Clock synchronization with entangled photons has been proposed as an idea and now demonstrated in an experiment.

One of the important issues in the theory of special relativity is the synchronization of clocks. How close can be the time at one clock, t1, be to the time at a second clock, t2? Modern clocks have improved to such a level that the resolution and accuracy of the comparison techniques have become the limiting factors to determine the degree of synchronization, t1-t2. New ideas, exploiting the novel aspects of entangled photons, say that quantum mechanics can overcome the classical limit in regard to clock synchronization

Physicists at the University of Maryland, Baltimore County, have now confirmed the idea by doing an experiment in which two entangled photons are sent respectively to two detectors some distance apart. Pairs of entangled photons are produced in a nonlinear crystal and will retain a special quantum correlation between themselves (belonging, as they do, to a single quantum state) even if they were to move apart to distances of trillions of km.

The Maryland physicists (contact Alejandra Valencia, avalen1@umbc.edu) synchronized two distant clocks, each attached to a photodetector, by building up a statistical sampling of the clock responses, first sending a photon from one emerging beam to one detector while its mate went to the other detector, and then switching the entangled pairs to the opposite detectors. In this way, two clocks 3 km apart were synchronized within a picosecond.

Synchronicity is of course critical in many areas of telecommunications, especially in GPS.

(Valencia et al., Applied Physics Letters, 27 September 2004)