is a solution of the Schrödinger equation but is not generally an eigenstate because E n and E n ′ are not generally equal. We say that | Ψ Ψ ⟩ ⟩ is made up of a superposition of energy eigenstates. Now consider the more concrete case of an electron that has either spin up or down. We now index the eigenstates with the spinors in the z ^ ^ basis: