Wednesday, February 1, 2012

The impossibility of quantum computers

A blog is debating whether quantum computers are the perpetual motion machines of the 21st century:
Gil Kalai and Aram Harrow are world experts on mathematical frameworks for quantum computation. They hold opposing opinions on whether or not quantum computers are possible. ...

Are quantum computers feasible? Or are their underlying models defeated by some fundamental physical laws? ...

As an aside, let me briefly say why I tend to regard universal quantum computers as unrealistic. An explanation for why universal quantum computers are unrealistic may require some change in physics theory of quantum decoherence. On the other hand, universal quantum computers will be physical devices that are able to simulate arbitrary quantum evolutions, where the word “simulate” is understood in the strong sense that the computer will actually create an identical quantum state to the state created by the evolution it simulates, and the word “arbitrary” is understood in the strong sense that it applies to every quantum evolution we can imagine as long as it obeys the rules of quantum mechanics. As such, quantum computers propose a major change in physical reality.
The perpetual motion analogy is a good one. Quantum computer research keep claiming progress, but it is like the progress of free energy researchers. Yes, they may find some slight gain in efficiency, but the research says nothing toward showing that the goal is attainable.

It is just not true that the skeptics of quantum computing are really skeptics of quantum mechanics. All of quantum mechanics can be true without quantum computers.

Update: Physicist Scott Aaronson is betting that Quantum Computing is possible:
I hereby offer $100,000 for a demonstration, convincing to me, that scalable QC is impossible in the physical world.

... Whether Bigfoot exists is a question about the contingent history of evolution on Earth. By contrast, whether scalable QC is possible is a question about the laws of physics. It’s entirely conceivable that future developments in physics would conflict with scalable QC in the same way relativity conflicts with faster-than-light communication and the Second Law conflicts with perpetuum mobiles. It’s such a development in physics that I’m offering $100k for.
I am not sure about this distinction. The post has four conjectures that are consistent with quantum mechanics and all of the experimental evidence, but make quantum computers impossible. The Second law of thermodynamics was demonstrated by people trying to build perpetual motion machines. We do have other arguments for it, but I doubt that they would cause someone to pay off a $100k bet.

Update: Aaronson now argues "that the burden is on the QC skeptics to answer" how quantum computing is impossible, based on his 2003 paper and his PhD thesis. This opinion seems bizarre to me, considering that quantum computing is contrary to every experiment that has even been done, but I guess that I will have to study his paper. Unfortunately, he has already reneged on his $100k offer.

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