When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. Known as the Doppler limit, T d = ℏ ℏ Γ Γ / 2 k b , this was given by the lowest possible temperature attainable considering the cooling of two-level atoms by Doppler cooling and the heating of atoms due to momentum diffusion from the scattering of laser photons. Here, Γ Γ , is the natural line-width of the atomic transition, ℏ ℏ , is the reduced Planck constant and, k b , is the Boltzmann constant.