title: Experimental verification of an indefinite causal order.
abstract: Investigating the role of causal order in quantum mechanics has recently revealed that the causal relations of events may not be a priori well defined in quantum theory. Although this has triggered a growing interest on the theoretical side, creating processes without a causal order is an experimental task. We report the first decisive demonstration of a process with an indefinite causal order. To do this, we quantify how incompatible our setup is with a definite causal order by measuring a "causal witness." This mathematical object incorporates a series of measurements that are designed to yield a certain outcome only if the process under examination is not consistent with any well-defined causal order. In our experiment, we perform a measurement in a superposition of causal orders-without destroying the coherence-to acquire information both inside and outside of a "causally nonordered process." Using this information, we experimentally determine a causal witness, demonstrating by almost 7 SDs that the experimentally implemented process does not have a definite causal order.
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria.
authors.forename: Giulia
authors.initials: G
authors.lastname: Rubino
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria.
authors.forename: Lee A
authors.initials: LA
authors.lastname: Rozema
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria.; Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Boltzmanngasse 3, Vienna A-1090, Austria.
authors.forename: Adrien
authors.identifier: 0000-0002-1819-4574
authors.initials: A
authors.lastname: Feix
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria.; Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Boltzmanngasse 3, Vienna A-1090, Austria.
authors.forename: Mateus
authors.initials: M
authors.lastname: Araújo
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria.
authors.forename: Jonas M
authors.identifier: 0000-0003-3130-5723
authors.initials: JM
authors.lastname: Zeuner
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria.
authors.forename: Lorenzo M
authors.initials: LM
authors.lastname: Procopio
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria.; Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Boltzmanngasse 3, Vienna A-1090, Austria.
authors.forename: Časlav
authors.initials: Č
authors.lastname: Brukner
authors.affiliation: Vienna Center for Quantum Science and Technology, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria.
authors.forename: Philip
authors.identifier: 0000-0002-4964-817X
authors.initials: P
authors.lastname: Walther
keywords: Quantum Foundations; Quantum Information; Quantum Optics