Complementarity and quantum erasure in an atom interferometer

We report on experiments with an atom interferometer, where the atom's spin is-used as a marker for the atom's way through the interferometer. A measu rement on this which-way marker allows us to acquire which-way knowledge. T he choice of the measured observable determines how much which-way knowledg e is obtained. In a quantum eraser, in particular, the observable is chosen so that no which-way knowledge is obtained. This allows us to regain inter ference fringes in subensembles of atoms, which are sorted according to the result of the measurement on the which-way marker. Here we focus on interm ediate situations where the which-way information is only partially erased. We quantitatively compare the which-way knowledge obtained in such a measu rement with the visibilities of the resulting interference patterns. The ex perimental data are in good agreement with two fundamental inequalities, on e recently derived by Bjork and Karlsson [1], the other introduced here. (C ) 2000 Elsevier Science B.V. All rights reserved.