Quantum Theory Cannot Violate a Causal Inequality

Tom Purves, A J Short

Research output: Contribution to journalArticle (Academic Journal)peer-review

13 Citations (Scopus)
136 Downloads (Pure)


Within quantum theory, we can create superpositions of different causal orders of events, and observe interference between them. This raises the question of whether quantum theory can produce results that would be impossible to replicate with any classical causal model, thereby violating a causal inequality. This would be a temporal analog of Bell inequality violation, which proves that no local hidden variable model can replicate quantum results. However, unlike the case of nonlocality, we show that quantum experiments can be simulated by a classical causal model, and therefore cannot violate a causal inequality.
Original languageEnglish
Article number110402
JournalPhysical Review Letters
Issue number11
Publication statusPublished - 10 Sept 2021

Bibliographical note

Funding Information:
T. P. acknowledges support from the EPSRC.

Publisher Copyright:
© 2021 American Physical Society.

Structured keywords

  • Bristol Quantum Information Institute


  • Spacetime topology & causal structure
  • Quantum information processing
  • Quantum foundations
  • Quantum control


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