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Quantum computers are a grand idea. By harnessing the famous strangeness of quantum mechanics, they should be able to perform some (though not all) calculations far faster than any ordinary computer. But building one has proven tricky. The idea was first floated in the 1970s. Four decades later quantum computers are still small, fragile devices confined to the laboratory bench—with one exception. In 2011, to a great fanfare, a Canadian firm called D-Wave announced a commercially available quantum computer, the $10m D-Wave One. Deals with Google, NASA and Lockheed Martin, a weapons firm, followed.

Admittedly, D-Wave’s device is a very specialised sort of computer, restricted to a single area of mathematics called discrete optimisation. But it was big news, and many scientists were rather sceptical. In the past couple of years the firm has published enough papers about its device to convince academics that it has indeed built a quantum-mechanical machine. Now the question is whether it is any faster than the competition.

One set of benchmarks, published in May, suggested that it was: they found that D-Wave’s machine was up to 3,600 times faster than its non-quantum rivals. But it was not quite a fair fight, for the classical machines were running off-the-shelf programs. With a bit of clever tweaking, they could have been made much faster. When a group led by Matthias Troyer, a computer scientist at the Swiss Federal Institute of Technology, tried such souped-up algorithms, they found that D-Wave’s lead vanished. D-Wave, in turn, pointed out that the researchers had been using an old version of its system.

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