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

TL;DR

This paper demonstrates that for Rubidium-87 atoms, efficient laser cooling using quasi-dark states is achievable on the D2 transition, resulting in significantly lower temperatures and higher phase-space density than standard methods.

Contribution

It shows that dark-states cooling can be effectively implemented on the D2 line of Rubidium-87, expanding the applicability of dark-states cooling beyond the D1 transition.

Findings

Achieved temperatures as low as 4.0 microKelvin.

Increased phase space density by nearly an order of magnitude.

Demonstrated effective quasi-dark state cooling on the D2 transition.

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 $n S_{1/2}\rightarrow n P_{1/2}$. 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, $5 S_{1/2}\rightarrow 5 P_{3/2}$. 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.