Paradox-free classical non-causality and unambiguous non-locality without entanglement are equivalent

TL;DR

This paper establishes a fundamental link between classical non-causality in process functions and quantum nonlocality without entanglement, providing a comprehensive characterization and revealing new connections with non-signaling inequalities.

Contribution

It provides the first complete recursive characterization of process functions and their equivalence to unambiguous product bases, linking non-causality to quantum nonlocality without entanglement.

Findings

Process functions are equivalent to unambiguous complete product bases.

Non-causality in process functions mirrors quantum nonlocality without entanglement.

The results extend to arbitrary local dimensions and parties.

Abstract

Closed timelike curves (CTCs) challenge our conception of causality by allowing information to loop back into its own past. Any consistent description of such scenarios must avoid time-travel paradoxes while respecting the no-new-physics principle, which requires that the set of operations available within any local spacetime region remain unchanged, irrespective of whether CTCs exist elsewhere. Within an information-theoretic framework, this leads to process functions: deterministic classical communication structures that remain logically consistent under arbitrary local operations, yet can exhibit correlations incompatible with any definite causal order - a phenomenon known as non-causality. In this work, we provide the first complete recursive characterization of process functions and of (non-)causal process functions. We use it to establish a correspondence between process functions and unambiguous complete product bases, i.e., product bases in which every local state belongs to a unique local basis. This equivalence implies that non-causality of process functions is exactly mirrored by quantum nonlocality without entanglement (QNLWE) - the impossibility of perfectly distinguishing separable states using local operations and causal classical communication - for such bases. Our results generalize previous special cases to arbitrary local dimensions and any number of parties, enable systematic constructions of non-causal process functions and unambiguous QNLWE bases, and reveal an unexpected connection between certain non-signaling inequalities and causal inequalities.