title: Quantum mechanical which-way experiment with an internal degree of freedom.
abstract: For a particle travelling through an interferometer, the trade-off between the available which-way information and the interference visibility provides a lucid manifestation of the quantum mechanical wave-particle duality. Here we analyse this relation for a particle possessing an internal degree of freedom such as spin. We quantify the trade-off with a general inequality that paints an unexpectedly intricate picture of wave-particle duality when internal states are involved. Strikingly, in some instances which-way information becomes erased by introducing classical uncertainty in the internal degree of freedom. Furthermore, even imperfect interference visibility measured for a suitable set of spin preparations can be sufficient to infer absence of which-way information. General results are illustrated with a proof-of-principle single-photon experiment.
authors.affiliation: Faculty of Physics, University of Warsaw, ul. Hoża 69, PL-00-681 Warszawa, Poland.
authors.forename: Konrad
authors.initials: K
authors.lastname: Banaszek
authors.forename: Paweł
authors.initials: P
authors.lastname: Horodecki
authors.forename: Michał
authors.initials: M
authors.lastname: Karpiński
authors.forename: Czesław
authors.initials: C
authors.lastname: Radzewicz