The fundamental lower limit of Sisyphus cooling is the recoil temperature, T r , set by the energy of the photon emitted in the decay from the J' to J state. This limit is k b T r = h 2 M λ λ 2 though practically the limit is a few times this value because of the extreme sensitivity to external magnetic fields in this cooling scheme. Atoms typically reach temperatures on the order of μ μ K , as compared to the doppler limit T D ≃ ≃ 250 μ μ K .