Comment on "Revisiting dynamics of quantum causal structures --when can causal order evolve?''

• URL: http://arxiv.org/abs/2009.10027v1
• Date: Mon, 21 Sep 2020 17:04:53 GMT
• Title: Comment on "Revisiting dynamics of quantum causal structures --when can causal order evolve?''
• Authors: Esteban Castro-Ruiz, Flaminia Giacomini and \v{C}aslav Brukner
• Abstract summary: We show that the formalism of higher order transformations is not general enough to capture its examples. A crucial example of Ref. [2] has already been explicitly considered and shown to be compatible with its results.
• Score: 0.0
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: In the last few years, there has been increasing interest in quantum processes with indefinite causal order. Process matrices are a convenient framework to study such processes. Ref. [1] defines higher order transformations from process matrices to process matrices and shows that no continuous and reversible transformation can change the causal order of a process matrix. Ref. [2] argues, based on a set of examples, that there are situations where a process can change its causal order over time. Ref. [2] claims that the formalism of higher order transformations is not general enough to capture its examples. Here we show that this claim is incorrect. Moreover, a crucial example of Ref. [2] has already been explicitly considered in Ref. [1] and shown to be compatible with its results.

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