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
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.
Good point. I didn't sufficiently delineate what counts as a scientific problem and what counts as an engineering problem in QC.
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.
Sure, I'm not disagreeing that this processor is noisy, just providing enough context to say that it's fine. Historically, these devices improve enough to be under threshold at which point it doesn't matter that they are noisy cause error correction protocols can be run on top of them.
This processor is state-of-the-art for silicon quantum computing. It's where modalities like superconducting were 15 years ago, and superconducting does not create noise these days https://www.nature.com/articles/s41586-024-08449-y
Quantum theory is so unlikely to be wrong that if large-scale fault tolerant quantum computers could not be built, the effort to try to build them will not be a dead end, but instead a revolution in physics.
Quantum theory says that quantum computers are physically plausible. Quantum theory lies in the realm of physics, not mathematics. As a physical theory, it makes predictions about what is plausible in the real world. One of those predictions is that it's possible to build a large-scale fault tolerant quantum computer.
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.
Depends on what we mean by "early days on hardware".
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.
"early days" means that the 1998 computer didn't have qubits that were below the error correction threshold. Now we have hundreds of qubits below threshold. We'll need millions of qubits like these for quantum computing to be useful. If that take decades, this is the "early days" relatively.
Silicon is not one of the leading modalities for quantum computers, but it has progressed a lot in the past ~2-3 years. Here are a few key advancements that have happened as of late:
Yes, QC is far enough that it's "anyone's guess", but the field is actively working on sliding the answer to this problem from "anyone's guess" to "a bit more certain". It will never be 100% certain until the useful QC appears but we can decrease the probability of our predictions being pure guesswork. As an example, DARPA is funding a project to find the first high impact QC applications https://www.darpa.mil/work-with-us/publications-highlighting... along with finding when the first hardware to run those applications can be built https://www.darpa.mil/work-with-us/quantum-benchmarking-init....
QC startups should be funded because industry is a crucial component of QC progress and large-scale QC labs (Google, IBM e.t.c) can't work on all the ideas. The ideas that come from startups do accelerate QC development.
One application we care about is using quantum computers to build high resolution telescopes https://arxiv.org/abs/1107.2939. A wide area network is required because the telescopes need to be far apart.
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