There’s apparently an interpretation of the Schrödinger cat paradox that claims it is explained by quantum decoherence. I ran across this on Quora where someone had asked about decoherence. Someone from SLAC gave a mathematically correct answer, but attributed it to decoherence. But if we use the usual quantum definition of decoherence, then it actually does not explain what happens. To understand all this let’s review coherence and decoherence.

Quantum systems have the ability to exist in superpositional states. As a simple analogy, suppose we have a bag that contains a bunch of black and white marbles. Now suppose that we remove a marble from the bag and hold it in our hand, but don’t look at it. So we don’t (yet) know whether it is a black marble or a white marble. If the marble were a quantum mechanical object, we would say that, until we open our hand, that marble exists in a ‘coherent’ superposition of black and white states. Suppose our bag only has two marbles in it. If we pull out the white marble, we immediately know that the other marble in the bag is black. We say the marble in our hand is in a ‘pure’ state because when we open our hand to look at it, it is either black or white (e.g. not striped, grey, etc.), i.e. we would say that the black and white outcomes are orthogonal to one another. So a pure state is one that allows for a coherent superposition of orthogonal states.

But sometimes the world isn’t quite so black and white (pardon the pun). Instead of marbles, suppose we’re dealing with, say, dogs. Pure breds would represent the orthogonal states. So suppose I go to a (strange) shelter where I’m not allowed to see into the pens where they keep the dogs and there is one dog per pen. Suppose this shelter has only two pure bred dogs – one dachsund and one poodle. If I choose the first door and it turns out to be a dachsund, then the poodle must be behind the second door. These are orthogonal states.

But what if the two dogs are mutts? Maybe the dog behind the first door has some dachsund in him as well as other things like beagle and basset hound. The same could be true of the dog behind the second door. He might have some poodle in him, but he could also have some dachsund in him. Since both dogs could have some dachsund him them, their states are not necessarily orthogonal. We would say such a state is a ‘mixed’ state.

Another way to look at this is to consider electromagnetic (light) waves. Imagine that we build a laser in which the light is produced by some atomic transition. If the light waves that are produced are all perfectly ‘in step’ with one another, then we say that our beam is ‘coherent’ and represents a ‘pure’ state in that the light waves all have the same wavelength, frequency, etc. If, however, they don’t all have the same frequency, wavelength, etc. then we have a ‘mixed’ state.

Decoherence in quantum systems, then, is the process by which a pure state becomes a mixed state. What causes decoherence is something I will leave to another blog post. But suffice it to say that its definition is fairly consistent throughout quantum physics.

So now what about Schrödinger’s cat? For those unfamiliar with this gedankenexperiment, it goes something like this. Suppose we have a cat in a box. Inside the box is a vial of cyanide gas. Next to this vial is a small hammer attached to a mechanism. The mechanism is set to trigger the hammer (and thus break the glass, killing the cat) if a certain radioactive isotope decays. So the cat’s life (or death) is governed by a purely random process. Before we open the box, the original version of the paradox says that the cat exists in a superposition of alive and dead states. It shouldn’t be too difficult to see that these are clearly orthogonal states. You’re either alive or you’re dead. Ain’t no way to be a mixture of the two. Let’s assume it’s about a 50-50 shot that the cat is alive/dead. The state prior to opening the box is

.

This is a coherent, pure state since it is composed of orthogonal terms (there is no dispute about this). When we open the box the cat is either alive or dead. Let’s be humane and assume it is alive. Then the state after we open the box is

.

This is clearly not a mixed state! Therefore, decoherence does not explain what happens to Schrödinger’s cat!

Update: I forgot to mention that entanglement doesn’t describe this either. Prior to measurement, the cat’s state is not entangled! By definition, an entangled state is a non-factorable state. In other words, you either need to be comparing a single observable for two objects, or two observables for a single object. Here we have a single observable (life/death) for a single object.

Update II: Matt Leifer pointed out on Google+ that technically decoherence is not merely a transition from a pure state to a mixed state. It is actually an irreversible transition from a pure state to a mixed state. I should also point out that decoherence is often cited as one mechanism by which the classical world arises from the quantum world. Matt and Thaddeus Ladd pointed out a number of other points, but for the purposes of my post (debunking a myth), I disagree with them regarding the importance of those points.