Time-dependent perturbation theory
= Time-dependent perturbation theory
Expanding a state in unperturbed energy eigenstates with their free phases converts the time-dependent Schrodinger equation into coupled equations for slowly varying amplitudes. To first order, the perturbation on the right-hand side acts on the initial unperturbed state.
This is the time-dependent branch of <perturbation theory (quantum mechanics)>; its corrections describe transition amplitudes.