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Comment by citizenpaul | Hacker News Reader

I got curious about quantum computers so I ordered and read (most of) Quantum Computation and Quantum Information: 10th Anniversary Edition.

It pretty much proved to me that quantum computing is at best 100+ years away or at worst a pipe dream fantasy. Until decoherence at scale is solved there is no way quantum computing will be useful beyond current computing abilities. There is not even a hint that this problem will be solved anytime soon.

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I co-authored that book (written in the late 1990s). I don't know when quantum computing will happen - I only follow developments very, very loosely now - but think you're being much too pessimistic. It is a very (very!) challenging problem, and it's still early days, but there's also been steady and impressive progress for many years.

How do people know that real-world QC is even possible?

I understand enough of nuclear physics and quantum physics to see that fusion is mostly a technical/engineering problem while QC is widely speculative.

The biggest reason I think its possible is that there are a lot of different ways you could build a quantum computer. Different groups are exploring building physical qubit systems out of transmons in superconductors, topological systems, linear optics, trapped ions, quantum dots, NMR driven spins, cavity QED and probably many other setups I don't know about.

It would be weird (and scientifically quite interesting) if all of these approaches fail for some reason.

I am willing to bet that a practical/useful Quantum Computer won't happen, ever.

Entanglement as a physical phenomenon is simply too fragile, and as the foundation of this whole stuff it is likely not well understood enough.

Theory is just literature if it does not match reality.

Our theories are only models, they are good enough up to some extent, and then they are wrong.

You may well be correct in that bet. I lean that way myself more than most of my colleagues. On the other hand your reasoning is quite wrong, entanglement is both very well understood and fairly robust. Scale and gate fidelity are significant problems in building quantum computers, entanglement isn't really (by this I mean that building good entangling gates is hard, but the entanglement once you've made it is ok).

The theory (which is just quantum mechanics) matches the reality better than any other known physical theory ever has. It is certainly not "just literature". Of course it is wrong in some sense, since it doesn't appear to cover gravity, but it is also right in some pretty meaningful sense.

QC is just as solid of theoretical footing at fusion. It is mostly a technical/engineering/materials problem with plenty of room for clever physics to make it easier.

I don’t see how you can read and understood Nielsen and Chuang in one sitting, unless you are already a quantum computation theorist. I also don’t see how reading what is essentially an algorithms textbook can lead you to develop an informed opinion about the state of quantum computer engineering…

it’s like reading saying “I was curious about how computer software works so I ordered and read CLRS and I don’t think faster computers are anywhere on the horizon in 100 years…”

It was not one sitting... That's a textbook, it took a while. Probably over a year. I was also doing it along learning some of the math involved in another course. Also my SO is a physicist so I had some help.

The theory is great. The problem is that it all hinges on a scientific breakthrough that has not happened yet. I don't see it happening soon. Just my not totally uneducated opinion. I have no horse in the race I think the people claiming it will work "soon" are being a bit dishonest with themselves as well as everyone else. For all we know it will end up taking several other scientific breakthroughs to get all the parts needed. I personally think that is the case and why I say it will not be in our lifetime.

Makes sense!

they didn't say or give the implicature that it was in one sitting. might be months

Except published research demonstrates continual improvements in coherence time and implementations of error correction protocols. So "not even a hint" is at best hyperbolic or at worst just wrong.

But it is solved theoretically, by quantum error correction. I'm not denying there are plenty of problems but can you justify that decoherence is the limiting problem for architectures other than superconducting qubits?

I felt this way about AI in college. And yet here we are.

To expand on this - I went into my first AI class in college thinking it'd be amazing. I left with a very bland taste, having solved n-queens a bunch of different ways and written genetic algorithms to "evolve" images.

I read Kurzweil before this. I had thought we were decades away from digital immortality. Taking the AI course and algorithm analysis was quite disappointing. Reality set in. Things are harder than we hand wave away.

I then started taking bio, read papers on neuroscientists decoding visual signals from mammalian LGN, and went deep down the biology rabbit hole. That only further convinced me that Kurzweil was wishfully wrong. Here are systems more complex than anything previously described to me in my entire life.

But now we're confronted with a pace of innovation that is frankly quite humbling. Things I had written off no longer seem impossible.

I'm intensely excited for the future.

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