[WhitneyLand]

What’s the argument for it not being possible?

Superposition is real, the speed up will be real once it scales up.

I know there’s a lot of details to figure out but I’m not aware of any that seem insurmountable given all the effort going into it and incremental progress that continues to be made.

[bawolff]

I think the current argument against them is that some people think that the error rate of qubits will grow exponentially with size of machine in such a way that the error rate will outrun the ability of error correcting codes to fix errors. (Not a quantum scientist. I probably misunderstand and i have no idea how likely it is)

> Superposition is real, the speed up will be real once it scales up.

To be clear, they only speed up very select problems. It is not a general speed up.

[tromp]

I just find it hard to believe that physical reality will let us do reliable computations with superpositions of 2^1024 states. I suspect that the theory of quantum physics is some excellent approximation of physical reality that will breakdown at such absurd levels of precision.

[Ono-Sendai]

My argument would be superposition, is a kind of mathematical abstraction, that is not directly useful in the real world. Quantum computers are a kind of analog computer, and suffer from analog computer problems.

[bawolff]

??

You dont believe that the experiments observing superposition are real?

(Im only attacking the, its a mathematical abstraction, bit. Whether or not superposition can be pragmatically harnessed to make qc is a separate question)

[Ono-Sendai]

clearly some kind of superposition is real, in the same sense that classical waves can superimpose. However there is a wild gulf between that and what is commonly accepted with multi-particle quantum mechanics, which is that the universe is N-dimensional, where N is the number of particles, and that arbitrary configurations in some slice of those N dimensions can be independent of some other slice. This hasn't really been tested, and in fact quantum computing is the first real test of it. And it's not looking good for that theory.

[bawolff]

So to be clear - your view is that quantum computers can't exist because quantum mechanics is false?

[Ono-Sendai]

Well, I would say QM is an approximation of some better and more accurate theory. In my view, multi-particle quantum mechanics is pretty clearly a kind of gross hack - using a very large dimensional space so you can write down a linear theory - instead of using a smaller dimensional space with non-linear interactions. Quantum computing assumes the very large dimensional space is actually real. So we'll see if that is the case.

[Viliam1234]

How would you explain the fact that the experiments with current quantum computers support the theory?

[rolisz]

Joscha Bach has some interesting arguments against it: http://bach.ai/quantum-computers-wont-work/

[dekken_]

> Superposition is real

Really? I wasn't aware science told us what was real.

It's called the "Copenhagen interpretation" for a reason.

[Green_man]

Do other QM interpretations predict that quantum computers wouldn't be able to benefit from superposing N qubits over 2^N (possible) output states? As far as I understand, it's not just Copenhagen that allows for this.

Would an interpretation need to reject Bell's theorem to get there? Or could it not include superposition but still keep all of Bell's theorem?

[PeterWhittaker]

Rovelli’s work on relational QM suggests to me that superposition-as-real in the Copenhagen interpretation is false, a misunderstanding of how to incorporate the observer into the equation.

Once the observer (or the affected system, same thing, in the end) is incorporated, a lot of the things about superposition that needed interpretation disappear.

Read Helgoland for more.

(My physics degree was a long time ago and my math is quite out of date. While Rovelli doesn’t discuss QC in the book, his take on what superposition is not has strong implications for QC AFAICT.)

[dekken_]

Thanks for the suggestion. I have his other book "Order of time" (beside me no less) that I have yet to finish but have liked so far.

[PeterWhittaker]

That’s a good one, next in my reread list.