Light Particles Are Duplicated


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Light Particles Are Duplicated More Than a Mile Away Along Fiber

By KENNETH CHANG

Employing a facet of quantum mechanics that Albert Einstein called "spooky action at a distance," scientists have taken particles of light, destroyed them and then resurrected copies more than a mile away. Previous experiments in so-called quantum teleportation moved particles of light about a yard. The findings could aid the sending of unbreakable coded messages, which is limited to a few tens of miles.

The new experiment used longer wavelengths of light than earlier ones, letting the scientists copy the light through standard glass fiber found in fiber optic cables. "The central issue is to move to telecom fibers and telecom wavelengths and telecom technology," said Dr. Nicolas Gisin, a physics professor at the University of Geneva and the senior author of an article today in the journal Nature. "This then allows us to go the long distance." The experiments are a primitive realization of the transporter in the "Star Trek" television series that beams people from starship to planet. In coming years, it may be possible to use teleportation to imprint the exact quantum configuration of one atom to another. But teleporting something from the everyday world like a person that contains more than a trillion trillion atoms is highly unlikely, if not impossible.

Even with the light particles, photons, about one in a thousand were received at the other side. "You're not very sure to arrive," a researcher, Dr. Hugo Zbinden, said about human teleportation. Still, the experiments show that scientists can overcome a seemingly insurmountable conceptual barrier, the Heisenberg uncertainty principle. The principle states that the location and velocity of a particle cannot both be precisely measured at the same time. That would seem to make it impossible to teleport anything, even single particles, because without knowing their exact specifications they cannot be copied somewhere else.

Devised in 1993 by scientists led by Dr. Charles H. Bennett of the I.B.M. Thomas J. Watson Research Center in Yorktown Heights, N.Y., quantum teleportation produces pairs of "entangled" light particles that can be thought of as a pair of encoding and decoding rings. A message is combined with the encoding light particle. That combination goes to the recipient, who uses the decoding photon to decipher the message. Because no one else has the decoding photon, no one else can decipher the message.

Other encoding techniques using quantum cryptography are simpler, and a more immediate use for teleportation would be as a repeater. Photons almost all peter out after traveling about 50 miles through optical fiber. Teleportation would enable the creation of copies every 50 miles or so, letting the message be sent across an unlimited distance.

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