title: Quantum erasure with causally disconnected choice.
abstract: The counterintuitive features of quantum physics challenge many common-sense assumptions. In an interferometric quantum eraser experiment, one can actively choose whether or not to erase which-path information (a particle feature) of one quantum system and thus observe its wave feature via interference or not by performing a suitable measurement on a distant quantum system entangled with it. In all experiments performed to date, this choice took place either in the past or, in some delayed-choice arrangements, in the future of the interference. Thus, in principle, physical communications between choice and interference were not excluded. Here, we report a quantum eraser experiment in which, by enforcing Einstein locality, no such communication is possible. This is achieved by independent active choices, which are space-like separated from the interference. Our setup employs hybrid path-polarization entangled photon pairs, which are distributed over an optical fiber link of 55 m in one experiment, or over a free-space link of 144 km in another. No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.
authors.affiliation: Institute for Quantum Optics and Quantum Information,  Austrian Academy of Sciences, A-1090 Vienna,  Austria.
authors.forename: Xiao-Song
authors.initials: XS
authors.lastname: Ma
authors.forename: Johannes
authors.initials: J
authors.lastname: Kofler
authors.forename: Angie
authors.initials: A
authors.lastname: Qarry
authors.forename: Nuray
authors.initials: N
authors.lastname: Tetik
authors.forename: Thomas
authors.initials: T
authors.lastname: Scheidl
authors.forename: Rupert
authors.initials: R
authors.lastname: Ursin
authors.forename: Sven
authors.initials: S
authors.lastname: Ramelow
authors.forename: Thomas
authors.initials: T
authors.lastname: Herbst
authors.forename: Lothar
authors.initials: L
authors.lastname: Ratschbacher
authors.forename: Alessandro
authors.initials: A
authors.lastname: Fedrizzi
authors.forename: Thomas
authors.initials: T
authors.lastname: Jennewein
authors.forename: Anton
authors.initials: A
authors.lastname: Zeilinger