verified to be
limited by the correlation measurement device and ideally can be better than 10
fs. Such results shine a light to the application of quantum clock
synchronization in the real high-accuracy timing system.

Quantum Physics;Optics