We build computers and skyscrapers and space ships, and we clone animals, and so on. At root you can regard all of these too as computations, because when you build a space ship and fly it to a different place, you get new information, or rather a different perspective on the same information, which is just what happens when you input information into a computer and look at the output. However, flying in a spaceship is not quite the same, even computationally speaking, as putting a camera on the space ship and letting it go somewhere, and watching, because, for instance, there's a time delay, so the machine gets harder to interact with if it's far away. Experience is inherently interactive, so there's a fundamental difference, imposed by the laws of physics, between the information processing you can do by going there vicariously using a robot and what you can do going there in person.

I've been thinking about those questions; that is, what sorts of computations do physical processes correspond to; which of these 'computations' can be arranged with what resources? And which sorts can't be arranged at all? What little we know about this new subject consists of a few broad limitations such as the finiteness of the speed of light. The theory of computability and complexity theory give us more detail on the quantum side. But a big technological question in my field at the moment is, can useful quantum computers actually be built? The basic laws of physics seem to permit them. We can design them in theory. We know what physical operations they would have to perform. But there is still room for doubt about whether one can build them out of actual atoms and make them work in a useful way. Some people are still pessimistic about that, but either way, that debate is not really a scientific one at the moment, because there is no scientific theory about what can and can't be built. Similar questions are raised by the whole range of nanotechnology that has been proposed in principle. So that's where a quantum constructor theory is needed.

EDGE: Why specifically a quantum constructor theory?

DEUTSCH: Because quantum theory is our basic theory of the physical world. All construction is quantum construction.

EDGE: What is distinctive about a quantum computer, compared to the computers we know today?

DEUTSCH: Quantum computing is information processing that depends for its action on some inherently quantum property, especially superposition. Typically we would superpose a vast number of different computations — potentially more than there are atoms in the universe — and then bring them together by quantum interference to get a result. Other quantum computations, notably quantum cryptography, couldn't be done by classical computers even in theory.