In the derivation of these formulae, we define ρ ρ ¯ ¯ g e ≡ ≡ ρ ρ g e e − − i δ δ t and ρ ρ ¯ ¯ e g ≡ ≡ ρ ρ e g e i δ δ t . It was also explicitly assumed that spontaneous emission is described by an exponential decay of the coefficient ρ ρ e g ( t ) with decay constant γ γ 2 . Ω Ω is the Rabi frequency, which is