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

Quantum theory, like all physical theories, makes predictions. In this case, quantum theory predicts that if the physical error rate of qubits is below a threshold, then error correction can be used to increase the quality of a logical at arbitrarily high levels. This prediction can be false. We currently don't know all of the potential noise sources that will prevent us from building a quantum logic gate that is of similar quality as a classical logic gate.

Building thousands of these logical qubits is an engineering problem similar to Dyson spheres and space elevators. You're right that the lower levels of building 1 really good logical qubit doesn't mean that we can build thousands of them.

If our case, even the lower-levels haven't been validated. This is what I meant when I implied that the project of building a large-scale QC might teach us something new about physics.

The way to test out this theory is to try out an experiment to see if this is so. If this experiment fails, we'll have to figure out why theory predicted it but the experiment didn't deliver.

If we mean "we've have been working on this for almost 3 decades. That's a very long time to be working on something!". I agree.

If we mean "We just now only have a few logical qubits that outperform their physical counterparts and we'll need thousands of these logical qubits to run anything useful" then we are still in the early days.

It's not only early days in hardware, it's early days in practical applications as well: https://arxiv.org/abs/2511.09124

Quantum theory predicts that they will work given enough time. If they don't work, there is something about physics that we are missing.

There are no real world use cases today. The hardware is not advanced enough yet, but it's improving exponentially.

- Intel can now do 2D which means a Surface code can be run on these devices: https://arxiv.org/abs/2412.14918

- HRL can now do 2D as well: https://arxiv.org/abs/2502.08861

- They are solving the wiring problem: https://www.nature.com/articles/s41565-023-01491-3

- Their interconnects are high fidelity: https://www.nature.com/articles/s41586-025-09827-w

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