Schrodinger's Kittens

Published online 22 November 2007 |

Nature
| doi:10.1038/news.2007.277

News

Schrödinger's kittens enter the classical world

Theory shows how quantum weirdness could still be seen on a large scale.

Philip Ball

Watch closely enough, and a compass needle might occasionally jump instantaneously between directions.Getty

The
particles that make up the world obey the rules of quantum theory,
allowing them to do counterintuitive things such as being in several
different places or states at once, so why don’t we see this sort
of bizarre behaviour in the world around us? The explanation commonly
offered in physics textbooks is that quantum effects apply only at very
small scales, and get smoothed away at the everyday scales we can
perceive.

But that’s not necessarily so, say two
physicists in Austria. They claim that we’d be experiencing
quantum weirdness all the time — balls that don’t follow
definite paths, say, or objects 'tunnelling' out of sealed containers
— if only we had sharper powers of perception.

Johannes
Kofler and Časlav Brukner of the University of Vienna and the
Institute of Quantum Optics and Quantum Information, also in Vienna,
say that the emergence of the 'classical' laws of physics, deduced by
the likes of Galileo and Newton, from quantum rules happens not as
objects get bigger, but because of the ways we measure these objects¹. If we could make every measurement with as much precision as we liked, there would be no classical world at all, they say.

Killing the cat

Austrian physicist Erwin Schrödinger famously illustrated the
apparent conflict between the quantum and classical descriptions of the
world. He imagined a situation where a cat was trapped in a box with a
small flask of poison that would be broken if a quantum particle was in
one state, and not broken if the particle was in another.

Quantum
theory states that such a particle can exist in a superposition of both
states until it is observed, at which point the quantum superposition
‘collapses’ into one state or the other. Schrödinger
pointed out that this means that the cat is neither dead nor alive
until someone opens the box to have a look — a seemingly absurd
conclusion.

Physicists generally resolve this paradox by
invoking a process called decoherence: the destruction of quantum
superposition as quantum particles interact with their environment. The
more quantum particles there are in a system, the harder it is to
prevent decoherence. So somewhere in the process of coupling a single
quantum particle to a macroscopic object like a flask of poison,
decoherence sets in and the superposition is destroyed. This means that
Schrödinger’s cat is always unambiguously in a
‘realistic’ state, either alive or dead, and not both at
once.

But that’s not the whole story, say Kofler and
Brukner. They think that although decoherence typically intervenes in
practice, it need not do so in principle.

Bring the cat back

“We
prefer to say that the [kittens] are neither dead nor alive, but in a
new state that has no counterpart in classical physics.”

Johannes Kofler and Časlav Brukner

The
fate of Schrödinger’s cat is an example of what in 1985
physicists Anthony Leggett and Anupam Garg called macrorealism².
In a macrorealistic world, they said, objects are always in a single
state and we can make measurements on them without altering that state.
Our everyday world seems to obey these rules. According to the
macrorealistic view, "there are no Schrödinger cats allowed" says
Kofler.

But Kofler and Brukner have proved that a quantum state can get as 'large' as you like, without conforming to macrorealism.

The
two researchers consider a system akin to a magnetic compass needle
placed in a magnetic field. In our classical world, the needle rotates
with a smooth movement that can be described by classical physics. But
in the quantum world, the needle could be in a superposition of
different alignments, and would just jump instantaneously into a
particular alignment once we tried to measure it.

So why
don’t we see quantum jumps like this? The researchers show that
it depends on the precision of measurement. If the measurements are a
bit fuzzy, so that we can’t distinguish one quantum state from
several other, similar ones, this smoothes out the quantum oddities
into a classical picture. Kofler and Brukner show that, once a degree
of fuzziness is introduced into measured values, the quantum equations
describing an object’s behaviour turn into classical ones. This
happens regardless of whether there is any decoherence caused by
interaction with the environment.

Watch the kitten

Kofler says that we should be able to see this transition between
classical and quantum behaviour. The transition would be curious:
classical behaviour would be punctuated by occasional quantum jumps, so
that, say, the compass needle would mostly rotate smoothly, but
sometimes jump instantaneously.

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