There's also the question of error correction. The one physical qubit is probably not enough to act as a qubit in genuine quantum computation, because of the problem of errors and decoherence. So you need to implement quantum error correction, and quantum error correction is going to require several physical qubits for every logical qubit of the computer. When I said you need 100 to 200, that probably means several hundred, or perhaps 1,000 or more, physical qubits.
WN: To get an effective 100 or 200 qubits.
Deutsch: Yes, and that is what would have to count as the watershed for quantum computation, for being a distinctive new technology with its own genuine uses.
WN: That's actually D-Wave's stated goal as well: essentially 1,000 qubits in two years. Do you think engineering-wise, and this is not completely within your realm, they will be able to maintain enough coherence at that level to create a practical computer.
Deutsch: As you said that really isn't my field. Maintaining coherence itself isn't quite enough. They've got to maintain coherence in the operation that I spoke of; that is, the arbitrary superposition, the arbitrary entanglement, and so on....
I don't know. The technologies I've seen so far have got way fewer than 1,000. They've got way fewer than 16. I always have to ask whether the claimed number of qubits are qubits that I would count as qubits by these stringent criteria, or whether it's merely two-state systems that can in some sense act in a quantum way. Because that's a much more lenient criterion.
WN: I don't have the sophistication to answer that, for D-Wave at least. If I were to ask you to cast your mind forward, saying everything goes well, what does a world that combines ubiquitous quantum computing and classical computing look like? And you've said that quantum computing would never replace classical computing.
Deutsch: It's not anywhere near as big a revolution as, say, the internet, or the introduction of computers in the first place. The practical application, from a ordinary consumer's point of view, are just quantitative.
One field that will be revolutionized is cryptography. All, or nearly all, existing cryptographic systems will be rendered insecure, and even retrospectively insecure, in that messages sent today, if somebody keeps them, will be possible to decipher ... with a quantum computer as soon as one is built.
Most fields won't be revolutionized in that way.
Fortunately, the already existing technology of quantum cryptography is not only more secure than any existing classical system, but it's invulnerable to attack by a quantum computer. Anyone who cares sufficiently much about security ought to be instituting quantum cryptography wherever it's technically feasible.
Apart from that, as I said, mathematical operations will become easier. Algorithmic search is the most important one, I think. Computers will become a little bit faster, especially in certain applications. Simulating quantum systems will become important because quantum technology will become important generally, in the form of nanotechnology.
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