Denote the two ground states and the excited state of the electron | g − − 1 ⟩ ⟩ , | g 1 ⟩ ⟩ , and | e 0 ⟩ ⟩ , respectively. The atom also has overall momentum, so the overall state of the atom is a product state of its internal state and its momentum, as shown in the figure. In the presence of counter-propagating beams of opposite polarization, the internal states experience the atom-light interaction Hamiltonian