Quantum

Indefinite causal order with output-signalling instruments

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Based on a feed excerpt; full article content has not been acquired. Source evidence retrieved 29/09/2026, 05:13:24 UTC.

Briefings

A 2026 arXiv preprint investigates indefinite causal order by restricting communication to classical outcomes of local operations. The study establishes a causal polytope with new inequalities violated by entanglement-breaking instruments, revealing noncausal correlations that satisfy all input-signalling inequalities. This demonstrates that limiting information flow can expose forms of indefinite causal order previously inaccessible under less restrictive assumptions.

Key points

• The study uses a semidefinite-programming framework to bound achievable violations of causal inequalities.
• Violations are demonstrated using measure-and-reprepare instruments requiring no coherent transmission through laboratories.

Context and limitations

The evidence is an arXiv preprint from September 2026, and the causal polytope bounds are subject to future refinement.

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