Incompleteness is necessary for activation of nonlocality without entanglement

• URL: http://arxiv.org/abs/2605.29775v1
• Date: Thu, 28 May 2026 11:20:14 GMT
• Title: Incompleteness is necessary for activation of nonlocality without entanglement
• Abstract summary: A set of product states is said to exhibit "quantum nonlocality without entanglement" if it is locally indistinguishable.<n>Recent studies have highlighted the phenomenon of "genuine activation of hidden nonlocality"<n>Our results provide a rigorous structural understanding of local-to-nonlocal transitions in quantum state discrimination.
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• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: A set of orthogonal product states is said to exhibit "quantum nonlocality without entanglement" if it is locally indistinguishable, i.e. no sequence of local operations and classical communication (LOCC) can perfectly discriminate the states. Building on this foundational idea, recent studies have highlighted the phenomenon of "genuine activation of hidden nonlocality", where a set of initially distinguishable orthogonal states becomes locally indistinguishable through orthogonality-preserving LOCC transformations. In this letter, we establish that any complete orthogonal product basis that is initially locally distinguishable remains so under all orthogonality-preserving local projective measurements, thereby ruling out activation via orthogonality-preserving local projective measurements and classical communication. We further introduce and formalise the notions of "strongly local sets", namely locally distinguishable sets that remain non-activable under all bipartitions. Interestingly, the study of "local activability" of distinguishable sets is useful to characterise the boundary between LOCC distinguishability and its irreversible loss in multipartite systems. Our results provide a rigorous structural understanding of local-to-nonlocal transitions in quantum state discrimination.

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