Then we can neglect ρ ρ e e = 0 , and set ρ ρ g g = 1 . Indeed, the idea behind this is that if the atom doesn't see any light, then to a first degree approximation in E , the atom will be in the ground state and not in the excited state forcing us to set ρ ρ e e = 0 , ρ ρ g g = 1 .