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Conference on Lasers and Electro-Optics

Experimental Tests of Indefinite Causal Orders

Lee A. Rozema, Giulia Rubino, Adrien Feix, Mateus Araújo, Časlav Brukner, Philip John Walther

Published Jan 1, 2017• 0 citations
Publisher

Abstract

It has been predicted that quantum processes without a defined causal order exist. Here we present an overview of our experimental research program creating and characterizing such processes, discussing also their potential applications.
DOI
10.1364/cleo_qels.2017.ftu1f.6

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Quantum processes can exhibit scenarios beyond a fixed order of events. We propose information inequalities that, when violated, constitute sufficient conditions to certify quantum processes without a fixed causal order -- causally separable or indefinite causal ordered processes. The inequalities hold valid for a vast class of information measures. Nevertheless, we take under scrutiny the von Neumann, α−α-α−Rényi entropies with parameter α∈[1/2,1)∪(1,∞)α\in [1/2,1) \cup (1,\infty)α∈[1/2,1)∪(1,∞), and max- and min-entropies. We also discuss how the strong subadditivity of quantum (von Neumann) entropy, used along with the information inequality developed here, implies relevant witnesses of causally separable and indefinite causal ordered processes in marginal scenarios. Importantly, we show the violation of these inequalities for the quantum switch, a paradigmatic example of a process with indefinite causal order. Our approach contributes to the important research direction of information-theoretic characterization of quantum processes beyond fixed causal orders.