IBM challenges a recent result in quantum computing

IT WAS HAILEDNASAIBM by many, including , as a landmark result in quantum computing. In September a scientific paper appeared accidentally on a 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 for a classical one.On October 23rd the paper reappeared, intentionally this time, in . But a few days before that some researchers at —which, like Google and several other information-technology firms, including Intel and Microsoft, is also conducting quantum-computing research—posted their own paper to arXiv, an online repository. In it, they cast doubt on Google’s claim.