Mechanical systems all tangled up

The motion of two ion pairs is linked through “spooky action at a distance”

Researchers have linked the vibrations of two separated atom pairs, catching sight of a strange quantum effect called entanglement in a 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’s campus in Boulder, Colo.

BIG ENTANGLEMENT An illustration of two entangled mechanical systems shows each behaving like two balls connected by a spring. Researchers entangled the pairs’ motions (represented by purple), even though the two magnesium ions (green) and two beryllium ions (red) are far from each other. The arrows show the internal spin states of the beryllium ions. Illustration by John Jost with help from Jason Amini

LASER LAB The ion trap used to entangle the ion pairs sits in the far right corner of the quantum information processing lab, one of the National Institute for Standards and Technology labs in Boulder, Colo. IMAGE: John Jost

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

The springlike, oscillating connection between two tiny atoms shares mechanical properties with macroscopic systems such as violin strings and pendulums in grandfather clocks. By entangling the motion of one pair of atoms with the motion of another pair, Jost and his colleagues may open the door for “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 Joint Quantum Institute and 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 his colleagues co-opted two pairs of positively charged ions — each pair had one beryllium ion and one magnesium ion. Electrodes held the ions in place while researchers entangled the two beryllium ions’ internal states, known as the 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, setting up a system in which, the researchers suggest, 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 the entanglement even as it is transferred to the larger system may come in handy as researchers search for signs of quantum entanglement in bigger systems. “It is applicable to tests of entanglement and why we don’t see it in everyday life,” Jost says.

The separation between the quantum world and the macroscopic world is still unclear and 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 and larger mechanical systems. Quantum mechanics shouldn’t care whether a system involves a couple atoms or trillions of atoms, Monroe says. “The quantum physics is exactly the same.”

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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