A causal modelling analysis of Bell scenarios in space-time: implications of jamming non-local correlations for relativistic causality principles

V. Vilasini, Roger Colbeck

Published 2023-11-30Version 1

Bell scenarios involve space-like separated measurements made by multiple parties. The standard no-signalling constraints ensure that such parties cannot signal superluminally by choosing their measurement settings. In tripartite Bell scenarios, relaxed non-signalling constraints have been proposed, which permit a class of post-quantum theories known as jamming non-local theories. To analyse whether no superluminal signalling continues to hold in these theories and, more generally, the role of non-signalling constraints in preserving relativistic causality principles, we apply a framework that we have recently developed for defining information-theoretic causal models in non-classical theories and their compatibility with relativistic causality in a space-time. We show that any theory that generates jamming correlations in a Bell scenario between space-like separated parties must necessarily do so through causal fine-tuning and by means of superluminal causal influences. Moreover, within our framework, we show that jamming theories can also lead to superluminal signalling (contrary to previous claims) unless it is ensured that certain systems are fundamentally inaccessible to agents and their interventions. Finally, we analyse relativistic causality in Bell scenarios showing that no-signalling constraints on correlations are generally insufficient for ruling out superluminal signalling when general interventions are also allowed. In this way, we identify necessary and sufficient conditions for ruling out superluminal signalling in Bell scenarios, and demonstrate through examples that the non-signalling constraints on correlations are neither necessary nor sufficient for ruling out causal loops. These results solidify our understanding of relativistic causality principles in information processing tasks in space-time, involving classical, quantum or post-quantum resources.