Experimental verification of an indefinite causal order

By Giulia Rubino, Lee A. Rozema, Adrien Feix, Mateus Araújo, Jonas Zeuner, Lorenzo M. Procopio, Č. Brukner, Philip Walther. Science Advances, 2016

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Researchers report on the observation and characterization of a quantum process that lacks a predefined causal order. 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.

Subfields: Artificial Intelligence, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/ef4fc7fdaf0a9d358a96909faf69b3b30bc12358