Connecting processes with indefinite causal order and multi-time quantum states

Ralph Silva, Yelena Guryanova, Tony Short, Paul Skrzypczyk, Nicolas Brunner, Sandu Popescu

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

22 Citations (Scopus)
357 Downloads (Pure)


Recently, the possible existence of quantum processes with indefinite causal order has been extensively discussed, in particular using the formalism of process matrices. Here we give a new perspective on this question, by establishing a direct connection to the theory of multi-time quantum states. Specifically, we show that process matrices are equivalent to a particular class of pre- and post- selected quantum states. This offers a new conceptual point of view to the nature of process matrices. Our results also provide an explicit recipe to experimentally implement any process matrix in a probabilistic way, and allow us to generalize some of the previously known properties of process matrices. Furthermore we raise the issue of the difference between the notions of indefinite temporal order and indefinite causal order, and show that one can have indefinite causal order even with definite temporal order.
Original languageEnglish
Article number103022
Number of pages12
JournalNew Journal of Physics
Issue number10
Early online date20 Oct 2017
Publication statusPublished - Oct 2017

Structured keywords

  • QITG
  • Bristol Quantum Information Institute


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