A quantum computer can be in many states simultaneously, which in turn means that it can, in some sense, perform many different calculations at the same time. To be precise, a quantum computer with four qubits could be in 2^4 (ie, 16) different states at a time. As you add qubits, the number of possible states rises exponentially. A 16-bit quantum machine can be in 2^16, or 65,536, states at once, while a 128-qubit device could occupy 3.4 x 10^38 different configurations, a colossal number which, if written out in longhand, would have 39 digits. Having been put into a delicate quantum state, a quantum computer can thus examine billions of possible answers simultaneously. (One way of thinking about this is that the machine has co-operated with versions of itself in parallel universes.)