Fringe visibility and which-way information in an atom interferometer

We experimentally investigate the reduction of the fringe visibility in an atom interferometer due to the storage of which-way information. We focus o n the case of incomplete which-way information and use the distinguishabili ty D to quantify how much information is stored. For a given value of D, th e fringe visibility V is limited by the duality relation D-2 + V-2 less tha n or equal to 1. We have measured D and V independently. Combining the resu lts, we find good agreement with the duality relation.