Re: quantum computing, we know quantum advantage is real both for certain classes of problems, e.g. theoretically using Grover’s, and experimentally for toy problems like bosonic sampling. It’s looking like we’re past the threshold where we can do error correction, so now it’s a question of scaling. I’ve never heard anyone discuss a limit on computation per volume as applying to QC. We’re down to engineering problems, not physics, same as your brain vs computer case.
We’ve already done bosonic sampling that’s classically intractable. Google published it a few years ago. So yes, quantum supremacy has already been proven. It’s a useless toy problem, but one a classical computer just can’t do.
Yes, error correction will get harder the more we scale, but we’re pretty sure we’ve reached the point where we win by throwing more qubits at it. Again, now it’s engineering the scaling. No mean feat, it’ll take a long time, but it’s not like this is all speculation or fraud. The theory is sound