Adiabatic population transfer in a multilevel system.

We report an experimental observation of coherent population transfer in a multilevel system by adiabatic following through a trapped state. Raman transitions between ground-state sublevels are induced by two partially overlapping a± polarized laser beams, tuned to the hyperfine transition F=4→F'=4 of the cesium D 2 line. An efficiency of more than 50% is observed for the m F =+4→m F =-4 population transfer. The influence of other excited hyperfine levels on the transfer efficiency is pointed out. The feasibility of the technique with counterpropagating laser beams is also demonstrated