(color online) A simple approach to degenerate Raman sideband cooling in Rb, using the 5 S , ground state and 5 P , excited state. The black curved arrows show the different degenerate Raman transitions between the states, while the green arrows show the optical pumping to the excited states and the red arrow shows the spontaneous emission returning an atom to . The different states necessitate the use of both and light for optical pumping, or else atoms become stuck in and will not be cooled further. The final spontaneous decay should be forbidden by conservation of momentum, since the recoil energy is not great enough to change the atom vibrational state. This gives the transition a narrow linewidth.