Defesa de Tese de Doutorado #391 – Marcello Nery Garcia Vidal de Barros – 21/01/2022

Guided by the intuition of coherent superposition of causal relations, recent works presented quantum processes without classical common-cause and direct-cause explanation, that is, processes which cannot be written as probabilistic mixtures of quantum common-cause and quantum direct-cause relations (CCDC). In this work, we analyze the minimum requirements for a quantum process to fail to admit a CCDC explanation and present “simple” processes, which we prove to be the most robust ones against general noise. These simple processes can be performed by preparing a maximally entangled state and applying the identity quantum channel, thus not requiring an explicit coherent mixture of common-cause and direct-cause, exploiting the possibility of a process to have both relations simultaneously.

We then prove that, although all bipartite direct-cause processes are bipartite separable operators, there exist bipartite separable processes which are not direct-cause. This shows that the problem of deciding whether a process is direct-cause is not equivalent to entanglement certification, and points out the limitations of entanglement methods to detect non-classical CCDC processes. We also present a semidefinite programming hierarchy that can detect and quantify the non-classical CCDC robustnesses of every non-classical CCDC process.

Among other results, our numerical methods allow us to show that the simple processes presented here are likely to be also the maximally robust against white noise. Finally, we explore the equivalence between bipartite direct-cause processes and bipartite processes without quantum memory, to present a separable process which cannot be performed as a process without quantum memory.

Tópico: Defesa de Tese Marcello Nery Garcia Vidal de Barros
Hora: 21 jan. 2022 09:00 da manhã São Paulo

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