Published online 22 November 2007 | Nature | doi:10.1038/news.2007.277


Schrödinger's kittens enter the classical world

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

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.


But watching such quantum jumps between life and death for Schrödinger’s cat would require that we be able to measure precisely an impractically large number of quantum states. For a 'cat' containing 1020 quantum particles, say, we would need to be able to tell the difference between 1010 states – too many to be feasible.

Our experimental tools should already be good enough, the researchers say, to look for this transition in much smaller 'Schrödinger kittens': objects consisting of a smaller number of particles. What state would those 'kittens' be in? "We prefer to say that they are neither dead nor alive," say Kofler and Brukner, “but in a new state that has no counterpart in classical physics.” No one has yet explicitly looked for such effects. 

  • References

    1. Kofler, J. & Brukner, Č. Phys. Rev. Lett. 99, 180403 (2007) | Article | ChemPort |
    2. Leggett, A. J. & Garg, A. Phys. Rev. Lett. 54, 857-860 (1985) | Article | PubMed | ISI |
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