A new use for entanglement allows incredibly precise synchronization of widely separated clocks. Alejandra Valencia, Giuliano Scarcelli and Yanhua Shih of the University of Maryland have managed to use pairs of entangled photons to synchronize clocks separated by 3 km to an accuracy of 1 ps.

The technique works by splitting an ultraviolet photon in a nonlinear crystal to produce two red ones. Detection of one photon makes detection of the other happen only at a sharply determined time, and this correlation can be used to match up the clocks amazingly well.

The tests were done over long optical fibres. Some accuracy may be lost if this is attempted over open-air channels, but it is an exciting new approach to an old problem, with a great deal of potential applications for both fundamental and applied physics.

Further reading

Alejandra Valencia, Giuliano Scarcelli and Yanhua Shih 2004 App. Phys. Lett. 85 2655.

Compiled by Steve Reucroft and John Swain, Northeastern University