Monday, April 30, 2012

Impossibility of quantum computers

MIT computer scientist Scott Aaronson is betting that quantum computers are possible, while he concedes that they have not yet been built, while others argue that they are impossible. Now Jo˜ao N. C. Especial has added a comment:
Entanglement is consequently the indispensable resource for relevant Quantum Computation and the viability of such Quantum Computation rests on whether this predicted phenomenon does indeed exist in the real world.

It happens that a terrible mistake has been made in the interpretation of experimental Bell tests (experiments which aim to demonstrate Entanglement): The inequalities used to interpret the experiments’ results did not adequately take into account the non-idealities present in the real experimental setups, and hence, the wrong inequalities were used and the wrong conclusion, that Entanglement had been observed, was drawn.

In fact, when the correct inequalities are used to interpret them, all experimental results to this date are found compatible with local-realism and, hence, no evidence presently exists that Entanglement is a real phenomenon.

Aaronson replies on his blog:
(1) The author never really comes out and says which tests would convince him that entanglement is there. ...

(2) The broader point is that quantum mechanics is a wildly-successful framework that, if it’s wrong at all, has to be replaced by something radically different. Yes, it’s important that physicists continue to perform better and better experimental tests of whatever aspects of QM they can, looking for deviations between theory and experiment. But ultimately, I’d say the burden lies with the people who deny some aspect of QM — whether it’s quantum computing, or something even more basic like 2-particle entanglement — to suggest an alternative picture of reality that recovers QM’s century of experimental successes, but in which the aspect in question doesn’t appear. Indeed, that was precisely the point of my bet: that I’m tired of the “defense attorneys” who think they win if they can raise a single point that the prosecution can’t immediately answer, and who don’t acknowledge any need to offer their own alternative picture of the world.

Yes, QM is wildly successful, and some interpretations say that on the smallest scales the math predicts:
  • energy is not conserved
  • particles go backwards in time
  • particles can be in 2 places at once
  • nonlocal action-at-a-distance
  • entangled qubits
  • super-Turing computation
And yet none of this is ever observed on a macroscopic level. The closest is that the Bell test experiments are given as evidence for nonlocality, but they have loopholes, and there is no convincing demonstration of nonlocality.

I am one of the “defense attorneys” that Aaronson is tired of. I cannot prove that quantum computing is impossible any more than I can prove that perpetual motion machines, psychic telekinesis, and backwards time travel is impossible. I reject those ideas because they are contrary to common sense, contrary to established science, and because a lot of smart people have spent a lot of time and money on them and failed. Extraordinary claims require extraordinary evidence.

The math of QM involves idealizations that are not meant to be taken literally. Eg, an electron is sometimes a particle and sometimes a wave, according to complementarity. But a particle is not a wave and a wave is not a particle. If you take either the particle model or wave model too literally, you will get some false conclusions. These are just conceptual tricks. The observations are what is real, and QM predicts those well.

The quantum computing enthusiasts cite the wild success of QM experiments, but those experiments do not really justify quantum computing. For that, they resort to extrapolations of those mathematical idealizations that have never been confirmed. But none of those idealizations are real. They are just mathematical shortcuts for explaining observations.

There are experiments that claim dozens of qubits, but there is a Nobel Prize waiting for anyone who can convincingly demonstrate a single qubit.

Aaronson asks what would convince me. All it takes is an experiment that demonstrates a computation that is greater than what is possible with a Turing machine. I don't see much chance of that.

Update: Aaronson just posted a diatribe against Joy Christian, a (male) skeptic about Bell nonlocality, and concludes:

But let’s forget about money for now. Over the past few months, I’ve had a real insight: the most exciting potential application of scalable quantum computers is neither breaking RSA, nor simulating quantum physics, nor Grover’s algorithm, nor adiabatic optimization. Instead, it’s watching the people who said it was impossible try to explain themselves. That prospect, alone, would more than justify a Manhattan-project-scale investment in this field.
Aaronson is saying that he cannot prove that quantum computing has any validity, but he will settle for trying to show that there are flaws in Christian's argument that quantum computing is impossible.

Christian's mathematical models probably are wrong. But Christian being wrong does not make quantum computing possible. Only an experiment can prove quantum computing possible, and no one has been able to do that.

Update: Lumo piles on. Update: Aaronson says he will continue to be a unabashed doofus and admit when he doesn’t know something or makes a mistake.


  1. At the end of the comments of Aaronson's blog, Joy Christian said, "let me make a counter offer to you. I will give you 200,000 US dollars the day someone produces an actual, working, quantum computer in a laboratory recognizable by me. If I am still alive, I will send you 200,000 US Dollars, multiplied by an appropriate inflation factor. Go build a quantum computer."

    The big question that I have is why is everyone so interested in offering money for doing stuff they believe is impossible?

    That's just plain silly.

  2. Aaronson is a big-shot MIT professor. He just won a prestigious $500k Waterman Award. He has staked his career on claims about what quantum computers can and cannot do. If someone built a quantum computer, then his claims could be tested. If the public recognizes that quantum computers are impossible, then all of his research becomes irrelevant. He actively attacks anyone who says that quantum computers have been built, or who says that they are impossible. Draw your own conclusions.

  3. It seems to me like Aaronson is borrowing from the playbook of the Amazing Randi, a magician who claims that if anyone can prove to him that supernatural phenomena exist, he'll pay them some large sum of money. Then he uses the fact that he hasn't paid the money as proof that supernatural phenomena doesn't exist.

    I see it all as just a stunt designed to make oneself appear generous and great before one's peers. It's what made the Amazing Randi famous. This stunt was even tried in the Bible, Genesis 23, with Ephron.

  4. teresa.mendes@imagitec.ptMay 3, 2012 at 3:27 PM

    Roger: Thank you for your post. Take heart. your opinion is 100% sound.
    I would like to exchange a few emails with you.
    Would you mind?
    You have my email.