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'Spooky action at a distance'

By Peter N. SpottsStaff writer of The Christian Science Monitor / October 4, 2001

One of the most famous felines of the 20th century is slowly opening the door to discoveries that could revolutionize communications, and computers - and spawn ever more rigorous tests of the very foundations of physics itself.

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The cat is the potentially tragic hero(ine) in a quantum-physics paradox penned by Austrian physicist Erwin Schrödinger in 1935. The cat finds itself enclosed in a box with an atom. When the atom decays, the cat will die.

In quantum physics, the cat and atom exist in both states - alive and dead, decayed and undecayed - until someone opens the box and checks. Regardless of what the observer sees inside the box, the states of the cat and the atom are inextricably interlinked.

Among the notions Schrödinger tried to illumine with his paradox is a property he dubbed entanglement. In effect, the state of the cat "knows" the state of the atom - even at a distance. Thus, if an experimenter measures the state of one, he or she will know the state of the other without making the additional measurement - the relationship between their two states remains constant.

Entanglement forms the basis for key elements in the burgeoning field of quantum computing and communication. Whether quantum computers will ever be built remains an open question, some researchers say. But if such computers are built, achieving and maintaining entanglement will be critical for everything from processing data to transmitting it.

Hence the excitement over a report this week that physicists in Denmark have entangled two large clusters of atoms in neighboring containers. The feat, the team says, represents the first demonstration of entanglement between separated, large clusters of atoms, at room temperature, and for relatively long periods of time.

The Danish team's effort is not the first time scientists have entangled atoms, notes Eugene Polzik, who led the team at the University of Aahrus in Aarhus, Denmark.

Last year, for example, researchers at the National Institute of Standards and Technology in Boulder, Colo., reported that they had achieved entanglement with four atoms. The entangled atoms in the NIST experiment established their relationships through close-up interactions.

Milestone for quantum computing

"That was a milestone for quantum computing," says Dr. Polzik. "But it's not so good for quantum communications, where you need to have entangled particles miles apart."

His team advanced that prospect by using a laser to entangle atoms in two containers a few millimeters apart. His team gathered cesium atoms and confined them in a pair of glass containers. Each held a trillion atoms.

The researchers treated each sample with a laser to give each cluster's overall magnetic "spin" its own orientation. Then the team sent a single laser beam through the samples to entangle the disparate clouds. A similar laser shot half a millisecond later showed that while the orientation of each cloud's spin had shifted somewhat, the original relationship between the two clouds' orientations remained the same.

"This is a real step forward," says William Wootters, a physicist at Williams College in Williams, Mass., who studies quantum interactions and did not take part in the experiment.

The team's use of a laser to entangle the disconnected clouds of atoms holds the promise for longer-distance quantum communication, which requires a set of entangled particles at each end of the quantum "connection."

Like kids at an egg-toss contest, the team plans to continue to widen the gap between samples to see how far they can separate the clouds and still trigger entanglement.

Entanglement has an embattled history in physics, Dr. Wootters says.

Back when Herr Schrödinger was writing about stuffing cats and atoms into boxes, he also held that entanglement was the one feature of quantum theory that distinguished it from "classical" physics, in which cause and effect could be distinguished and one object is forbidden from influencing another object at a distance instantaneously.