Physicists Put the Quantum Into Mechanics

5 June 2009 (All day)

John Jost and Jason Amini/NIST

Spooky connection. Physicists forged a quantum link called entanglement between the mechanical oscillations of one pair of ions and another distant pair.

Quantum mechanics and its bizarre rules explain the structure of atoms, the formation of chemical bonds, and the switching of transistors in microchips. Oddly, though, in spite of the theory's name, physicists have never made an actual machine whose motion captures the quirkiness of quantum mechanics. Now a group from the National Institute of Standards and Technology (NIST) in Boulder, Colorado, has taken a step in that direction by forging a mind-bending quantum connection between two mechanical widgets. Their devices don't look like electric drills or other familiar machines, however: Each is a pair of ions oscillating in an electric field, like two marbles joined by a spring.

The link the researchers created is called entanglement, and it has been made before between certain internal properties of quantum particles, such as the inner gyrations of ions. The new work extends that link to the actual motion of the ions, which is a kind of micro-analog of the swinging of the pendulum of a grandfather clock. "For the first time, the mechanical motion itself has been entangled," says Rainer Blatt, an experimental physicist at the University of Innsbruck in Austria.

To appreciate what the NIST researchers have done, an aficionado has to get his head around two very weird concepts in quantum mechanics. First, quantum theory says that an object can literally be in two contradictory states at the same time. So whereas an office chair can spin either to the right or to the left, a quantum particle like an ion can literally spin in two opposite directions--call them up and down--at once. That mind-creasing "superposition" state lasts until an experimenter measures the ion's spin, at which point the ion instantly "collapses" to one direction or the other. Weirder still, two ions can be put into these uncertain two-ways-at-once states and then linked up so that, even though it's impossible to say which way either is spinning, their directions are completely correlated. For example, if the first one is measured and collapses into the up state, the second one will instantly collapse into the down state, even if it's light-years away. That connection is called entanglement, and anyone who finds it hard to swallow is in good company: Einstein famously called it "spooky action at a distance."

To extend such a connection to mechanical motion, NIST's John Jost, David Wineland, and colleagues used electric fields to trap two beryllium ions and two magnesium ions. They then applied a magnetic field and pulses of laser light to entangle the spins of the beryllium ions. After that, they separated the ions into two beryllium-magnesium pairs, which would be their mechanical widgets.

During this process, the beryllium spins remained entangled, and the researchers next transferred that link to the motion of the pairs. To do that, they zapped each beryllium with a laser again to "rotate" the down-spinning half of its split personality back to up while leaving the up-spinning half untouched. But they tuned the energy of the laser so that as the down-spinning part of the beryllium's state turned, the light would also excite the ions in the pair to oscillate. As a result, each beryllium ion spun only up, but each beryllium-magnesium pair was left in a state in which it was both oscillating and not-oscillating. Moreover, because the two beryllium spins started out entangled, the two oscillating–not-oscillating pairs ended up entangled, too, the researchers report this week in Nature.

"It's a completely amazing experiment," says Jack Harris of Yale University, one of a number of physicists striving to show quantum effects in vibrating beams and other "macroscopic" mechanical devices. The ion experiment hasn't beaten their efforts to the punch, he says, because although it entangles mechanical motion, the ions themselves are still quantum particles. "It's more the macroscopic than the mechanical that we're after," Harris says. Indeed, he and others hope to test whether some as-yet-undiscovered principle forbids quantum weirdness in objects containing many billion atoms.

For their part, NIST researchers hope to use ions to fashion a quantum computer that, thanks to quantum weirdness, could solve problems that stymie conventional computers. "A lot of the technologies we developed for this experiment are going to be crucial for making a quantum computer with trapped ions," Jost says. However, making a quantum computer will likely be even harder than making a rudimentary quantum machine.

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