A scheme to couple the motion of a single atom with a crystal membrane could enable quantum-mechanical effects to be seen on a larger scale than ever before.

The idea, which has been proposed by researchers in Austria, Germany and the US, could help to solve the mystery of why quantum effects only seem to appear at tiny dimensions while the everyday world is governed by classical physics.

"The real drive is to understand how far we can drive quantum mechanics into the macroscopic realm," says Klemens Hammerer, a theorist at the University of Innsbruck and lead author of the paper. "It's still not clear where the boundaries of quantum mechanics lie. How large can objects be and still behave quantum mechanically?"

From small to big

In seeking to test the boundaries of quantum mechanics, physicists have gone down two main routes. One involves "entangling" several quantum particles into a single large state, sometimes known as a "Schrödinger cat" state after the famous thought experiment devised by Erwin Schrödinger. The other is to couple a single quantum particle to something macroscopic, typically some form of mechanical oscillator.

The trouble with this second option is that a macroscopoic object can be up to 1013 times heavier than, say, a single atom. To achieve a strong coupling – that is, one that can survive the other noise present in a system – the mechanism or "spring" of the coupling is crucial.

Hammerer's group, which includes researchers at the Ludwig–Maximilians University of Munich, the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and the California Institute of Technology (Caltech), suggest that the spring could take the form of an optical trap.

In this set-up, two criss-crossing laser beams reflected between a pair of cavity mirrors would create a potential well, into which a single atom would be placed. A thin membrane made from a crystal with a high refractive index, such as silicon nitride, would then be placed next to the atom to act as the macroscopic object in the set-up.

Sprung movement

The effectiveness of the optical trap as a spring results from a process of amplification. Any movement of the atom that reflects its quantum state should shift the neighbouring crystal membrane. However, any shift in the membrane should change the resonance of the cavity, which in turn would force the membrane – and the atom – to move more. This amplified movement could be detected either by shining another laser on the atom, or by monitoring the amount of light leaving the cavity, which would reveal the state of the membrane, say the researchers.

Hammerer told physicsworld.com that he is collaborating with several experimentalists to implement the proposal, including Jeff Kimble of Caltech and Jun Ye at NIST, who are both co-authors of the paper. Already, says Hammerer, Kimble has managed to get atoms into traps in the cavities and is now working on adding a membrane.

Markus Aspelmayer, a quantum physicist at the University of Vienna, considers the researchers' proposal "a very important contribution". "Loosely speaking, atomic quantum physics could be transferred to the macroscopic domain of massive mechanical resonators," he says. "Hammerer and his collaborators show that this regime should be feasible even with present day experimental parameters. This poses a fascinating challenge to experimentalists and opens up the field of quantum-optomechanics to the atom physics community."