Genuinely multipartite noncausality

Alastair A. Abbott; Julian Wechs; Fabio Costa; Cyril Branciard

doi:10.22331/q-2017-12-14-39

The study of correlations with no definite causal order has revealed a rich structure emerging when more than two parties are involved. This motivates the consideration of multipartite "noncausal" correlations that cannot be realised even if noncausal resources are made available to a smaller number of parties. Here we formalise this notion: genuinely N-partite noncausal correlations are those that cannot be produced by grouping N parties into two or more subsets, where a causal order between the subsets exists. We prove that such correlations can be characterised as lying outside a polytope, whose vertices correspond to deterministic strategies and whose facets define what we call "2-causal" inequalities. We show that genuinely multipartite noncausal correlations arise within the process matrix formalism, where quantum mechanics holds locally but no global causal structure is assumed, although for some inequalities no violation was found. We further introduce two refined definitions that allow one to quantify, in different ways, to what extent noncausal correlations correspond to a genuinely multipartite resource.

Popular summary

Causality imposes restrictions on how different parties can communicate: a message sent in the future cannot reach a receiver in the past. It is an intriguing possibility, suggested by both quantum mechanics and general relativity, that nature might not enforce such tight restrictions. With access to the right “noncausal” resources, a group of players could win certain communication games with better odds than with any "causal" resource forcing them to play in a well-defined causal order. Here we consider new types of games, involving more than two players, that cannot be won even with noncausal resources, as long as one group of players is causally ordered with respect to another group. Winning such games would thus certify a "genuinely multipartite" noncausal resource that cannot be reduced to a noncausal resource shared by only some of the players. We show how to mathematically characterise such resources, which will be of help in the quest to understand whether we can one day experimentally observe — and even exploit — noncausal resources.

► BibTeX data

@article{Abbott2017genuinely,
  doi = {10.22331/q-2017-12-14-39},
  url = {https://doi.org/10.22331/q-2017-12-14-39},
  title = {Genuinely multipartite noncausality},
  author = {Abbott, Alastair A. and Wechs, Julian and Costa, Fabio and Branciard, Cyril},
  journal = {{Quantum}},
  issn = {2521-327X},
  publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}},
  volume = {1},
  pages = {39},
  month = dec,
  year = {2017}
}

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