There are 2 complex numbers, one of them the coefficient of |0> and |1>. The sum of the square of the magnitudes of these numbers is 1. Alternatively/equivalent

There are 2 complex numbers, one of them the coefficient of |0> and |1>. The sum of the square of the magnitudes of these numbers is 1. Alternatively/equivalently , the magnitude of the vector taken as a whole, is 1. So if you have (a+bi)|0> + (c+di)|1>, the magnitude of this vector is a^2 + b^2 + c^2 + d^2 = 1 Edit: however, the vector only has to have magnitude 1 if it is describing the entire system, I think?