We study the spectral narrowing induced by collisions in a dense cold atomic ensemble. We report on experiments showing a prolongation of the coherence time of optically trapped 87Rb atoms as the density increases, a phenomenon we call collisional narrowing in analogy to the motional narrowing effect in NMR. We derive an expression for the new dephasing time scale in terms of the collision rate and the inhomogeneous decay time. Remarkably, this time scale universally depends only on the atomic phase space density.
We report an experimental study of peak and phase-space density of a two-stage magneto-optical trap (MOT) of
6Li atoms, which exploits the narrower $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ ultraviolet (UV) transition at 323 nm following trapping and cooling on the more common
D2 transition at 671 nm. The UV MOT is loaded from a red MOT and is compressed to give a high phase-space density up to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Temperatures as low as $33\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ are achieved on the UV transition. We study the density limiting factors and in particular find a value for the light-assisted collisional loss coefficient of $1.3\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$
cm3/s for low repumping intensity.