DEUTSCH: If computers are going to continue to become more powerful, processors and memory devices must become smaller. For that reason alone, quantum processes must be harnessed. Whether to make quantum computers or not doesn't really matter. Even to make classical computers out of atomic-scale components you'd have to use quantum physics and ultimately the quantum theory of computation. And once you're making those, the same technology could probably also make quantum computers. And the incentive would be there because of the various inherent advantages of quantum computation.

EDGE: How would you build one?

DEUTSCH: Proposed technologies for building them are at present competing. We don't know which way it's going to go. It could be ion traps or it could be quantum dots, or other solid state devices, or it could be superconducting loops. It could be molecules, or something we don't know about yet.

At present the biggest quantum computer in the world has about 3 qubits. Not much practical use, and it requires quite a large apparatus to make it work. Yet with three qubits you can already implement quantum algorithms that no classical computer using three bits could mimic.

Quantum cryptographic devices already exist in the laboratory. Eventually that's going to give perfectly secure communication. No longer will cryptography depend on the difficulty, or the intractability, of guessing an unknown key. It will simply be physically impossible to discover the key if you don't have the relevant physical object. So that is the ultimate in cryptography.

EDGE: We know that historically, advances in cryptography have been suppressed by governments. Could it be that quantum computers will never come on the market because people will make sure they don't?

DEUTSCH: If so, I know nothing about it. Both in Britain and America there are government agencies working on quantum cryptography, and as far as I can tell, they participate in much the same way as they would if they were academic institutions. Presumably they have their secrets — I hope they do! — but I'm not aware of them having tried to prevent any of these technologies from being developed, let alone theoretical advances. But I do find it a bit surprising, now that you come to mention it, that there isn't already a quantum cryptographic device on the market.

EDGE: For e-business?

DEUTSCH: No. The trouble is that at the moment quantum cryptography is severely limited in range. It can't be done through open air. It's got to be done through fiber-optic cable, and I think the world record is about 100 kilometers. But still, you could wire up the City of London, or central Washington DC, with absolutely secure communications. I don't know why that hasn't been done. I doubt that it has anything to do with sinister machinations by the government, though. It's probably just that it takes a long time for an idea to become genuinely commercially viable.

EDGE: What if there was a critical situation such as a war which required security?

DEUTSCH: In that case we already know how to build absolutely secure communications if we want to, at ranges of a few kilometers. Longer ranges would present a problem, but at least one group at Los Alamos is working on a system that would allow you to bounce quantum-encrypted messages off a satellite, and that would essentially solve the problem.