[eigenket]

People have already said here most of what I want to say in this comment, but just to make it as explicit as possible:

Essentially the only reason anyone thinks that useful quantum computation is possible is because of things called threshold theorems, which state that as long as the noise in each qubit is less than some small but non-zero error rate you can add more qubits and use quantum error correction to make your computation arbitrarily precise. In other words as long as you're below the threshold rate quantum computers scale well.

Of course those threshold rates are very very small, and creating significiant numbers of qubits which are below the threshold rates is incredibly difficult, but theoretically it works.

[upofadown]

>...as long as you're below the threshold rate quantum computers scale well.

Last I heard, getting below that threshold was going to take one or two orders of magnitude of noise improvement. That seems unlikely.

Say you were at a VC presentation and the company said that they had this really great system and the only thing stopping their immense success was the requirement to reduce the noise by an order or two of magnitude. Oh, and by the way, we already have the system very close to absolute zero. So you ask them what they are planning to do and they tell you that they don't have the faintest idea. Noise is always the ultimate limit on the information that can be obtained from a system. The most reasonable interpretation of the situation is that a technology doesn't exist and that there is no reason to think it would ever exist.

But when it comes to quantum computers the optimism is boundless. I am not sure why.

[eigenket]

I agree with you that the general optimism towards quantum computing needs some tempering. It is certainly possible it doesn't work out and we never get a universal fault-tolerant quantum computer.

I think part of the reason for optimism is that there are a lot of different ways to make a quantum computer. Ion traps, linear optics, resonant superconducting circuits, topological quantum computing (if anyone ever actually discovers an anyon) and many more.

Different groups are working on wildly different directions right now, i.e. Google and IBM are doing superconducting stuff, microsoft loves topological stuff, researchers at various places are doing trapped ions and at least one company (psi-quantum) is working on linear optics.

It certainly wouldn't be surprising if several of these approaches fail to work, but the hope is that they don't all fail.

[bawolff]

> the only thing stopping their immense success was the requirement to reduce the noise by an order or two of magnitude

I mean, i know its different context, but in general, a 1-2 magnitude improvement in hardware capability does not sound crazy on its face for most of the computer industry. Most computer hardware has improved by that much over very short time periods historically.

[JBits]

I don't necessarily disagree with what you're saying but multiple research teams do already have error rates below the threshold. It's still early days for the most promising QC platforms, so it's too early to tell if making a QC is possible or not.