Navegando por Autor "Araújo, Rafael Chaves Souto"

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Ab initio experimental violation of Bell inequalities
(Physical Review Research, 2022-02-28) Poderini, Davide; Polino, Emanuele; Rodari, Giovanni; Suprano, Alessia; Araújo, Rafael Chaves Souto; Sciarrino, Fabio
The violation of a Bell inequality is the paradigmatic example of device-independent quantum information: The nonclassicality of the data is certified without the knowledge of the functioning of devices. In practice, however, all Bell experiments rely on the precise understanding of the underlying physical mechanisms. Given that, it is natural to ask: Can one witness nonclassical behavior in a truly black-box scenario? Here, we propose and implement, computationally and experimentally, a solution to this ab initio task. It exploits a robust automated optimization approach based on the stochastic Nelder-Mead algorithm. Treating preparation and measurement devices as black boxes, and relying on the observed statistics only, our adaptive protocol approaches the optimal Bell inequality violation after a limited number of iterations for a variety photonic states, measurement responses, and Bell scenarios. In particular, we exploit it for randomness certification from unknown states and measurements. Our results demonstrate the power of automated algorithms, opening a venue for the experimental implementation of device-independent quantum technologies
Algorithmic independence of initial condition and dynamical law in thermodynamics and causal inference
(IOP Publishing, 2016-09-27) Janzing, Dominik; Araújo, Rafael Chaves Souto; Schölkopf, Berhnard
Wepostulate a principle stating that the initial condition of a physical system is typically algorithmically independent of the dynamical law.Wediscuss the implications of this principle and argue that they link thermodynamics and causal inference. On the one hand, they entail behavior that is similar to the usual arrow of time. Onthe other hand, they motivate a statistical asymmetry between cause and effect that has recently been postulated in the field of causal inference, namely, that the probability distribution Pcause contains no information about the conditional distribution Peffect cause and vice versa, while Peffect may contain information about Pcause effect
Bell nonlocality using tensor networks and sparse recovery
(American Physical Society, 2020-05-20) Eliëns, Ian Sebastiaan; Brito, S. G. A.; Araújo, Rafael Chaves Souto
Bell’s theorem, stating that quantum predictions are incompatible with a local hidden variable description, is a cornerstone of quantum theory and at the center of many quantum information processing protocols. Over the years, different perspectives on nonlocality have been put forward as well as different ways to detect nonlocality and quantify it. Unfortunately, and in spite of its relevance, as the complexity of the Bell scenario increases, deciding whether a given observed correlation is nonlocal becomes computationally intractable. Here, we propose to analyze a Bell scenario as a tensor network, a perspective permitting us to test and quantify nonlocality, resorting to very efficient algorithms originating from compressed sensing and that offer a significant speedup in comparison with standard linear programming methods. We use that all nonsignaling correlations can be described by hidden variable models governed by a quasiprobability, a fact we prove with simple linear algebra methods
Bell scenarios with communication
(IOP Publishing, 2017-01-27) Brask, J. B.; Araújo, Rafael Chaves Souto
Classical and quantum physics provide fundamentally different predictions about experiments with separate observers that do not communicate, a phenomenon known as quantum nonlocality. This insight is a key element of our present understanding of quantum physics, and also enables a number of information processing protocols with security beyond what is classically attainable. Relaxing the pivotal assumption of no communication leads to new insights into the nature quantum correlations, and may enable new applications where security can be established under less strict assumptions. Here, we study such relaxations where different forms of communication are allowed. We consider communication of inputs, outputs, and of a message between the parties. Using several measures, we study how much communication is required for classical models to reproduce quantum or general no-signalling correlations, as well as how quantum models can be augmented with classical communication to reproduce no-signalling correlations
Bounding the sets of classical and quantum correlations in networks
(American Physical Society, 2019-10-03) Pozas-Kerstjens, Alejandro; Rabelo, Rafael; Rudnicki, Łukasz; Araújo, Rafael Chaves Souto; Cavalcanti, Daniel; Navascu´es, Miguel; Acín, Antonio
We present a method that allows the study of classical and quantum correlations in networks with causally independent parties, such as the scenario underlying entanglement swapping. By imposing relaxations of factorization constraints in a form compatible with semidefinite programing, it enables the use of the Navascu´es-Pironio-Acín hierarchy in complex quantum networks. We first show how the technique successfully identifies correlations not attainable in the entanglement-swapping scenario. Then we use it to show how the nonlocal power of measurements can be activated in a network: there exist measuring devices that, despite being unable to generate nonlocal correlations in the standard Bell scenario, provide a classical-quantum separation in an entanglement swapping configuration
Causal hierarchy of multipartite Bell nonlocality
(Verein zur Förderung des Open Access Publizierens in den Quantenwissenschaften, 2017-04-08) Araújo, Rafael Chaves Souto; Cavalcanti, D.; Aolita, L.
As with entanglement, different forms of Bell nonlocality arise in the multipartite scenario. These can be defined in terms of relaxations of the causal assumptions in local hidden-variable theories. However, a characterisation of all the forms of multipartite nonlocality has until now been out of reach, mainly due to the complexity of generic multipartite causal models. Here, we employ the formalism of Bayesian networks to reveal connections among different causal structures that make a both practical and physically meaningful classification possible. Our framework holds for arbitrarily many parties. We apply it to study the tripartite scenario in detail, where we fully characterize all the nonlocality classes. Remarkably, we identify new highly nonlocal causal structures that cannot reproduce all quantum correlations. This shows, to our knowledge, the strongest form of quantum multipartite nonlocality known to date. Finally, as a by-product result, we derive a non-trivial Belltype inequality with no quantum violation. Our findings constitute a significant step forward in the understanding of multipartite Bell nonlocality and open several venues for future research
Causal modeling the delayed-choice experiment
(American Physical Society, 2018-05-07) Araújo, Rafael Chaves Souto; Lemos, Gabriela Barreto; Pienaar, Jacques Lavin
Wave-particle duality has become one of the flagships of quantum mechanics. This counterintuitive concept is highlighted in a delayed-choice experiment, where the experimental setup that reveals either the particle or wave nature of a quantum system is decided after the system has entered the apparatus. Here we consider delayed-choice experiments from the perspective of device-independent causal models and show their equivalence to a prepare-and-measure scenario. Within this framework, we consider Wheeler’s original proposal and its variant using a quantum control and show that a simple classical causal model is capable of reproducing the quantum mechanical predictions. Nonetheless, among other results, we show that, in a slight variant of Wheeler’s gedanken experiment, a photon in an interferometer can indeed generate statistics incompatible with any nonretrocausal hidden variable model, whose dimensionality is the same as that of the quantum system it is supposed to mimic. Our proposal tolerates arbitrary losses and inefficiencies, making it specially suited to loophole-free experimental implementations
Causal Networks and Freedom of Choice in Bell’s Theorem
(PRX Quantum, 2021-11-03) Araújo, Rafael Chaves Souto; Moreno Filho, Marcos George Magalhães; Polino, Emanuele; Poderini, Davide; Agresti, Iris; Suprano, Alessia; Barros, Mariana Rodrigues; Carvacho, Gonzalo; Wolfe, Elie; Canabarro, Askery; Spekkens, Robert W.; Sciarrino, Fabio
Bell’s theorem is typically understood as the proof that quantum theory is incompatible with local-hidden-variable models. More generally, we can see the violation of a Bell inequality as witnessing the impossibility of explaining quantum correlations with classical causal models. The violation of a Bell inequality, however, does not exclude classical models where some level of measurement dependence is allowed, that is, the choice made by observers can be correlated with the source generating the systems to be measured. Here, we show that the level of measurement dependence can be quantitatively upper bounded if we arrange the Bell test within a network. Furthermore, we also prove that these results can be adapted in order to derive nonlinear Bell inequalities for a large class of causal networks and to identify quantumly realizable correlations that violate them.
Causal structures from entropic information: geometry and novel scenarios
(New Journal of Physics, 2014-04-03) Araújo, Rafael Chaves Souto; Luft, Lukas; Gross, David
Bell's theorem in physics, as well as causal discovery in machine learning, both face the problem of deciding whether observed data is compatible with a presumed causal relationship between the variables (for example, a local hidden variable model). Traditionally, Bell inequalities have been used to describe the restrictions imposed by causal structures on marginal distributions. However, some structures give rise to non-convex constraints on the accessible data, and it has recently been noted that linear inequalities on the observable entropies capture these situations more naturally. In this paper, we show the versatility of the entropic approach by greatly expanding the set of scenarios for which entropic constraints are known. For the first time, we treat Bell scenarios involving multiple parties and multiple observables per party. Going beyond the usual Bell setup, we exhibit inequalities for scenarios with extra conditional independence assumptions, as well as a limited amount of shared randomness between the parties. Many of our results are based on a geometric observation: Bell polytopes for two-outcome measurements can be naturally imbedded into the convex cone of attainable marginal entropies. Thus, any entropic inequality can be translated into one valid for probabilities. In some situations the converse also holds, which provides us with a rich source of candidate entropic inequalities
Certificação entrópica de não-classicalidade em uma rede quântica com fontes independentes
(Universidade Federal do Rio Grande do Norte, 2025-06-03) Alves, Vinicius Fernandes; Araújo, Rafael Chaves Souto; http://lattes.cnpq.br/1509277905143351; http://lattes.cnpq.br/0363881777719298; Melnikov, Dmitry; Maziero, Jonas
Neste trabalho, uma abordagem baseada na teoria clássica da informação e utilizada para certificar a não-classicalidade de correlações em uma rede quântica com duas fontes independentes que distribuem partículas em estados emaranhados para três laboratórios distantes, onde experimentadores têm acesso a certas escolhas de medição em suas respectivas partes do sistema. A certificação de não-classicalidade é feita otimizando o valor quântico de expressões entrópicas de bilocalidade sobre estados e medições e verificando os casos nos quais esse valor supera o limite clássico, expresso por meio das desigualdades bilocais entrópicas. A partir disso, são encontrados os valores de parâmetros dos estados e operadores de medição que maximizam as violações quâanticas dessas desigualdades, nos casos em que elas ocorrem. Tais violações implicam que as correlações em questão não são explicáveis por modelos clássicos compatíveis com essa estrutura em rede, além de demonstrarem a viabilidade da abordagem entrópica como uma alternativa para a certificação de não-classicalidade que se adapta particularmente bem a redes com fontes independentes.
Computational tools for solving a marginal problem with applications in Bell non-locality and causal modeling
(IOP Publishing, 2018-11-07) Gläßle, T.; Gross, D.; Araújo, Rafael Chaves Souto
Marginal problems naturally arise in a variety of different fields: basically, the question is whether some marginal/partial information is compatible with a joint probability distribution. To this aim, the characterization of marginal sets via quantifier elimination and polyhedral projection algorithms is of primal importance. In this work, before considering specific problems, we review polyhedral projection algorithms with focus on applications in information theory, and, alongside known algorithms, we also present a newly developed geometric algorithm which walks along the face lattice of the polyhedron in the projection space. One important application of this is in the field of quantum non-locality, where marginal problems arise in the computation of Bell inequalities. We apply the discussed algorithms to discover many tight entropic Bell inequalities of the tripartite Bell scenario as well as more complex networks arising in the field of causal nference. Finally, we analyze the usefulness of these inequalities as nonlocality witnesses by searching for violating quantum states
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 Souto
Bell’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 performed
Construção de líquidos de spin quirais por junções de cadeias de spin
(Universidade Federal do Rio Grande do Norte, 2021-03-26) Ferraz, Gabriel Campelo de Melo; Pereira, Rodrigo Gonçalves; ; http://lattes.cnpq.br/9293110312400359; ; http://lattes.cnpq.br/4860165617027764; Caldeira, Amir Ordacgi; ; http://lattes.cnpq.br/0685340886402006; Sales, Mariana Malard; ; http://lattes.cnpq.br/0925491332464897; Araújo, Rafael Chaves Souto; ; http://lattes.cnpq.br/1509277905143351; Macri, Tommaso; ; http://lattes.cnpq.br/4580166672949340
Nesse trabalho de tese, buscou-se apresentar um novo método teórico de construção de um líquido quântico de spin quiral (CSL) formado a partir de cadeias de spin de Heisenberg. As cadeias de spin, reunidas inicialmente em trios, eram acopladas na forma de junções Y. Em cada junção Y foi adicionada uma interação quiral entre os três spins das bordas de cada cadeia, além da interação de troca entre os spins vizinhos no bulk das cadeias. Através do estudo anterior de uma junção Y isolada investigou-se aqui sua extensão, estabelecendo o conjunto dessas junções na forma de uma rede hexagonal de cadeias de spin. A análise teórica foi baseada em baixas energias, através da técnica de bosonização e aplicando-se condições de contorno conforme invariantes aos modos bosônicos ou correntes de spin das cadeias. Concluiu-se que o CSL ´e do tipo de Kalmeyer-Laughlin, com suas excitações elementares correspondendo a spinons, excitações magnéticas fracionárias de estatística semiônica. O líquido de spin quiral encontrado também é acompanhado de uma condutância de spin de Hall quantizada e modos de borda nas fronteiras da rede. Estudou-se o espectro (com gap) dos spinons e a estabilidade do líquido quiral através da perturbação de backscattering entre os campos bosônicos quirais de hexágonos vizinhos, e de um modelo de tight-binding efetivo para spinons. A teoria foi capaz de prever uma transição de fase quando o gap de spinons se fecha, resultando na condensação dos spinons ou formação de um CSL estável. Como perspectiva, o método desenvolvido neste trabalho pode ser estendido para outras fases topológicas da matéria e eventualmente ser útil na aplicação de computação quântica topológica.
Criteria for nonclassicality in the prepare-and-measure scenario
(Physical Review Research, 2020-10-20) Poderini, Davide; Brito, Samuraí Gomes de Aguiar; Nery, Ranieri Vieira; Sciarrino, Fabio; Araújo, Rafael Chaves Souto
Quantum communication networks involving the preparation, sharing, and measurement of quantum states are ubiquitous in quantum information. Of particular relevance within this context is to understand under which conditions a given quantum resource can give rise to correlations incompatible with a classical explanation. Here we consider the so-called prepare-and-measure scenario, in which a quantum or classical message with bounded dimension is transmitted between two parties. In this scenario we derive criteria witnessing whether a set of quantum states can lead or not to nonclassical correlations. Based on that, we show that quantum resources that can only give rise to classical correlations in the simplest prepare-and-measure scenario can have their nonclassicality witnessed if we increase the number of preparations or measurements.
Data-driven study of the COVID-19 pandemic via age-structured modelling and prediction of the health system failure in Brazil amid diverse intervention strategies
(Public Library of Science, 2020-07-30) Silva, Askery Alexandre Canabarro Barbosa da; Tenório, Elayne; Martins, Renato; Martins, Laís; Brito, Samuraí Gomes de Aguiar; Araújo, Rafael Chaves Souto
In this work we propose a data-driven age-structured census-based SIRD-like epidemiological model capable of forecasting the spread of COVID-19 in Brazil. We model the current scenario of closed schools and universities, social distancing of people above sixty years old and voluntary home quarantine to show that it is still not enough to protect the health system by explicitly computing the demand for hospital intensive care units. We also show that an urgent intense quarantine might be the only solution to avoid the collapse of the health system and, consequently, to minimize the quantity of deaths. On the other hand, we demonstrate that the relaxation of the already imposed control measures in the next days would be catastrophic
Detecting nonlocality of noisy multipartite states with the Clauser-Horne-Shimony-Holt inequality
(Physical Review A, 2014-04-14) Araújo, Rafael Chaves Souto; Acín, Antonio; Aolita, Leandro; Cavalcanti, Daniel; https://orcid.org/0000-0001-8493-4019
The Clauser-Horne-Shimony-Holt inequality was originally proposed as a Bell inequality to detect nonlocality in bipartite systems. However, it can also be used to certify the nonlocality of multipartite quantum states. We apply this to study the nonlocality of multipartite Greenberger-Horne-Zeilinger (GHZ), W, and graph states under local decoherence processes. We derive lower bounds on the critical local-noise strength tolerated by the states before becoming local. In addition, for the whole noisy dynamics, we derive lower bounds on the corresponding nonlocal content for the three classes of states. All the bounds presented can be calculated efficiently and, in some cases, provide significantly tighter estimates than with any other known method. For example, they reveal that 𝑁-qubit GHZ states undergoing local dephasing are, for all 𝑁, nonlocal throughout all the dephasing dynamics
Detecting quantum phase transitions in a frustrated spin chain via transfer learning of a quantum classifier algorithm
(Physical Review A, 2024-05-20) Ferreira-Martins, André Juan; Silva, Leandro; Palhares Júnior, Alberto Bezerra de; Pereira, Rodrigo; Soares-Pinto, Diogo O.; Araújo, Rafael Chaves Souto; Canabarro, Askery
The classification of phases and the detection of phase transitions are central and challenging tasks in diverse fields. Within physics, these rely on the identification of order parameters and the analysis of singularities in the free energy and its derivatives. Here, we propose an alternative framework to identify quantum phase transitions. Using the axial next-nearest-neighbor Ising (ANNNI) model as a benchmark, we show how machine learning can detect three phases (ferromagnetic, paramagnetic, and a cluster of the antiphase with the floating phase). Employing supervised learning, we demonstrate the feasibility of transfer learning. Specifically, a machine trained only with nearest-neighbor interactions can learn to identify a new type of phase occurring when next-nearest-neighbor interactions are introduced. We also compare the performance of common classical machine learning methods with a version of the quantum nearest neighbors (QNN) algorithm
Device-independent secret sharing and a stronger form of Bell nonlocality
(American Physical Society, 2020-05-27) Moreno, M. G. M.; Brito, Samuraí Gomes de Aguiar; Nery, Ranieri Vieira; Araújo, Rafael Chaves Souto
Bell nonlocality, the fact that local hidden-variable models cannot reproduce the correlations obtained by measurements on entangled states, is a cornerstone in our modern understanding of quantum theory. Apart from its fundamental implications, nonlocality is also at the core of device-independent quantum information processing, the successful implementation of which is achieved without precise knowledge of the physical apparatus. Here we show that a stronger form of Bell nonlocality, for which even some nonlocal hiddenvariable models cannot reproduce the quantum predictions, allows us to circumvent possible attacks in the implementation of secret sharing, a paradigmatic communication protocol in which a secret split amid many possibly untrusted parts can be decoded only if they collaborate among themselves
Device-independent test of a delayed choice experiment
(American Physical Society, 2019-08-12) Polino, Emanuele; Agresti, Iris; Poderini, Davide; Carvacho, Gonzalo; Milani, Giorgio; Lemos, Gabriela Barreto; Araújo, Rafael Chaves Souto; Sciarrino, Fabio
The wave or particle duality has long been considered a fundamental signature of the nonclassical behavior of quantum phenomena, especially in a delayed choice experiment, where the experimental setup revealing either the particle or the wave nature of the system is decided after the system has entered the apparatus. However, as counterintuitive as it might seem, usual delayed choice experiments do have a simple causal explanation. Here, we take a different route and under a natural assumption about the dimensionality of the system under examination, we present an experimental proof of the nonclassicality of a delayed choice experiment based on the violation of a dimension witness inequality. Our conclusion is reached in a device-independent and detection loophole-free manner, that is, based solely on the observed data and without the need of special assumptions about the measurement apparatus
Device-independent tests of entropy
(American Physical Society, 2015-09-08) Araújo, Rafael Chaves Souto; Brask, Jonatan Bohr; Brunner, Nicolas
We show that the entropy of a message can be tested in a device-independent way. Specifically, we consider a prepare-and-measure scenario with classical or quantum communication, and develop two different methods for placing lower bounds on the communication entropy, given observable data. The first method is based on the framework of causal inference networks. The second technique, based on convex optimization, shows that quantum communication provides an advantage over classical communication, in the sense of requiring a lower entropy to reproduce given data. These ideas may serve as a basis for novel applications in device-independent quantum information processing

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Navegando por Autor "Araújo, Rafael Chaves Souto"