In an article published in 2012 John Preskill, a theoretical physicist, posed a question: “Is controlling large-scale quantum systems merely really, really hard, or is it ridiculously hard?” Seven years later the answer is in: it is merely really, really hard.

Last week a paper on the matter was—briefly and presumably accidentally—published online. The underlying research had already been accepted by Nature, a top-tier scientific journal, but was still under wraps. The leak revealed that Google has achieved what Dr Preskill dubbed in his article, “quantum supremacy”. Using a quantum computer, researchers at the information-technology giant had carried out in a smidgen over three minutes a calculation that would take Summit, the world’s current-best classical supercomputer, 10,000 years to execute... [+]