Browsing by Author "Cavalcanti, Paulo J."

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    Decomposing all multipartite non-signalling channels via quasiprobabilistic mixtures of local channels in generalised probabilistic theories
    Cavalcanti, Paulo J.; Selby, John H.; Sikora, Jamie; Sainz, Ana Belen (IOP Publishing, 2022-10-07)
    Non-signalling quantum channels-relevant in, e.g., the study of Bell and Einstein-Podolsky-Rosen scenarios-may be decomposed as an affine combinations of local operations in bipartite scenarios. Moreover, when these channels correspond to stochastic maps between classical variables, such a decomposition is possible even in multipartite scenarios. These two results have proven useful when studying the properties of these channels, such as their communication and information processing power, and even when defining measures of the non-classicality of physical phenomena (such as Bell non-classicality and steering). In this paper we show that such useful quasi-stochastic characterizations of channels may be unified and applied to the broader class of multipartite non-signalling channels. Moreover, we show that this holds for non-signalling channels in quantum theory, as well as in a larger family of generalised probabilistic theories. More precisely, we prove that channels are non-signalling if and only if they can be decomposed as an affine combinations of corresponding local operations, provided that the underlying physical theory is locally tomographic-a property that quantum theory satisfies. Our results then can be viewed as a generalisation of references (Phys. Rev. Lett. 111 170403) and (2013 Phys. Rev. A 88 022318) to the multipartite scenario for arbitrary tomographically local generalised probabilistic theories (including quantum theory). Our proof technique leverages Hardy's duotensor formalism, highlighting its utility in this line of research.
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    Post-quantum steering is a stronger-than-quantum resource for information processing
    Cavalcanti, Paulo J.; Selby, John H.; Sikora, Jamie; Galley, Thomas D.; Sainz, Ana Belen (Nature Portfolio, 2022-06-30)
    We present the first instance where post-quantum steering is a stronger-than-quantum resource for information processing - remote state preparation. In addition, we show that the phenomenon of post-quantum steering is not just a mere mathematical curiosity allowed by the no-signalling principle, but it may arise within compositional theories beyond quantum theory, hence making its study fundamentally relevant. We show these results by formulating a new compositional general probabilistic theory - which we call Witworld - with strong post-quantum features, which proves to be a intuitive and useful tool for exploring steering and its applications beyond the quantum realm.