Physicists get good vibrations on entanglement: 'spooky action at a distance' achieved with oscillating ions.

Researchers have linked the vibrations of two separated atom pairs, catching sight of a strange quantum effect called entanglement in a mechanical system that approaches the scale of everyday life. This new link between two pairs of oscillating ions, reported in the June 4 Nature, "pushes the bounds on where entanglement can be seen," says study coauthor John Jost of the National Institute of Standards and Technology in Boulder, Colo.

Quantum entanglement, a mysterious connection between far-flung particles that Einstein called "spooky action at a distance," has been confined to the microworld of tiny particles including photons, atoms and "other things that are not easy to relate to," Jost says.

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The springlike, oscillating connection between the two tiny atoms shares mechanical properties with macroscopic systems such as violin strings and clock pendulums. By entangling the motion of one pair of atoms with the motion of another pair, Jost and colleagues may permit "quantumness" to creep into the real world.

"We all want to move quantum mechanics to the macroscopic world we live in," says Christopher Monroe, a quantum physicist at the University of Maryland in College Park. "But it's really hard, and that's why it hasn't been done."

For their mechanical system, Jost and colleagues co-opted two pairs of positively charged ions--each pair had one beryllium and one magnesium ion. Electrodes held the ions in place while researchers entangled the two beryllium ions' internal states, known as spin states.

The researchers then separated the ions into the two pairs and used a series of precisely tuned laser pulses to transfer the entanglement from the internal state of each beryllium ion to the oscillating motion between each beryllium ion and each magnesium ion. At this point, the pairs of ions vibrated in unison about .24 millimeters apart, setting up a system in which, the team says, a poke to one pair would have an effect on the other.

"This is an incredibly difficult experiment," Monroe says. "They move the atoms around very gently and keep all the little pieces in line."

Successfully playing puppeteer with entangled beryllium ions and preserving entanglement even as it is transferred to the larger system may come in handy as researchers search for signs of entanglement in bigger systems. "It is applicable to tests of entanglement and why we don't see it in everyday life," Jost says.

The line of separation between the quantum world and the macroscopic world is still unclear, and it's an issue that interests many researchers. Now that entanglement has been demonstrated in a mechanical system, says Monroe, scientists may be able to apply the findings to larger mechanical systems. Quantum mechanics shouldn't care whether a system involves a couple of atoms or trillions of atoms, he says. "The quantum physics is exactly the same."