Mixed state

A mixed state is a qubit that carries ordinary randomness on top of its amplitudes, so no single pair of amplitudes describes it. It comes from noise, from a careless preparation, or from entanglement with something you are not looking at. A measurement still returns one bit, and the bit follows the chances written in the mixture.

A Hadamard on the |+⟩ state returns the state certain to read 0, because the two routes to a reading of 1 cancel. The same gate on a half-and-half mixture of the states certain to read 0 and 1 still leaves an even chance, and Malcolm keeps those two sheets in different drawers.