## Séminaire de Physique Théorique

**Quantum processes with no definite causal order**

Cyril Branciard (Institut Néel, Grenoble)

Thursday 30 June 2016 14:00 - Tours - Salle 1180 (Bât E2)

**Résumé :**

In standard models of quantum computation, e.g. the quantum circuit model, operations are typically assumed to be performed one after another, in a definite causal order. However, quantum theory actually allows us to go beyond the framework of causally ordered circuits; it allows one to perform operations in some kind of “superposition of causal orders”—a situation of the kind “|A causes B> + |B causes A>”. In this talk, I will show how one can analyse this kind of situation, where quantum processes have no definite causal order, using the new formalism of "process matrices" introduced by Oreshkov, Costa and Brukner [Nat. Commun. 3, 1092 (2012)] and the tool of "causal witnesses" [Araújo et al., New J. Phys. 17, 102001 (2015)]. We will see, based on a concrete example, that so-called “causally nonseparable processes” represent a new kind of resource for quantum information that allows for new applications, beyond what is possible using standard (causally ordered) quantum circuits.

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