The golden rule is a straightforward consequence of the Schrödinger equation, solved to lowest order in the perturbation H' of the Hamiltonian. The total Hamiltonian is the sum of an “original” Hamiltonian H 0 and a perturbation: H = H 0 + H ′ ( t ) . In the interaction picture, we can expand an arbitrary quantum state’s time evolution in terms of energy eigenstates of the unperturbed system | n ⟩ ⟩ , with H 0 | n ⟩ ⟩ = E n | n ⟩ ⟩ .