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'Spooky action at a distance'

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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.

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.

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