We start with a three-state Λ-type system, where | 1 ⟩ ⟩ ↔ ↔ | 3 ⟩ ⟩ and | 2 ⟩ ⟩ ↔ ↔ | 3 ⟩ ⟩ are dipole-allowed transitions and | 1 ⟩ ⟩ ↔ ↔ | 2 ⟩ ⟩ is forbidden. In the rotating wave approximation, the semi-classical Hamiltonian is given by