2016/08/31 by Giulia Rubino, Lee A. Rozema, Adrien Feix +6 · 3 citations
Computer Science · Mathematics · Physics and Astronomy · #A priori and a posteriori #Causal model #Causal structure #Causality (physics) #Causation #Coherence (philosophical gambling strategy) #Computer science #Epistemology #Mathematical analysis #Mathematics #Order (exchange) #Physics #Process (computing) #Quantum Computing Algorithms and Architecture #Quantum Information and Cryptography #Quantum Mechanics and Applications #Quantum mechanics #Set (abstract data type) #Statistical physics #Statistics #Superposition principle #Witness #quant-ph
paper · pdf · doi:10.1126/sciadv.1602589
published as Sci. Adv., 3: e1602589 (2017) · 11 pages + a 9-page Appendix; 5 figures + 5 in the Appendix. Corrected definitions of unitaries in Appendix
arxiv created 2016/09/02 · openalex publication_date 2017/03/03 · arxiv updated 2017/03/31 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
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.