IBM challenges a recent result in quantum computing

IT WAS HAILED by many, including The Economist, as a landmark result in quantum computing. In September a scientific paper appeared accidentally on a NASA webpage. In it a team of researchers at Google described how they had used a quantum computer to complete, in three minutes, a calculation that would have taken a classical machine 10,000 years to crunch through. This feat, they claimed, marked the first demonstration of "quantum supremacy"--using a quantum computer to tackle a task unfeasible...