Hippolyte Dourdent - Unambiguous non-locality without entanglement = Deterministic classical non-causality

Unambiguous non-locality without entanglement = Deterministic classical non-causality

This seminar, given by Hippolyte Dourdent, will happend on 17 December 2025, at 13:0. It will take place in Room 25-26 105.

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Abstract

Process functions generalize deterministic classical communication by removing the assumption of a fixed causal structure between free local operations without generating paradoxes. Kunjwal and Baumeler [PRL 131, 120201 (2023)] showed that any Boolean local operations with a process function lacking a global past can be encoded in a multi-qubit product basis that cannot be projected onto using local operations with classical communication—an instance of quantum nonlocality without entanglement (QNLWE). We extend this result to arbitrary dimensions and any number of parties by linking the unique fixed-point condition defining process functions to a simple unambiguity condition requiring locally disjoint operations. After refining previous characterisations of process functions and characterising the notion of (non-)causal process functions, we show that: (i) every unambiguous product basis yields a process function; (ii) every process function admits an encoding in an unambiguous product basis; and (iii) non-causal process functions corresponds to unambiguous QNLWE bases. This establishes a systematic framework for constructing and analyzing these objects. Notably, it implies that certain causal inequalities maximally violated by a process function correspond directly to non-signaling inequalities. Furthermore, it provides a new interpretative perspective on paradox-free deterministic classical communication, formulated in terms of event labeling.