Quantum mechanical which-way experiment with an internal degree of freedom

• Published in: Nature communications 2013-10, Vol.4 (1), p.2594-2594, Article 2594
• Main Authors: Banaszek, Konrad, Horodecki, Paweł, Karpiński, Michał, Radzewicz, Czesław
• Format: Article
• Language: English
• Subjects: 639/766/483/1139; 639/766/483/640; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms3594

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. Quantum mechanics dictates that the interference pattern cast by particles after passing through a double slit depends on how much information it is possible to know about which slit they went through. Banaszek et al . show how this behaviour extends to a system’s internal degrees of freedom.