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At the Collider exhibition in London’s Science Museum there is a mock-up of an office corridor at Cern, the huge Geneva laboratory where the Higgs boson was unmasked. A poster on a door, featuring the photograph of a cat, proclaims: “Lost cat”. Then, underneath: “Please return dead and alive to Erwin Schrödinger.”

The cat – a feline in a locked box that is both dead and alive until the box is opened – was a thought experiment devised by physicist Schrödinger to expose the counterintuitive weirdness of quantum theory. The theory posits that an entity can exist simultaneously in any number of states until the point at which it is observed, whereupon it will “collapse” into one state – either purring or deceased in the case of the trapped tabby, which is incarcerated with a poison that either has or has not been released through radioactive decay.

Geniuses have spotted that tapping into the quantum realm could release fantastic amounts of computing power. Where a classical bit must be either 0 or 1, a quantum bit, called a qubit, can be 0 or 1 – or, crucially, a mixture of both. Freed of this “either-or” constraint, even a modest quantum computer would vastly outpace the fastest supercomputer. Cracking the world’s toughest encryption software, which would take a supercomputer about 1m years, would be an hour’s work.

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