Spooky timing: quantum-linked photons coordinate clock ticks.

Check out the closest half-dozen timepieces, and they're almost certain to disagree by at least a few seconds. That variation doesn't cut it for global data networks or fleets of satellites, where even microsecond differences among clocks can wreak havoc. Thanks to sophisticated computations, high-speed electronics, and in some eases relativity theory, the far-flung clocks of those systems can tick within just a few nanoseconds of each other, despite separations of thousands of kilometers.

A new experiment indicates that much tighter synchronization of distant doers may be possible by exploiting another powerful realm of physics--quantum mechanics. Yanhua Shih and his colleagues of the University of Maryland at Baltimore have tapped a phenomenon known as entanglement (SN: 7/17/04, p. 46), which is one of the weirdest features of that branch of physics.

In a laboratory test reported in the Sept. 27 Applied Physics Letters, the scientists used entangled photons of two red hues to determine, with a precision of 1 picosecond, the time difference between clocks several kilometers apart.

"It's a promising first step toward the use of entanglement in clock synchronization," says Seth Lloyd of the Massachusetts Institute of Technology. The work also raises prospects of dramatic improvements in numerous cluck-dependent technologies, including the Global Positioning System (GPS), comments Jonathan P. Dowling of Louisiana State University in Baton Rouge.

When two particles are entangled, they exhibit a quantum property, such as energy or magnetic field orientation, in a complementary manner (SN: 12/8/01 p. 364). The Baltimore researchers created pairs of energy-entangled photons by firing an ultraviolet laser into a type of crystal that splits one incoming UV photon into two outgoing red ones.

Even over cosmic distances, the energies of such entangled particles remain correlated, summing to the energy of the original UV photon, says Shih. Albert Einstein deemed such correlations "spooky."

In the Baltimore experiment, the researchers simulate a 3-km span between two-clocks by connecting 1.5 km of spooled optical-fiber cable between the crystal and each of two detector stations composed of a photodetector and a clock. A beam splitter sends each of the two red photons to one of the stations. The photon's arrival triggers the detector and prompts the clock to record the time. Because entangled photons arrive at both stations within an extraordinarily short time, an analysis of arrival times leads to a highly precise calculation of the time difference between the clocks, Shih explains. Unlike ordinary light pulses, entangled photons don't spread as they travel, he adds.

Dowling says that synchronizing clocks in GPS satellites to within a picosecond might make it possible to locate objects at the Earth's surface within millimeters. The Baltimore team plans to further test its approach with photons transmitted through air, Shih says.

Clock-synchronization specialist Judah Levine of the National Institute of Standards and Technology in Boulder, Colo., sees no apparent advantage of the new method over established, nonquantum techniques. What's more, he says, atmospheric fluctuations may lower the accuracy of the new technique.

The variability of atmospheric conditions presents a challenge, Shih admits, but he expects his method to be robust.

In any case. Dowling suggests, the approach might find a role in future space-based observatories (SN: 11/30/02, p. 339) that will require extraordinarily tight synchronization of clocks on widely separated platforms.