Navegando por Autor "Duarte, Cristhiano"
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Artigo Concentration phenomena in the geometry of Bell correlations(American Physical Society, 2018-12-19) Duarte, Cristhiano; Brito, Samuraí Gomes de Aguiar; Amaral, Barbara; Araújo, Rafael Chaves SoutoBell’s theorem shows that local measurements on entangled states give rise to correlations incompatible with local-hidden-variable models. The degree of quantum nonlocality is not maximal though, as there are even more nonlocal theories beyond quantum theory still compatible with the nonsignaling principle. In spite of decades of research, we still have a very fragmented picture of the whole geometry of these different sets of correlations. Here we employ both analytical and numerical tools to ameliorate that. First, we identify two different classes of Bell scenarios where the nonsignaling correlations can behave very differently: In one case, the correlations are generically quantum and nonlocal while in the other quite the opposite happens as the correlations are generically classical and local. Second, by randomly sampling over nonsignaling correlations, we compute the distribution of a nonlocality quantifier based on the trace distance to the local set. With that we conclude that the nonlocal correlations can show a concentration phenomenon: Their distribution is peaked at a distance from the local set that increases both with the number of parts or measurements being performedArtigo Device-independent witness for the nonobjectivity of quantum dynamics(Physical Review A, 2023-09-05) Araújo, Rafael Chaves Souto; Rodari, Giovanni; Moreno Filho, Marcos George Magalhães; Polino, Emanuele; Nery, Ranieri Vieira; Suprano, Alessia; Duarte, Cristhiano; Sciarrino, Fabio; Poderini, DavideQuantum Darwinism offers an explanation for the emergence of classical objective features (those we are used to at macroscopic scales) from quantum properties at the microscopic level. The interaction of a quantum system with its surroundings redundantly proliferates information to many parts of the environment, turning it accessible and objective to different observers. However, given that one cannot probe the quantum system directly, only its environment, how to determine whether an unknown quantum property can be deemed objective? Here we propose a probabilistic framework to analyze this question and show that objectivity implies a Bell-like inequality. Among several other results, we show quantum violations of this inequality, a device-independent proof of the nonobjectivity of quantum correlations. We also implement a photonic experiment where the temporal degree of freedom of photons is the quantum system of interest, while their polarization acts as the environment. Employing a fully black-box approach, we achieve the violation of a Bell-like inequality, thus certifying the nonobjectivity of the underlying quantum dynamics in a fully device-independent frameworkArtigo Efficient and operational quantifier of nondivisibility in terms of channel discrimination(Physical Review A, 2025-02-04) Nery, Ranieri Vieira; Bernardes, Nadja Kolb; Cavalcanti, Daniel; Araújo, Rafael Chaves Souto; Duarte, CristhianoThe understanding of open quantum systems is crucial for the development of quantum technologies. Of particular relevance is the characterization of divisible quantum dynamics, seen as a generalization of Markovian processes to the quantum setting. Here, we propose a way to detect divisibility and quantify how nondivisible a quantum channel is through the concept of channel discrimination. We ask how well we can distinguish generic dynamics from divisible dynamics. We show that this question can be answered efficiently through semidefinite programming, which provides us with an operational and efficient way to quantify nondivisibilityArtigo Events in quantum mechanics are maximally non-absolute.(Quantum: the open journal for quantum science, 2022-08-24) Moreno Filho, Marcos George Magalhães; Nery, Ranieri Vieira; Duarte, Cristhiano; Araujo, Rafael Chaves SoutoThe notorious quantum measurement problem brings out the difficulty to reconcile two quantum postulates: the unitary evolution of closed quantum systems and the wave-function collapse after a measurement. This problematics is particularly highlighted in the Wigner's friend thought experiment, where the mismatch between unitary evolution and measurement collapse leads to conflicting quantum descriptions for different observers. A recent no-go theorem has established that the (quantum) statistics arising from an extended Wigner's friend scenario is incompatible when one try to hold together three innocuous assumptions, namely no-superdeterminism, parameter independence and absoluteness of observed events. Building on this extended scenario, we introduce two novel measures of non-absoluteness of events. The first is based on the EPR2 decomposition, and the second involves the relaxation of the absoluteness hypothesis assumed in the aforementioned no-go theorem. To prove that quantum correlations can be maximally non-absolute according to both quantifiers, we show that chained Bell inequalities (and relaxations thereof) are also valid constraints for Wigner's experimentArtigo Geometry of the quantum set on no-signaling faces(American Physical Society, 2019-03-11) Rai, Ashutosh; Duarte, Cristhiano; Brito, Samuraí Gomes de Aguiar; Araújo, Rafael Chaves SoutoSince Bell’s theorem we know that quantum mechanics is incompatible with local hidden-variable models, the phenomenon known as quantum nonlocality. However, despite steady progress over the years, a precise characterization of the set of quantum correlations has remained elusive. There are correlations compatible with the no-signaling principle and still beyond what can be achieved within quantum theory, which has motivated the search for physical principles and computational methods to decide the quantum or postquantum behavior of correlations. Here, we identify a feature of Bell correlations that we call quantum voids: faces of the no-signaling set where all nonlocal correlations are postquantum. Considering the simplest possible Bell scenario, we give a full characterization of quantum voids, also understanding its connections to known principles and its potential use as a dimension witnessArtigo Semidefinite tests for quantum network topologies(Physical Review Letters, 2020-09-10) Åberg, Johan; Nery, Ranieri Vieira; Duarte, Cristhiano; Araújo, Rafael Chaves SoutoQuantum networks play a major role in long-distance communication, quantum cryptography, clock synchronization, and distributed quantum computing. Generally, these protocols involve many independent sources sharing entanglement among distant parties that, upon measuring their systems, generate correlations across the network. The question of which correlations a given quantum network can give rise to remains almost uncharted. Here we show that constraints on the observable covariances, previously derived for the classical case, also hold for quantum networks. The network topology yields tests that can be cast as semidefinite programs, thus allowing for the efficient characterization of the correlations in a wide class of quantum networks, as well as systematic derivations of device-independent and experimentally testable witnesses. We obtain such semidefinite tests for fixed measurement settings, as well as parties that independently choose among collections of measurement settings. The applicability of the method is demonstrated for various networks, and compared with previous approaches.