Published online 3 June 2009 | Nature | doi:10.1038/news.2009.540

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Quantum entanglement of mechanical vibrations brings 'quantum computing' another step nearer.

Entangled oscillations could boost the power of future quantum computers. John Jost / Jason Amini

The line between the quantum realm and the world of classical mechanics we inhabit has just got a little bit blurrier. Physicists have shown that mechanical vibrations can be entangled, which promises to help along attempts to build quantum computers.

Physicists have succeeded in tying together the fate of pairs of particles in the lab before. Manipulating the properties of one immediately affects its entangled partner, no matter how far apart they are — a phenomenon known as quantum entanglement. But so far they have only been able to entangle fairly esoteric quantum properties, such as an atom's internal spin state, says John Jost at the US National Institute of Standards and Technology in Boulder, Colorado.

Now Jost and his colleagues have succeeded in entangling a property that we are familiar with from everyday experience — vibrations1. They have entangled two pairs of ions, held about a quarter of a millimetre apart in an ion trap, so that the pairs oscillate in step with each other. In each pair, a beryllium ion was partnered with a magnesium ion. "You can think of them like two balls connected by a spring that vibrate back and forth in unison," says Jost.

“This is the first time we've seen entanglement in a mechanical system.” John Jost

US National Institute of Standards and Technology

The first step to achieving these synchronized vibrations relied on standard techniques to entangle the spins of the beryllium ions in each pair. The trick was then to transfer this conventional form of entanglement into the vibration of the ion pairs, using lasers. "Depending on its internal spin state, the beryllium ions will absorb certain frequencies of laser light, which excites them and sets them vibrating," explains Jost. The two entangled pairs of beryllium and magnesium ions then began to vibrate in lockstep (see video).

"This is the first time we've seen entanglement in a mechanical system," says Jost.

Pendulum power

"It's like entangling two little pendulums," says Rainer Blatt, an expert on manipulating quantum systems at the University of Innsbruck, Austria. "This is first-class experimental physics."

It's possible to synchronize two macroscopic pendulums without employing any weird quantum effects. So the team's final step was to demonstrate entanglement by transferring it back into the beryllium spins again, and testing that the spins were once again correlated.

The particles were held a fraction of a millimetre apart in an ion trap. John Jost

The work, published in Nature today, has important implications for scaling up quantum effects to build quantum computers, says physicist Vlatko Vedral at the University of Oxford, UK. In current quantum ion-trap experiments, one ion is able to hold one 'qubit' — the quantum equivalent of a classical bit of information — encoded in one of its internal properties, such as its spin. However, says Vedral, the new demonstration suggests that this storage capacity could be doubled by also encoding information in the ion's vibration. "It's really having this extra memory capacity that could give more processing power."

Pushing the boundaries

The team's work also addresses the reach of the quantum realm. "The question is: can anything be entangled?" says Vedral. "That's really what's at stake here, and this group has moved quantum effects up a level, into something more tangible."

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The next step, says Vedral, should be to repeat the experiment using larger and heavier oscillators. In 2007, Vedral and his colleagues proposed that entanglement could, in theory, show up in the vibrations of microscale mirrors2. Tests to demonstrate this are under way at the University of Vienna, in Austria.

Jost's methods will make it much easier to create and detect entanglement between ions and mirrors, says Blatt. "Then, of course, really macroscopic devices become available for entanglement studies," he says. "Thus the techniques will certainly help us in understanding the quantum-classical boundary."