EDGE: What is the importance of this work?
DEUTSCH: Apart from quantum cryptography, it's unlikely to have practical applications in the near or medium-term future. It's theoretical. But as such it does give us some immediate benefits. One is the benefit of looking backwards. Let me give you a recent example from my own work.
Quantum mechanics, in the traditional formulation, seems to have a 'non local' character: that is, things you do HERE instantaneously affect things that happen THERE. It has been known from the beginning that this 'non locality' can't be used to send signals or anything. But still, philosophically, what are we to make of it? What sort of reality is quantum mechanics telling us we live in? And of course it's hard not to wonder: "well, if something gets there instantaneously, it is going faster than light. So in another reference frame it's travelling into the past. So it could create paradoxes; couldn't that solve the problem of consciousness, explain telepathy, summon up ghosts...?" you name it. This non-locality idea is one of the things that's helped to fuel the appalling mysticism and double-talk that's grown up around quantum mechanics over the decades.
But once you understand that this is all about information processing, it becomes much easier to stop hand-waving and start calculating where the information actually goes in quantum phenomena. That's what Patrick Hayden and I did. The results (recently published in Proceedings of the Royal Society click here) blow the 'quantum non-locality' misconception clean out of the water. Doing things HERE can only affect things THERE (visibly or invisibly) once the information about what you've done here has travelled there in some information-carrying physical object. Nothing instantaneous; nothing non local, nothing mystical.
EDGE: What about the famous experiments that demonstrate quantum non-locality in the lab?
DEUTSCH: They don't. They demonstrate quantum entanglement: one of the fundamental quantum phenomena, but a local one. It turns out that when it look as though there's a non-local effect as in Bell-inequality experiments what's really happening is that some of the information in quantum objects has become inaccessible to direct observation. And in our analysis we actually track how this information travels during entanglement phenomena. It never exceeds the speed of light, and it always interacts in a purely local way.
Well, I am currently working on two spin-offs of that paper. One is work on the structure of the multiverse making precise what we mean by such previously hand-waving terms as 'parallel', 'universes' and 'consists of'. It turns out that the structure of the multiverse is largely determined by the flow of quantum information within it, and I am applying the techniques we used in that paper to analyse that information flow. The other is a generalization of the quantum theory of computation, to allow it to describe exotic types of information flow such as we expect to exist in black holes and at the quantum gravity level. This is all in the context of my growing conviction that the quantum theory of computation is quantum theory.
Speaking of that, another spinoff from the quantum theory of computation is that it provides the clearest and simplest language, and mathematical formalism, for setting out quantum theory itself. I'm planning a series of lectures on video which I think will be quite revolutionary. They will constitute a course in quantum theory for an audience that has no previous knowledge of it say, university-entry level all the way to leading-edge issues in quantum computation, in just twelve lectures (we're currently looking for a sponsor for them, by the way!).