Schrödinger's cat comes closer

Object big enough to see with microscope could be in two places at once.
1 October 2003

PHILIP BALL

Schrödinger's theoretical cat was dead and alive at the same time. © GettyImages

The physicist Erwin Schrödinger famously said that quantum theory would allow the existence of a cat that was simultaneously living and dead.

Now a team of physicists has published the recipe for making a large object - not cat-sized, but certainly bacterium-sized - in such a quantum quandary¹. A tiny mirror, they propose, can be in two places at once.

Scientists are resigned to atom-sized entities being capable of such feats. But they generally assume that at larger scales a phenomenon called decoherence intervenes, stamping out quantum weirdness and fixing everyday objects to a single, definite location.

William Marshall of the University of Oxford and his coworkers outline a scheme for evading decoherence to achieve a quantum superposition of states in an object with around a hundred trillion atoms. This is about a billion times larger than anything demonstrated previously.

It's not the first proposal for achieving quantum effects in a big system. But unlike others, it is feasible with current technology. For example, mirrors like those Marshall and colleagues invoke can be made just ten thousandths of a millimetre square - about the size of a red blood cell, weighing around five billionths of a gram.

Cat's cradle

The plan goes like this. The mini-mirror, pasted on the end of a tiny arm, is hooked up to a conventional quantum object: a single photon of light in a quantum superposition. The photon is made to bounce back and forth between the small mirror and a much larger one, making the small mirror oscillate on its springy arm.

Under normal circumstances, this would be like trying to use the flapping of a fly's wing to push a yacht's sail during a storm. Vibrations of the mirror caused by heat would swamp any influence of the lone photon.

The researchers propose to calm this stormy background by cooling the apparatus to less than two thousandths of a degree above absolute zero. The mirrors would also be in a very high vacuum so as not to be disturbed by colliding gas molecules.

Beam time

In the hypothetical experiment, the light beam passes through a beam splitter, a kind of semi-mirror that lets some photons through and reflects others. Any photon can end up on one of two possible paths. Or it is possible to arrange things so that a photon effectively follows both paths at once, in a quantum superposition.

This enables the photon to interfere with itself, just as two light beams interfere when they cross paths, creating light and dark bands where their waves add or cancel out.

The photon can transfer its superposition to the small mirror, so that it is in two positions at once. When this happens, the photon's self-interference disappears. The researchers calculate that the system will cycle back and forth between a superposition of photon states (in which case one can detect an interference pattern) and a superposition of mirror positions (for which there is no photon interference pattern).

1. Marshall, W., Simon, C., Penrose, R. & Bouwmeester, D. Towards quantum superpositions of a mirror. Physical Review Letters, 91, 130401, doi:10.1103/PhysRevLett.91.130401 (2003). |Article|