[mlyle]

I don't think most critics are doubting quantum mechanics. The question is whether quantum computers can reasonably (as an engineering challenge, as a factor of cost, etc) be scaled and can be adapted to take on important real-world problems better than classical computer systems in practice.

We are getting more and more proof points and eliminating a lot of the doubt but this has not been shown yet.

[tagrun]

Actually, we had people who claimed for about four decades that it is fundamentally impossible to have quantum speed up, basically equating it to a perpetual motion machine. We still have such famous people around (who aren't physicists, of course), now a loud minority, and "quantum supremacy" was coined because of/for them.

What you're describing is mainly the new generation of people who grew up hearing about quantum computers on the news about experimental realization of small-scale (a few qubits) quantum computers.

[mlyle]

Eh, I haven't really heard them. I'm 40 and have followed this from near the beginning (starting with reading Science News in the late 80's-- even then the criticism was pretty muted).

I am not positive we are going to get quantum computers with error correction on boolean qubits that can do all the meaningful tasks we hope quantum computers can do. I think it's more likely than not, but it is not close to happening and may never happen. I am not even 100% certain (but it is very very likely) that it is physically realizable.

In my view, this current milestone is kind of contrived.

And even if we do, it's not clear what subset of tasks currently performed on classical computers will be superseded by quantum computation. That's perhaps one of the biggest problems: normal computing has had a whole lot of use cases to pay the research and capital costs.

[tagrun]

Well, I'm envious that you didn't have to deal with those people.

I am involved on the theory side of the implementation of different kinds of solid state qubits, so you may say I'm biased, but the question really isn't whether we will ever get it or not, the question is when. We already have had exponential growth in single qubit coherence times in the past decade, we have very good entangling gates, and there isn't any fundamental reason why the number of qubits can't be increased. It's not like there is an invisible great barrier ahead of us, and nothing in the physics of these devices say we can't.

By the way, they aren't using quantum error correction methods right now, basically because it's not worth it: you need a lot of physical qubits to encode a high quality logical qubits.

[mlyle]

Right-- so if we need 10-1000 qubits per high-quality error corrected boolean-ish qubit and 20,000 error-corrected qubits to really tackle interesting real world problems, we need 3-5 orders of magnitude improvement.

The fact that we have exponential improvements to parts of the process is nice. But exponential improvement doesn't continue forever. Extrapolating from the current status 5 orders of magnitude out seems reckless.

[tagrun]

I don't know what exactly your belief is rooted on (clearly not physics), but of course you're free to put your money wherever you want, just be aware that:

- Qubits are not boolean. Classical error correction does not work in quantum computers. You need quantum error correction.

- The exponential improvements are in single qubit coherence times, and a stagnation in those improvements doesn't prevent an increase in the number of qubits. (it'd be nice to have, though, because then eventually we wouldn't even need error correction)

- No, it doesn't take 1000 or even 100 physical qubits to have a high fidelity qubit (it can actually be 1:1 with dynamical error correction) or 20000 error corrected qubits to tackle real world problems (Shor's algorithm isn't the only interesting thing out there, simulating some quantum systems requires far less qubits and probably more interesting for people in natural sciences)

- Even if we were 3-5 orders of magnitudes away in # of qubits, it's still a matter of time, and there is still nothing fundamental in physics or material science that prevents having millions of physical qubits.

- Now I feel like I'm dealing with one of those non-physicists people again who claim quantum computers are a pipe dream, only this time instead of a faulty physical argument, it has no real technical basis at all, so this is where I'd like to stop. Enjoy the rest of your day