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Alice's Restaurant at the End of the Universe

Photonics Spectra
Jan 1997
Daniel J. Dufresne

INNSBRUCK, Austria -- Physicists are approaching a practical application for a remarkable property of photons, something Einstein called "spooky action at a distance." Science recently reported that researchers at the University of Innsbruck successfully transmitted ternary information using entangled photon pairs -- an improvement over previous achievements that transmitted binary information. Anton Zeilinger, head of the Innsbruck group, said his team is also making progress developing experiments proving the phenomenon's ability to "teleport" the polarization of a photon.
William Wootters, professor of physics at Williams College in Williamstown, Mass., said this kind of teleportation could find use in quantum computing. Transmitting between two points without physically moving information packets could bypass system noise.
When scientists excite certain atoms, a percentage of photon pairs released will emerge "entangled," said Wootters. The two photons travel in opposite directions and, according to quantum physics, are unpolarized until measured. Once one entangled photon is polarized, the entanglement ends and its counterpart effectively and instantly assumes the opposite polarization, regardless of their relative positions in the universe.
In teleportation, one photon from an entangled pair is sent to a transmitter ("Bob") and the other to a receiver ("Alice"). At Bob, a third, unknown photon interacts with the first photon. Through an exotic quantum measurement process, Bob entangles the unknown photon with the first photon of the original pair. Simultaneously, this measurement disentangles the original two and, with Alice's help, turns her photon into an exact copy of the unknown photon. Through the "magic" of entanglement, Bob only needs to make one of four rough measurements to get Alice to give her photon the precise polarization of the initially unknown photon, Wootters said.
Does this mean faster-than-light communication could take place between Earth and a distant star if we sent Bob with one entangled photon in a spaceship while Alice and hers stayed on Earth? Not according to Zeilinger: "It is possible to transmit information faster than the speed of light, but at the expense of not being able to decode it until receiving classical information which can only travel at the speed of light." Alice's photon's polarization would appear random to us and, for all Bob could know, Alice might not live here anymore.


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