Monday, June 10, 2013

Still no quantum computer

Scott Aaronson continues to deny the existence of quantum computers:
D-Wave founder Geordie Rose claims that D-Wave has now accomplished its goal of building a quantum computer that, in his words, is “better at something than any other option available.” This claim has been widely and uncritically repeated in the press, so that much of the nerd world now accepts it as fact. However, the claim is not supported by the evidence currently available. It appears that, while the D-Wave machine does outperform certain off-the-shelf solvers, simulated annealing codes have been written that outperform the D-Wave machine on its own native problem when run on a standard laptop. More research is needed to clarify the issue, but in the meantime, it seems worth knowing that this is where things currently stand.
Aaronson is a big quantum computing enthusiast, and he is doing a public service by throwing cold water on exaggerated claims.

I think that it is unlikely that a useful quantum computer will ever be built. There are a lot of smart people with million dollar research grants trying to prove me wrong. Aaronson makes his mistake with this argument:
This talk will assume what David Deutsch calls the “momentous dichotomy”:

Either a given technology is possible, or else there’s some principled reason why it’s not possible.

Example application: Quantum computing
This sort of reasoning can lead you to all sorts of science fiction ideas.

I never understood a principled reason why Maxwell's demon is impossible. And yet it seems to be impossible.

Meanwhile Aaronson has released a long new paper on The Ghost in the Quantum Turing Machine. This discusses free will and other topics, and he seems to say some sensible things. I hope to post more about it later.

Update: This paper is an elaboration of his previous views. To read those, see his lecture (that was a draft for his book), or this summary of another lecture:
This backward causation, or retrocausality, was the “loony” aspect of Aaronson’s talk. Except there’s nothing loony about it. It is a concept that Einstein’s special theory of relativity made a live possibility. Relativity convinced most physicists that we live in a “block universe” in which past, present, and future are equally real. In that case, there’s no reason to suppose the past influences the future, but not vice-versa. Although their theories shout retrocausality, physicists haven’t fully grappled with the implications yet. It might, for one thing, explain many of the mysteries of quantum mechanics.

In a follow-up email, Aaronson told me that the connection between free will and cosmic initial state was also explored by philosopher Carl Hoefer in a 2002 paper. What Aaronson has done is apply the insights of quantum mechanics. If you can’t clone a quantum state perfectly, you can’t clone yourself perfectly, and if you can’t clone yourself perfectly, you can’t ever be fully simulated on a computer. Each decision you take is yours and yours alone. It is the unique record of some far-flung collection of particles in the early universe. Aaronson wrote, “What quantum mechanics lets you do here, basically, is ensure that the aspects of the initial microstate that are getting resolved with each decision are ‘fresh’ aspects, which haven’t been measured or recorded by anyone else.”
He has a quantum-inspired view of free will, and is not squarely in the pro or anti camps.

Update: There are comments on Aaronson's free will paper here, and possibly here.


  1. I'm convinced that the inability to build a quantum computer will prove that quantum mechanics is just an approximation of nature, just like Newtonian mechanics is. Ultimately, nature is a discrete cellular automata.

  2. The principled reason why Maxwell's demon is impossible is Landauer's principle.