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Making lemonade with quantum lemons

Physicists harness forces of destruction to link isolated atoms

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12:01pm, June 29, 2010
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In a feat of physics judo, researchers have harnessed the same forces that usually destroy long-range quantum links between particles to generate those connections.

This counterintuitive approach may produce “extremely long-lived” quantum links, researchers report in a paper appearing online at the physics website arXiv.org. Long-lasting, room temperature quantum effects are ideal for designing systems that can reliably exchange quantum information.

One of the weirdest outcomes of quantum mechanics is entanglement — a mysterious link in which what happens to one object seems to instantaneously affect its partner, even though the two may be separated by some distance. Pairs of entangled objects can serve as powerful messengers, since the information they share is revealed instantaneously, no matter how far apart the two objects are. But this strange link is notoriously delicate, falling apart when the outside environment impinges on either end.

In the new experiment, a team of researchers from the Niels Bohr Institute in Copenhagen and the Max Planck Institute for Quantum Optics in Garching, Germany, figured out how to produce entanglement in two isolated clouds of cesium atoms using a carefully calibrated prod.

The work extends previous attempts to entangle two groups of atoms, says Seth Lloyd, a quantum physicist at MIT who was not involved in the research. “I would call it evolutionary rather than revolutionary,” he says, “but the experimental demonstration is nicely done, and it is all in all a pretty piece of work.” 

In the laboratory, the researchers arranged two blobs of cesium atoms in separate glass cells. Next, the scientists shot a precisely tuned laser through both clouds of atoms. As the laser excited the cesium atoms, energy left in the form of photons — normally the kiss of death for entangled particles.

But in this case the photons’ departures, carefully calibrated to fill a void in the surrounding electromagnetic field, eventually caused a property of the two atom groups called spin to become entangled.

The two groups of cesium atoms were kept in this entangled state at room temperature for about 15 milliseconds, the researchers report in their paper. Other types of atoms, such as ytterbium, may be amenable to much longer entanglement times, the authors write.

Quantum physicist Christopher Monroe points out that the new study doesn’t allow much control over the individual state of each quantum system, something that would be necessary for quantum communication. “This general method of making entanglement through dissipation is a fairly new theoretical development, so the experiment is interesting,” says Monroe, of the University of Maryland in College Park. “That said, this type of entanglement is still not really useful for most quantum information applications, where you may need to manipulate the state you are creating at will.”

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