Combined quantum-state preparation and laser cooling of a continuous beam of cold atoms

Abstract

We use two-laser optical pumping on a continuous atomic fountain in order to prepare cold cesium atoms in the same quantum ground state. A first laser excites the $F=4$ ground state to pump the atoms toward $F=3$ while a second $\ensuremath{\pi}$-polarized laser excites the $F=3\ensuremath{\rightarrow}{F}^{'}=3$ transition of the ${D}_{2}$ line to produce Zeeman pumping toward $m=0$. To avoid trap states, we implement the first laser in a two-dimensional optical lattice geometry, thereby creating polarization gradients. This configuration has the advantage of simultaneously producing Sisyphus cooling when the optical lattice laser is tuned between the $F=4\ensuremath{\rightarrow}{F}^{'}=4$ and $F=4\ensuremath{\rightarrow}{F}^{'}=5$ transitions of the ${D}_{2}$ line, which is important to remove the heat produced by optical pumping. Detuning the frequency of the second $\ensuremath{\pi}$-polarized laser reveals the action of a mechanism improving both laser cooling and state-preparation efficiency. A physical interpretation of this mechanism is discussed.