It could turn out to be a milestone for quantum computing. Last week, D-Wave Systems of Burnaby in British Columbia, Canada, announced the first sale of a commercial quantum computer, to global security firm Lockheed Martin, based in Bethesda, Maryland.

Yet perhaps fittingly for a quantum device, uncertainty persists around how the impressive black monolith known as D-Wave One actually works. Computer scientists have long questioned whether D-Wave's systems truly exploit quantum physics, and although the company last month published a paper in Nature (M. W. Johnson et al. Nature 473, 194–198; 2011) to help verify its quantum credentials, some say that the technique used is still in doubt.

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