where p = -iℏ∇ and the A 2 0 term is just a zero point energy that will be ignored going forward. Since in this perspective light is just a classical, oscillating electromagnetic field with perfect periodicity, a time analog to Bloch's theorem can be used to express a state that satisfies the time-dependent Schrödinger equation, Ψ α (t) , in terms of static states, |ψ α n ⟩ as,