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A recent framework for studying causal structures introduced the notion of a process: a resource shared between some parties that is compatible with standard quantum mechanics in each party’s laboratory, but that does not presuppose the existence of a global causal order between them. These processes can be used to perform several tasks that are impossible in standard quantum mechanics: they allow for the violation of causal inequalities, and provide an advantage for computational and communication complexity. Nonetheless, no physical process is known that allows for the violation of a causal inequality. There is therefore considerable interest in determining which processes are physical and which are just mathematical artefacts of the framework. Here we make the first step in this direction, by proposing a purification postulate: processes are physical only if they are purifiable. We derive necessary conditions for a process to be purifiable, and show that several known processes do not satisfy them.

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Mateus Araujo, University of Vienna
Seminarroom 0.02, ETP
Contact: David Gross