Superposition

Superposition, in the usual 0-and-1 arrangement, means both amplitudes of a qubit are nonzero, so the state is neither a plain 0 nor a plain 1. The chances of the two readings are the squared sizes of those amplitudes, and the superposition ends once a measurement has returned a single bit.

Keith wrote “both at once” in the margin and underlined it twice. Malcolm crossed the phrase out, wrote “two numbers” underneath, and would not return the pencil until the squares were on the same line.