Schrödinger’s cat is not dead yet

[This is an article I’ve written for news@nature. One of the things I found most interesting was that Schrödinger didn’t set up his ‘cat’ thought experiment with a gun, but with an elaborate poisoning scheme. Johannes Kofler says “He puts a cat into a steel chamber and calls it "hell machine" (German: Höllenmaschine). Then there is a radioactive substance in such a tiny dose that within one hour one atom might decay but with same likelihood nothing decays. If an atom decays, a Geiger counter reacts. In this case this then triggers a small hammer which breaks a tiny flask with hydrocyanic acid which poisons the cat. Schrödinger is really very detailed in describing the situation.” There’s a translation of Schrödinger’s original paper here, but as Johannes says, the wonderful “hell machine” is simply translated as “device”, which is a bit feeble.]

Theory shows how quantum weirdness may still be going on at the large scale.

Since the particles that make up the world obey the rules of quantum theory, allowing them to do counter-intuitive things such as being in several different places or states at once, 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 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 Caslav 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 the quantum world is an issue not of size but of measurement [1]. 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 through a process called decoherence, which happens when quantum particles interact with their environment. Decoherence destroys the delicately poised quantum state and leads to classical behaviour.

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 macroscopically ‘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 back the cat

The fate of Schrödinger’s cat is an example of what in 1985 physicists Anthony Leggett and Anupam Garg called macrorealism [2]. 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 around the direction of the field in a process called precession. That movement can be described by classical physics. But in the quantum world, there would be no smooth rotation – 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 the observed objects turn into classical ones. This happens regardless of whether there is any decoherence caused by interaction with the environment.

Having kittens

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.

Seeing the transition for macroscopic objects like Schrödinger’s cat would require that we be able to distinguish an impractically large number of quantum states. For a ‘cat’ containing 10**20 quantum particles, say, we would need to be able to tell the difference between 10**10 states – just too many to be feasible.

But our experimental tools should already be good enough to look for this transition in much smaller ‘Schrödinger kittens’ consisting of many but not macroscopic numbers of particles, says Kofler and Brukner.

What, then, becomes of these kittens before the transition, while they are still in the quantum regime? Are they alive or dead? ‘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.’

References

1. Kofler, J. & Brukner, C. Phys. Rev. Lett. 99, 180403 (2007).
2. Leggett, A. & Garg, A. Phys. Rev. Lett. 54, 857 (1985).