Quantum computers? Don't hold your breath

Quantum computing will never work. At least, that's the view of one physicist who thinks that unavoidable noise will always stand in its way.

In theory, a quantum computer could be far more powerful than any existing device. Making it work, however, means protecting the quantum particles used in its calculations from the disrupting noise of the outside world. To date, this has been achieved only for a few particles for fractions of a second. A useful device would have to be noiseless for far longer, and use hundreds or thousands of particles.

Michael Dyakonov of the University of Montpellier in France believes this feat is akin to achieving perpetual motion. It has been assumed till now that errors caused by noise can be fixed. In reality, Dyakonov argues, such errors would grow far too rapidly with the number of particles, making correction impractical. What's more, he says, correction schemes make unproven assumptions: for example, that the various errors introduced by imperfect devices will be independent of one another, and so largely cancel each other out. "I have serious doubts about the possibility of large-scale quantum computations," he says (www.arxiv.org/quant-ph/0610117).

Others think Dyakonov has got it wrong. "It is true that the quantum computing community should be cautiously optimistic, rather than confident," says Andrew Steane of the University of Oxford. "But his arguments are largely misleading."

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