The research, described in the June 4 issue of Nature,* involves a bizarre intertwining between two pairs of vibrating ions (charged atoms) such that the pairs vibrate in unison, even when separated in space. Each pair of ions behaves like two balls connected by a spring (see figure), vibrating back and forth in opposite directions. Familiar objects that vibrate this way include pendulums and violin strings.
The NIST achievement provides insights into where and how "classical" objects may exhibit unusual quantum behavior. The demonstration also showcased techniques that will help scale up trapped-ion technology to potentially build ultra-powerful computers relying on the rules of quantum physics. If they can be built, quantum computers may be able to solve certain problems, such as code breaking, exponentially faster than today’s computers. (For further details, see: http://www.nist.gov/public_affairs/quantum/quantum_info_index.html.)
"Where the boundary is between the quantum and classical worlds, no one really knows," says NIST guest researcher John Jost, a graduate student at the University of Colorado at Boulder and first author of the paper. "Maybe we can help answer the question by finding out what types of things can—and cannot be—entangled. We’ve entangled something that has never been entangled before, and it’s the kind of physical, oscillating system you see in the classical world, just much smaller."