Dark-state cooling in the presence of gravity: Gravitational limit of laser cooling

Dark-state cooling of atoms in the presence of a uniform external (gravity) field is investigated. The external field leads to a finite lifetime for the dark state and thereby to a lower limit for the temperatures obtainable. The minimum temperature in the presence of a gravity field is found to be k(B)T(grav)similar to Mg lambda/2 pi (where M is the atom mass, lambda the wavelength of the cooling laser). This limit is shown to be a universal limit for any laser-cooling scheme.