Your Symmetry Broke My Quantum Computer?

An article in Scientific American (of all places….I stopped reading Scientific American when they started a section on science/pseudoscience. Sure I agree with them, but I don’t want to read a science magazine to read about how science is different from pseudoscience, I already know that. Plus they stopped the amateur science section and mathematical recreations section: really the two best reasons to read Scientific American in the good old days) on a mechanism for decoherence due to symmetry breaking.

Jeroen van den Brink and his colleagues at Leiden University in the Netherlands, however, suggest that even perfect isolation would not keep decoherence at bay. A process called spontaneous symmetry breaking will ruin the delicate state required for quantum computing. In the case of one proposed device based on superconducting quantum bits (qubits), they predict that this new source of decoherence would degrade the qubits after just a few seconds.

The paper in question, published in Physical Review Letters (and available as quant-ph/0408357cond-mat/0408357) presents an interesting mechanism for decoherence. What is most interesting about this decoherence mechanism is the rate they obtain for decoherence: [tex]$t_D={N h over k_B T}$[/tex], where N is the number of microscopic degress of freedom, and h, k_B, and T should be recognizable to every physicist 😉

What does this mean for quantum computers? Well the above might indicate that this is some fundamental limit for quantum computing (and in particular for superconducting implementations of quantum computers for which this result will hold). But I don’t think this is true. I’ll let the article explain why:

Not everyone agrees that the constraint of a few seconds is a serious obstacle for superconducting qubits. John Martinis of the University of California at Santa Barbara says that one second “is fine for us experimentalists, since I think other physics will limit us well before this timescale.” According to theorist Steven M. Girvin of Yale University, “if we could get a coherence time of one second for a superconducting qubit, that would mean that decoherence would probably not be a limitation at all.” That is because quantum error correction can overcome decoherence once the coherence time is long enough, Girvin argues. By running on batches of qubits that each last for only a second, a quantum computer as a whole could continue working indefinitely.

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4 Responses to Your Symmetry Broke My Quantum Computer?

  1. Your arXiv identifier is wrong — it’s actually in cond-mat, as the hyperlink itself shows.

    The e-print never quite says that quantum computation is impossible. Nonetheless, as it is interpreted by Scientific American, it is yet another paper in the general direction of, “we don’t understand fault tolerance, therefore quantum computation is impossible.”

  2. It reminds me of a political joke that I read in the book, “From Beirut to Jerusalem” (but which has undoubtedly been adapted to many situations in history).

    A man walks up to a policeman standing on a corner in Beirut.

    Man: Help me! A Swiss man stole my Syrian watch!

    Policeman: A Swiss man? A Syrian watch? That doesn’t make any sense. You mean to say that a Syrian man stole your Swiss watch.

    Man: You said it, mister, not me.

    In this case, the abstract of cond-mat/0408357 has the statement, “It is shown that spontaneous symmetry breaking imposes a fundamental limit to the time that a system can stay quantum coherent.” It is true that they don’t quite say that quantum computation is impossible, and maybe they don’t even mean it. But who could be blamed for reading it between the lines? They analyze one physical system that they call “representative”, but then in other places they call it a “fundamental limit” that applies to “a system”. As in all systems? Just systems that don’t have quantum fault tolerance? What are they really saying here?

  3. Dave Bacon says:

    Oops. Fixed to cond-mat now.

    I don’t think the authors intend the paper to be an impossibility of quantum computation paper, but it sure seems that scientific american wants to interpret it that way.

  4. A condensation-ghost came forth.
    Imperilled fear condensed and morphed
    And shifted. But inside its shell–
    Its mask of gaseous molten hell–
    There was a ghastly brooding core
    Of singularity wanting for
    Humanity to be extinct,
    And Earth to be a razed precinct.
    That’s why Time Cube must now be sought.
    For as it percolates our thought,
    It will dissolve addictions base,
    Propelling us to freer space
    In which future’s commencement’s wings
    Shall view an abyss of bad things
    And turn it all to greater good
    A Cubic vision which we should
    Seek to achieve. It lies in store:
    4-days, Cube-time, and so much more.

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