Phase-dependent which-way information

We introduce an observable for reading out a which-way detector in a Young-type interferometer whose eigenstates either contain full which-way information or none at all. We calculate the which-way knowledge KK that can be retrieved from this observable and find that KK depends on the phase difference δ   that the interfering quantum object accumulates on its passage through the interferometer. It turns out that for certain phase shifts K(δ)K(δ) equals one, independent of the visibility of the interference pattern. In addition, within a certain range of phase shifts K(δ)K(δ) can be larger than the which-way information obtained from the observable recognized to provide optimal which-way information on average.

► We study the which-way knowledge in a Young-type interferometer.
► We investigate a particular which-way detector to derive the which-way knowledge.
► The which-way knowledge depends on the phase accumulated in the interferometer.
► The which-way knowledge can be independent of the interference patternʼs visibility.
► The which-way knowledge can be larger than previously thought.