Google's quantum revolution is still in the slow lane. A milestone speed test of the D-Wave Two, a commercial quantum computer recently purchased by the internet giant, suggests that the machine performs no better than an ordinary PC.

The test marks the first head-to-head comparison of the D-Wave Two and a conventional computer running an optimised algorithm. Critics say the D-Wave machine is simply not able to exploit quantum mechanics to calculate faster than a regular computer. But the latest test only looked at one specific type of computing problem, and some experts think D-Wave might still show improvements in other areas.

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