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If you drop a glass cup on the ground, it will break and shatter into pieces. This happens all the time and is consistent with quantum mechanics. But it never happens that a shattered glass cup rearranges itself from the ground into someone's hand as a whole glass cup, even though this is also consistent with quantum mechanics. We see from this example that not everything that is consistent with quantum mechanics is possible.

As far as I know, scalable quantum computing has never been demonstrated either backwards in time or forwards in time. So a fortiori, I would think that this would be good enough evidence to suggest that scalable quantum computing is impossible. Yet, some physicists believe that scalable quantum computing is still possible. Why?

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closed as primarily opinion-based by Emilio Pisanty, Kyle Kanos, Brandon Enright, tpg2114, BebopButUnsteady Feb 25 at 0:44

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit the question.

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You mean aside from the bit where small scale quantum calculating elements can be built and run today? –  dmckee Aug 30 '12 at 14:02
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May not be the most well formulated question, but certainly does not deserve a -1. Added a +1. @Craig Feinstein: Wiki is a bit outdated, but still useful. en.wikipedia.org/wiki/Quantum_computer –  Antillar Maximus Aug 30 '12 at 14:25
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@Craig so based on your comment, it sounds like you're interested in whether quantum computing is scalable, not just whether it's possible. (Because it clearly is possible; primitive implementations of Shor's algorithm do exist.) Is that accurate? –  David Z Aug 30 '12 at 18:08
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Related: physics.stackexchange.com/q/7412/2451 –  Qmechanic Aug 30 '12 at 20:05
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Well, I down voted you because your 'question' is trite and argumentative. If everything but the title was removed it would be an improvement. –  user2963 Aug 31 '12 at 12:29

6 Answers 6

up vote 6 down vote accepted

The reason is Shor error correction. Shor demonstrated that by using 9 bits for every bit, you can reverse any decoherence event on any of the 9 bits by doing some measurements on auxiliary quantities. Before the existence of error correction, it was plausible to say (and Unruh did say) the quantum computers are unphysical, because they require no error in a macroscopic system. This is an impossible position to hold past 1996.

The error correction method has been made more efficient since, by Shor and collaborators, and the upshot is that if you make a small quantum computer which is coherent for long enough time, and you can encode some dozens of qubits robustly so that you can reverse the errors faster than they occur, you can scale up the computation indefinitely without problems.

This makes quantum computation feasable for sure, and there is no way to argue that it is impossible without arguing that quantum mechanics fails.

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That's only on paper and 16 years ago. –  Craig Feinstein Aug 31 '12 at 3:20
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@CraigFeinstein: It's a series of papers for one, with many citations and extensions (including a demonstration that at some finite small decoherence rate quantum computation becomes feasable), and it completely solves the problem. Why would you need another? –  Ron Maimon Aug 31 '12 at 8:36
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16 years ago and also an active field since then. See for instance the QEC 2011 conference. –  Emilio Pisanty Aug 31 '12 at 14:01
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@CraigFeinstein: The error correction is based on the assumption of uncorrelated errors. The evidence for this is that this is the same assumption as for classical error correction, and classical error correction works. If quantum error correction fails, it is not likely because the correlations in the environment are correlated by magic, but rather it is because quantum mechanics is not exact. –  Ron Maimon Aug 31 '12 at 16:53
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@CraigFeinstein: You are not alone in this belief--- this is at the heart of what makes QM mysterious, and it has finally been distilled to an experimental question. I am not sure if quantum mechanics is exact. If you look at the question 't Hooft asks "Why do some people dismiss simple quantum models?", you will see that like you, he believes quantum mechanics is not exact. I gave a proposal for a hidden variable scheme which in my opinion works, but I genuinely don't know if nature is quantum mechanical or not, at these system sizes. Nobody knows. We'll find out with a quantum computer. –  Ron Maimon Aug 31 '12 at 17:28

You might find this lecture by Scott Aaronson insightful.

http://www.scottaaronson.com/democritus/lec14.html

He basically makes an very convincing defence of quantum computing against arguments made by quantum computing skeptics. I certainly found it very interesting when I read it!

Also, your "entropic" argument is not entirely correct. A physical system will not change its state to one with much lower entropy spontaneously (except with overwhelmingly low probability). However, it can be brought to a state of lower entropy by performing work on it. This for example is what a refrigerator does!

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My argument is not really about entropy. It's about things that don't happen in real life, but can happen in theory. Scalable QM can happen in theory, but there is no evidence that it can happen in real life. Yet, it seems that some physicists believe that it must happen in real life or else QM theory is wrong. –  Craig Feinstein Aug 31 '12 at 3:26
    
What do you mean precisely by something that can happen in theory but cannot happen in real life? This is a very troublesome statement. As Aaronson points out, Immanuel Kant even wrote a complete treatise to demolish it. In any case, it is true that there might be some fundamental reason why a quantum computer cannot be built, but all we have is evidence that IT CAN be built. –  Juan Miguel Arrazola Aug 31 '12 at 19:14
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This whole thing is very confusing to me. I'll have to concede now that I think quantum mechanics must break down once you get things to be on the level of scalable quantum computing. There's just no way I can conceive of it working. Thank you for your answer. –  Craig Feinstein Aug 31 '12 at 20:58

Your information is slightly outdated. Quantum computing is possible. They even sucessfully implemented Shor's Algorithm to factorize a product of primes. In 2009.

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Demonstration in "Real Life". Others have given you many other sources, what more do you want?

Survey of NMR Quantum Computing

D-Wave, a commercial vendor

Quantum Information Processing with Trapped ions

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very good, thank you –  Craig Feinstein Aug 31 '12 at 13:46

Several prominent physicists are on record as being pessimistic about the prospects for unlimited scalability of Quantum Computing - See for example http://arxiv.org/abs/quant-ph/0311039

(I found that link when web-searching on "Gerard 't Hooft" and "sudden death", as I recall him speculating in an on-line lecture that at some calculation/complexity threshold, quantum computing devices would undergo an unaccountable but unavoidable "sudden death", the quantum equivalent of a core dump!)

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That's what I believe and have always believed. But I haven't been able to prove it. –  Craig Feinstein Sep 2 '12 at 18:07

I would say it is not "some" but "most" physicists who believe that quantum computing is in principle possible (although many think the engineering challenges will be very hard and have doubts about whether the resulting computers would be worth the cost). The reasons are:

  1. The threshold theorem, which applies even to mildly correlated errors, e.g. see 1207.6131.
    What about skepticism about whether the assumptions are met? (a) I am not aware of serious proposals for error models that would violate this in the sort of systems being considered for quantum computing. Note that there are error models that violate this in chaotic or turbulent systems, like plasma. But these are separated by phase transitions from the systems we can build. (b) We have very good small-scale systems, e.g. see 1402.4848. Again no one has even a proposal for how this will fall apart when this is scaled up. (c) If correlated noise stopped quantum computing, then it would also stop classical computers, which use very similar techniques of error correction (once you see each bit as being encoded in repetition code involving many electrons).

  2. Maybe quantum mechanics is false? The problem is that it's been tested a ton over the last century in a wide range of regimes to a ridiculously high precision. No one can propose a modification that even works on paper that would stop quantum computers but allow all previous observations. One problem is that small modifications that add things like nonlinearity would lead to crazy consequences, like time travel.

In short, the hypothesis that quantum computing is possible is based on a foundation of (a) math, and (b) very well-tested observation. I think it is comparable to the hypothesis that a human mission to Mars is possible.

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But if there is an entanglement of many qubits together, I would expect a great correlation of errors between the qubits, not the mild correlation that the threshold theorem assumes. After all, entanglement implies correlation in some sense. This is why I am still skeptical. –  Craig Feinstein Feb 24 at 16:44
    
Having the evolution depend on the state as you suggest would violate the linearity of the Schrodinger equation, so this is considered by physicists to be a radical and unlikely possibility. If this happened, we probably would have seen it in earlier quantum experiments. I mean maybe a radical change in the laws of quantum mechanics is always possible, but no one has put forward a proposal that matches existing experiment and differs from the Schrodinger equation. –  Aram Harrow Feb 26 at 17:35
    
So quantum mechanics implies that quantum computing is scalable? –  Craig Feinstein Feb 28 at 22:01
    
can you please explain further how it would violate the linearity of the schroedinger equation? –  Craig Feinstein Feb 28 at 22:13
    
The Schrodinger eq says that d psi / dt = -i H psi where H is the Hamiltonian and psi is the wavefunction. If the state is entangled, that is a statement about psi. If errors occur at some rate, this is a statement about H. If H depends on psi, then this becomes a nonlinear differential equation. –  Aram Harrow Mar 3 at 3:12

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