[vtomole]

Quantum theory predicts this: https://en.wikipedia.org/wiki/Threshold_theorem. An experiment can show that this prediction is false. This is a scientific problem not an engineering one. Physical theories have to be verified with experiments. If the results of the experiment don't match what the theory predicts then you have to do things like re-examine data, revise the theory e.t.c.

[Dylan16807]

But that theorem being true doesn't mean "they will work given enough time". That's my objection. If a setup is physically possible but sufficiently thorny to actually build, there's a good chance it won't be built ever.

In the specific spot I commented, I guess you were just talking about the physics part? But the GP was talking about both physics and physical realization, so I thought you were also talking about the combination too.

Yes we can probably test the quantum theory. But verifying the physics isn't what this comment chain is really about. It's about working machines. With enough reliable qubits to do useful work.

[vtomole]

You're right. I didn't sufficiently separate experimental physics QC from engineering QC.

On the engineering end, the question on if a large-scale quantum computer can be built is leaning to be "yes" so far. DARPA QBI https://www.darpa.mil/research/programs/quantum-benchmarking... was made to answer this question and 11 teams have made it to Stage B. Of course, only people who believe DARPA will trust this evidence, but that's all I have to go on.

On the application front, the jury is still out for applications that are not related to simulation or cryptography: https://arxiv.org/abs/2511.09124