I think that in future, quantum mechanics textbooks will use quantum computations as their introductory examples, rather than calculating the energy levels of the hydrogen atom and suchlike, which contain a high proportion of irrelevant stuff. Quantum computation gets down to basics, because quantum computation is the basics.
EDGE: But for you, the main application of the theory is to change our sense of the nature of reality?
DEUTSCH: Yes. However useful the theory as such is today and however spectacular the practical applications may be in the distant future, the really important thing is the philosophical implications epistemological and metaphysical and the implications for theoretical physics itself. One of the most important implications from my point of view is one that we get before we even build the first qubit [quantum bit]. The very structure of the theory already forces upon us a view of physical reality as a multiverse. Whether you call this the multiverse or 'parallel universes' or 'parallel histories', or 'many histories', or 'many minds' there are now half a dozen or more variants of this idea what the theory of quantum computation does is force us to revise our explanatory theories of the world, to recognize that it is a much bigger thing than it looks. I'm trying to say this in a way that is independent of 'interpretation': it's a much bigger thing than it looks.
EDGE: What do you mean by 'bigger'?
DEUTSCH: What I mean is suppose we were to measure 'amounts' of reality, the sizes of things, in terms of the amount of information needed to describe them. To specify the positions of the atoms in this room, I need three numbers for each atom. The more atoms I want to describe, the more numbers I need. The more accurately I want to do it, the more decimal places I need to give. So that requires a certain amount of information. I can think of doing that for the whole universe. That may sound a lot of information, because there are 10^80-odd atoms in the known universe, not to mention the other degrees of freedom. So it may seem unimaginably vast. Yet it is minuscule compared to the amount of information that would be needed to specify the computational state of a single quantum computer, sitting on some future laboratory bench. So in terms of world view, or conceptual model, a quantum computer is a much bigger object than the whole of the classical universe. This fact forces quite a change in our world view.
EDGE: So the theory tells us that a quantum computer is in itself a universe.
DEUTSCH: It would be an object far more complex than the whole of the classical universe. The whole of physical reality is like that too, of course, and we sometimes call it the multiverse. We see, very roughly, a classical universe out there because most of the multiverse is not directly accessible. You can only infer the existence of hidden quantum information indirectly, as in the entanglement experiments I mentioned.
To many people this conclusion was already compelling even before quantum computers. The many-universes interpretation was proposed in 1957. But you can construe all the earlier arguments as being computational arguments too. The people making them didn't think of them as such, but that's what they were. They were saying: we look around us and we see something that's approximately a classical universe, and we might expect that if you take quantum mechanics into account, that might add a certain amount of extra 'stuff' like relativity did which behaves differently but there's still roughly the same 'amount' of reality as we thought there was. But that's not what happens when you take quantum mechanics into account. Reality becomes a vastly, exponentially bigger and more complex thing than it was under classical physics.
EDGE: How can we tell that there's so much of this 'hidden information' in a quantum system?