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'Spooky action at a distance'

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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.

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