At last, we are discovering what quantum computers will be useful for

Over the past decade, quantum computing has grown into a billion-dollar industry. Everyone seems to be investing in it, from tech giants, such as IBM and Google, to the US military. But Ignacio Cirac at the Max Planck Institute of Quantum Optics in Germany, a pioneer of the technology, has a more sober assessment. "A quantum computer is something that at the moment does not exist," he says. That is because building one that actually works – and is practical to use – is incredibly difficult.