They call it Q-Day: the day when a quantum computer, one more powerful than any yet built, could shatter the world of privacy and security as we know it.

It would happen through a bravura act of mathematics: the separation of some very large numbers, hundreds of digits long, into their prime factors.

That might sound like a meaningless division problem, but it would fundamentally undermine the encryption protocols that governments and corporations have relied on for decades. Sensitive information such as military intelligence, weapons designs, industry secrets and banking information is often transmitted or stored under digital locks that the act of factoring large numbers could crack open.