Abstract
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 measurement on this which-way marker allows us to acquire which-way knowledge. The choice of the measured observable determines how much which-way knowledge 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 interference fringes in subensembles of atoms, which are sorted according to the result of the measurement on the which-way marker. Here we focus on intermediate situations where the which-way information is only partially erased. We quantitatively compare the which-way knowledge obtained in such a measurement with the visibilities of the resulting interference patterns. The experimental data are in good agreement with two fundamental inequalities, one recently derived by Björk and Karlsson [1], the other introduced here.
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