Unbounded cooling force in travelling waves excitation of a four-level system

Summary form only given. In a two-level system under traveling wave excitation, the cooling force is bounded by the spontaneous force, given by the product of the photon momentum and the excited-state decay rate. This because the force in such a scheme originates from absorption of directed photons followed by emission of spontaneous photons in random direction. In the case of a standing wave excitation, this limit no longer holds. The atoms can, in principle, absorb photons from one direction, while emitting coherent photons in the other direction via stimulated emission. This process, known as stimulated cooling, requires symmetry breaking leading to effects such as Doppleron cooling.