Schrödinger's cat comes closer
Object big enough to see with microscope
could be in two places at once.
1 October 2003
|Schrödinger's theoretical cat was
dead and alive at the same time.
The physicist Erwin Schrödinger famously said that quantum theory
would allow the existence of a cat that was simultaneously living and
Now a team of physicists has published the recipe for making a large
object - not cat-sized, but certainly bacterium-sized - in such a
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
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.
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
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.
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.
The mirror would have to be in a very high vacuum as not
to be disturbed by colliding gas molecules
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).