where d → → is the dipole moment operator of the atom. The total Hamiltonian for the atom-light system is therefore H = H 0 + H 1 . The atom does not have a dipole moment when it is in an energy eigenstate, so ⟨ e | d → → | e ⟩ = ⟨ g | d → → | g ⟩ = 0. This means that defining d → → eg := ⟨ e | d → → | g ⟩ allows the dipole operator to be written as