Page "Quantum superposition" Paragraph 136

Every physical quantity has a Hermitian linear operator associated to it, and the states where the value of this physical quantity is definite are the eigenstates of this linear operator.

The linear combination of two or more eigenstates results in quantum superposition of two or more values of the quantity.

If the quantity is measured, the value of the physical quantity will be random, with a probability equal to the square of the coefficient of the superposition in the linear combination.

Immediately after the measurement, the state will be given by the eigenvector corresponding to the measured eigenvalue.