Multiple-beam Ramsey interference and quantum decoherence

We study the effect of photon scattering from a path of a four-beam atomic interference setup, which is based on a cesium atomic beam and two subseque nt optical Ramsey pulses projecting the atoms onto a multilevel dark state. While in two-beam interference, any attempt to keep track of an interferin g path reduces the fringe contrast, we demonstrate that photon scattering i n a multiple-path arrangement cannot only lead to a decrease, but - under c ertain conditions - also to an increase of the interference contrast. The r esults are confirmed by a density-matrix calculation. We are aware that in all cases the "which-path" information carried away by the scattered photon s leads to a loss of information that is contained in the atomic quantum st ate. An approach to quantify this "which-path" information using observed f ringe signals is presented; it allows for an appropriate measure of quantum decoherence in multiple-path interference.