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Pessimistically I think it's most comparable to fusion. Theoretically possible but very difficult. I'm biased because I'm in the industry, but nothing has cropped up that I've seen that requires a miracle.

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> I'm biased because I'm in the industry, but nothing has cropped up that I've seen that requires a miracle.

Scaling is itself the open question. Gravitational effects start creeping in when you scale up sensitive entangled systems and we don't have a good understanding of how gravity interacts with entanglement. Entangled systems above a certain size may just be impossible.

That would itself be a tremendous theoretical breakthrough for all of physics.

The difference is that whenever it’s daytime and there aren’t many clouds in the sky, you can see an example of fusion working at scale…

And every time you use a transistor, observe a green plant living, or see that your hand does not pass through the table when you tap it, you see quantum mechanical effects working at scale. Every time you use a telescope, you see quantum information processing (interference) at scale. The control over that process is the difficult part, same as with fusion.

Transistors, plants, hands, and tables are all macroscopic. We see quantum mechanical effects, but we do not see stable superpositions. None of these real-world examples seem to shield a quantum state from decoherence in the way a quantum computer needs to. The sun demonstrates clearly that fusion is controllable (albeit in a regime we struggle to match). I don't think your examples show that a quantum state can be controlled at the scale we need it to be, and I don't know of any extant phenomena that do. But I am no expert, yell at me if I'm wrong.

On the contrary, we have plenty of examples of long-lived (many hours, days, or more) examples of superposition in many solid state materials and chemical systems. For a quantum computer you need both (1) something that can retain superposition and (2) be easily controllable. Point 2 is the difficult one, because if you can control it (interact with it), the "environment" can interact with it and destroy it as well.

All of the examples of macroscopic effects above are possible thanks to effects explainable only through the existence of superposition. It is just that they are not particularly controllable and thus not of interest for storing quantum information.

Another fun point: the example you are focusing on, fusion happening in the sun, is only possible due to the quantum tunneling effect, which is itself dependent on "superposition" being a real thing. Looking past the clouds at our star is already an example of quantum mechanics working, which is very much an experimental observation of an effect possible only thanks to the existence of superposition.

Thanks, I'll stand partially corrected. It was a badly written comment, or muddled thoughts, or both. I didn't mean to doubt the existence of superposition or our ability to achieve it for sustained periods.

But doesn't the point stand? I meant to say that none of the examples seem to demonstrate both 1) and 2). Do we know of any natural system which can maintain and precisely manipulate a quantum state like a quantum computer needs to?

Well no, but we also don't know any natural system that does fusion using an amount of material smaller than about a billion billion billion tonnes. Getting from A (what we know works) to B (what we'd like to work) is the hard problem. That's called research.

In don't think the argument is whether nuclear fusion is possible at all. The question is whether it is possible to reliably control it and whether Tue captured energy output will be worth the inputs required to run the system.

Quantum state is the miracle in my opinion. By definition it can never really be confirmed.

You cannot observe the initial state because that collapses the super position. Said more simply, we can only see the end result and make educated guesses as to how it happened and what the state was prior to the experiment.

There's plenty of valid criticisms of the quantum computing industry but the idea that quantum mechanics is whole-cloth invalid is nonsense. You can absolutely verify a quantum state through tomography

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