Quantum computers will break the encryption that protects the internet

AS EVERY SCHOOLCHILD knows, some sorts of mathematics are harder than others. In the classroom, that is annoying. Outside, it can be useful. For instance, given two prime numbers, however large, multiplying them together to find their product is easy. But the reverse—factorising that product back into its constituent primes without knowing in advance what those primes are—is hard, and becomes rapidly harder as the number to be factorised gets bigger.Factorising numbers into their constituent primes may sound esoteric, but the one-way nature of the problem—and of some other, closely related mathematical tasks—is the foundation on which much modern encryption rests. Such encryption has plenty of uses. It defends state secrets, and the corporate sort. It protects financial flows and medical records. And it makes the $2trn e-commerce industry possible. Without it, credit-card details, bank transfers, emails and the like would zip around the internet unprotected, for anyone so minded to see or steal.