$\Lambda$-enhanced grey molasses on the $D_2$ transition of Rubidium-87 atoms

Abstract

Laser cooling based on dark states, i.e. states decoupled from light, has proven to be effective to increase the phase-space density of cold trapped atoms. Dark-states cooling requires open atomic transitions, in contrast to the ordinary laser cooling used for example in magneto-optical traps (MOTs), which operate on closed atomic transitions. For alkali atoms, dark-states cooling is therefore commonly operated on the $D_1$ transition $n{S}_{1/2}\to n{P}_{1/2}$. We show that, for ${}^{87}\text{Rb}$, thanks to the large hyperfine structure separations the use of this transition is not strictly necessary and that $"$quasi-dark state$"$ cooling is efficient also on the $D_2$ line, $5S_{1/2}\to 5P_{3/2}$. We report temperatures as low as $(4.0\pm 0.3)\mu$K and an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling.