When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. Known as the Doppler limit, , 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, , is the natural line-width of the atomic transition, , is the reduced Planck constant and, , is the Boltzmann constant.
Experiments at the National Institute of Standards and Technology, Gaithersburg, found the temperature of cooled atoms to be well below the theoretical limit.In 1988, Lett et al.  directed Sodium atoms through an optical molasses and found the temperatures to be as low as ~40μk; 6 times lower than the expected 240μk doppler cooling limit. Other unexpected properties found in other experimentsincluded significant unexpected insensitivity to laser alignment of the counter-propagating beams.