Quantum-coherent mixtures of causal relations

Abstract

Understanding the causal influences that hold among parts of a system is critical both to explaining that system's natural behaviour and to controlling it through targeted interventions. In a quantum world, understanding causal relations is equally important, but the set of possibilities is far richer. The two basic ways in which a pair of time-ordered quantum systems may be causally related are by a cause-effect mechanism or by a common cause acting on both. Here, we show a coherent mixture of these two possibilities. We realize this nonclassical causal relation in a quantum optics experiment and derive a set of criteria for witnessing the coherence based on a quantum version of Berkson's effect, whereby two independent causes can become correlated upon observation of their common effect. The interplay of causality and quantum theory lies at the heart of challenging foundational puzzles, including Bell's theorem and the search for quantum gravity.

25 pages (10 main, 15 supplementary information), 7 main figures, 3 supplementary figures

References in corpus (8)

• Quantum Non-Markovianity: Characterization, Quantification and Detection
• Beyond Bell's Theorem: Correlation Scenarios
• Towards Quantum Gravity: A Framework for Probabilistic Theories with Non-Fixed Causal Structure
• Information-Theoretic Implications of Quantum Causal Structures
• Inferring causal structure: a quantum advantage
• Experimental violation of Svetlichny's inequality
• Quantum Dynamics as an analog of Conditional Probability
• Experimental realization of linear-optical partial SWAP gates

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