New article published by the Bruker Group:


Non-causal correlations certify the lack of a definite causal order among localized space-time regions

In stark contrast to scenarios where a single region influences its own causal past, some processes that distribute non-causal correlations satisfy a series of natural desiderata: logical consistency, linear and reversible dynamics, and computational tameness. Here, we present such processes among arbitrary many regions where each region influences every other but itself, and show that the above desiderata are altogether insufficient to limit the amount of "acausality" insufficient to limit the amount of "acausality" of non-causal correlations. This leaves open the identification of a principle that forbids non-causal correlations. Our results exhibit qualitative and quantitative parallelsqualitative and quantitative parallels with the non-local correlations due to Ardehali and Svetlichny.

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Featured image: This figure shows various correlation sets for n parties. Bi-causal inequalities bound the set of correlations that are obtained by partitioning the n parties in two sets, and where only one set can influence the other. Correlations that lie outside this set are genuinely multi-party non-causal. The processes reported in this work establish the non-causal correlations Gn. The degree of violation of the respective bi-causal inequality increases with n. More so, the processes do not require any quantum-theoretic features, and are therefore obtained with classical processes.