The hardest idea in quantum mechanics to hold in a non-physicist’s head is that a thing can be in two states at once. Not flickering between them too quickly to see but both, genuinely, at the same time, until someone measures and it settles into one. That contradiction turns out to be the best way to describe the decades-old argument over the state of quantum computing itself. Quantum has been a technology that the Nobel Committee, the National Security Agency and half of Silicon Valley agree will rewrite what a computer can do. It is also, simultaneously, a thing so…