At this point, the atom is moving in the +z direction with some velocity, and eventually moves into a region with σ σ + light. The atom, still in its M J = − − 1 2 state that it was pumped into, now experiences the opposite AC Stark shift as it did in σ σ - light, and the M J = 1 2 state is now lower in energy than the M J = − − 1 2 state. The atom is pumped to the M J ′ = 1 2 excited state, where it spontaneously emits a photon and decays to the M J = + 1 2 state. As before, this energy level has been lowered by the AC Stark shift, and the atom loses another U 0 of energy.