Sunday, February 19, 2012

Quantum computer hype in Vancouver

The Vancouver Sun shows a picture of computer scientist Scott Aaronson and brags about the vast computational advantages of quantum computers:
“If this all works well, this is five years off. ...

“But who knows what will happen in the future. That's how people used to think about regular computers.”
The field has a dramatic mix of views. One company says that it is in commercial production of 100-qubit computers. A qubit is the basic building block of a quantum computer. If we could make qubits that could be chained together, then we could make quantum computers. We cannot make quantum computers, and predictions vary from saying that it will be possible in 5 years, to saying that it is forbidden by the laws of physics.
Aaronson thinks some problems in mathematics will remain beyond the limits of quantum computing and indeed, he thinks a breakthrough in development of a large-scale quantum computer is many decades, rather than a few years, away.

“Even quantum computers would have significant limitations. If you're just looking for a needle in a haystack. . . then not even a quantum computer will give you an exponential speedup for this problem,” Aaronson said.

He said that view is contrary to most popular representations of quantum computers as infinitely capable.

Nonetheless, he rejects skeptics who doubt they will get built – he's offering a $100,000 prize to anyone who can demonstrate that it's impossible to build a large scale quantum computer.
Aaronson says that I was one of the skeptics that pushed him over the edge to offer his prize.

Czech String theorist Lubos Motl has another view:
Many people have said many stupid things about quantum computation. Many but not all anti-quantum zealots of course believe that quantum computation is impossible because the world must be fundamentally classical and a classical computer of a given size can't solve certain problems after a certain number of steps.
That is not my view. I believe that the world is fundamentally quantized and not classical.
Other people believe in quantum computers but they have used them to "prove" bizarre "interpretations" of quantum mechanics. In particular, David Deutsch has said that the likely existence of quantum computers proves the many-worlds interpretation of quantum mechanics.
At least Deutsch does recognize that a true quantum computer would demonstrate a phenomenon that has never been seen before in any experiment. Because if it had, then that would be a proof of many-worlds, and everyone agrees that there is no proof of many-worlds. And I do not believe that there will ever be any proof of many-worlds, as it is a silly and unscientific idea.

Motl goes on with a little rant about the nature of science:
The only way to rule out a hypothesis is to actually find a wrong prediction that the hypothesis makes, one that disagrees with observations. Various anti-quantum zealots love to use some "alternative", non-scientific ways to falsify theories. A theory doesn't agree with their medieval prejudices so they just decide it means that the theory has to be abandoned. But science hasn't worked in this way for 500 years. What a prejudiced bigot a priori thinks is totally irrelevant in science.
That's right. A-priori opinions about how the world works are irrelevant, whether then come from Einstein or anyone else.

My main reason for being skeptical about scalable quantum computing is that decades of research by really smart people and 100s of millions of dollars have gone into this, and no one has even figured out a way to show that it is even possible. If the dramatic and implausible consequences were really possible, then I would expect to see some evidence of it. At this point, it seems more likely that some law of physics is preventing it, such as the laws of thermodynamics that prevent perpetual motion machines.

Motl concludes:
Quantum computers demonstrate the power of quantum mechanics, the fundamental framework underlying the laws of physics in our Universe, very crisply and if and when they will be produced, the world may substantially change, at least at some level. Let's hope the change won't be catastrophic.
Okay, but that is wishful thinking. If he follows his own advice, then he has to admit that the world may not follow his a-priori prejudices.

Update: Aaronson says he visited D-Wave Systems near Vancouver, and he reiterated his skepticism about their quantum computer:
It remains true, as I’ve reiterated here for years, that we have no direct evidence that quantum coherence is playing a role in the observed speedup, or indeed that entanglement between qubits is ever present in the system. ...

So I hereby retire my notorious comment from 2007, about the 16-bit machine that D-Wave used for its Sudoku demonstration being no more computationally-useful than a roast-beef sandwich. ...

... the fundraising pressure is always for more qubits and more dramatic announcements, not for clearer understanding of its systems. So, let me try to get a message out to the pointy-haired bosses of the world: a single qubit that you understand is better than a thousand qubits that you don’t. There’s a reason why academic quantum computing groups focus on pushing down decoherence and demonstrating entanglement in 2, 3, or 4 qubits: because that way, at least you know that the qubits are qubits! Once you’ve shown that the foundation is solid, then you try to scale up. ...

For the first time, I find myself really, genuinely hoping — with all my heart—that D-Wave will succeed in proving that it can do some (not necessarily universal) form of scalable quantum computation. For, if nothing else, such a success would prove to the world that my $100,000 is safe, and decisively refute the QC skeptics who, right now, are getting even further under my skin than the D-Wave boosters ever did.
In other words, he accused them of being charlatans 5 years ago for over-hyping quantum computing when they cannot even prove that they have one true qubit. But now he hopes that they will make a quantum computer so that he can prove me and the other quantum skeptics wrong! This is pathetic. Aaronson seems to realize that the field is going nowhere.


  1. It is entirely within the laws of QM and gravity that a shattered glass plate can rise up from the ground into someone's hand and be restored into a whole plate. This is because QM and gravity are time-reversible. It's only from the laws of thermodynamics that it is very improbable that this can happen in the real world. Yet nobody is foolish enough to try to invent technology which allows this to be done in practice.

    Then why should one think that quantum computing, which is also consistent with QM and I would think should work backwards in time, must also work forwards in time?

  2. That is an excellent point. A lot of quantum computer research is at a temperature near absolute zero, where I guess they think that they can get away from the laws of thermodynamics.