The atom is now pumped to the M J ′ = − − 1 2 excited state, where it spontaneously emits a photon and decays to the M J = − − 1 2 ground state. The key concept is that in the presence of σ σ − − light, the AC stark shift lowers the M J = − − 1 2 further in energy than the M J = + 1 2 state. In going from the M J = + 1 2 to the M J = − − 1 2 state, the atom has indeed lost U 0 in energy, where U 0 = E M J = + 1 2 − − E M J = − − 1 2 approximately equal to the AC Stark shift U 0 ≃ ≃ ℏ ℏ Ω Ω 2 4 δ δ where omega is the Rabi frequency and delta is the detuning.