Marcelo Terra Cunha

All noncontextuality inequalities for the n-cycle scenario

The problem of separating classical from quantum correlations is in general intractable and has been solved explicitly only in few cases. In particular, known methods cannot provide general solutions for an arbitrary number of settings. We provide the complete characterization of the classical correlations and the corresponding maximal quantum violations for the case of n >= 4 observables X_0, ...,X_{n-1}, where each consecutive pair {X_i,X_{i+1}}, sum modulo n, is jointly measurable. This generalizes both the Clauser-Horne-Shimony-Holt and the Klyachko-Can-Binicioglu-Shumovsky scenarios, which are the simplest ones for, respectively, locality and noncontextuality. In addition, we provide explicit quantum states and settings with maximal quantum violation and minimal quantum dimension.

The geometry of entanglement sudden death

In open quantum systems, entanglement can vanish faster than coherence. This phenomenon is usually called sudden death of entanglement. In this paper sudden death of entanglement is discussed from a geometrical point of view, in the context of two qubits. A classification of possible scenarios is presented, with important known examples classified. Suggestions of theoretical and experimental examples are given and large dimensional and multipartite versions of the effect are briefly discussed.

Entanglement in single-particle systems

We address some of the most commonly raised questions about entanglement, especially with regard to the so-called occupation number entanglement. To answer unambiguously whether entanglement can exist in a one-atom delocalized state, we propose an experiment capable of showing violations of Bells inequality using only this state and local operations. We review previous discussions for one-photon non-locality and propose a specific experiment for creating one-atom entangled states, showing that the superselection rule of atom number can be overcome. As a by-product, this experiment suggests a means of creating an entangled state of two different chemical species. By comparison with a massless system, we argue that there should be no fundamental objection to such a superposition and its creation may be within reach of present technology.

Exclusivity principle forbids sets of correlations larger than the quantum set.

We show that the exclusivity (E) principle singles out the set of quantum correlations associated with any exclusivity graph assuming the set of quantum correlations for the complementary graph. Moreover, we prove that, for self-complementary graphs, the E principle, by itself (i.e., without further assumptions), excludes any set of correlations strictly larger than the quantum set. Finally, we prove that, for vertex-transitive graphs, the E principle singles out the maximum value for the quantum correlations assuming only the quantum maximum for the complementary graph. This opens the door for testing the impossibility of higher-than-quantum correlations in experiments.

Proposal of a Two-Qutrit Contextuality Test Free of the Finite Precision and Compatibility Loopholes

It has been argued that any test of quantum contextuality is nullified by the fact that perfect orthogonality and perfect compatibility cannot be achieved in finite precision experiments. We introduce experimentally testable two-qutrit violations of inequalities for noncontextual theories in which compatibility is guaranteed by the fact that measurements are performed on separated qutrits. The inequalities are inspired by the basic building block of the Kochen-Specker proof of quantum contextuality for a qutrit, despite the fact that their proof is completely independent of it.

Maximal violations and efficiency requirements for Bell tests with photodetection and homodyne measurements

We study nonlocality tests in which each party performs photodetection and homodyne measurements. The results of such measurements are dichotomized and a Clauser?Horne?Shimony?Holt inequality is used. We prove that in this scenario the maximal violation is attainable and fully characterize the set of maximally violating states. If we restrict our search to states composed of at most 2, 4 and 6 photons per mode, we find critical photodetection efficiencies of 0.48, 0.36 and 0.29. We also found an entangled variation of the famous cat states that has critical efficiency 0.32. These values are well within the limit of current photodetector technology, which suggests the present approach as a road for a loophole-free Bell experiment.

Quantum contextuality in a Young-type interference experiment

Gilberto Borges, Marcos Carvalho, Pierre-Louis de Assis, José Ferraz, Mateus Araújo, Adán Cabello, 1 Marcelo Terra Cunha, and Sebastião Pádua ∗ Departamento de F́ısica, Universidade Federal de Minas Gerais, Caixa Postal 702, Belo Horizonte, MG 30123-920, Brazil CEA-CNRS-UJF group ‘Nanophysique et Semiconducteurs’, Institut Néel, CNRS Université Joseph Fourier, 38042, Grenoble, France Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria Departamento de F́ısica Aplicada II, Universidad de Sevilla, E-41012 Sevilla, Spain Departamento de Matemática, Universidade Federal de Minas Gerais, Caixa Postal 702, Belo Horizonte, MG 30123-920, Brazil (Dated: May 11, 2014)

Estimating entanglement of unknown states

The experimental determination of entanglement is a major goal in the quantum information field. In general, the knowledge of the state is required in order to quantify its entanglement. Here we express a lower bound to the robustness of entanglement of a state based only on the measurement of the energy observable and on the calculation of a separability energy. This allows the estimation of entanglement dismissing the knowledge of the state in question.

Most incompatible measurements for robust steering tests

From a fundamental point of view, deciding whether a given entangled state is able to demonstrate Einstein-Podolsky-Rosen steering under arbitrary measurement conditions is an important problem to which some effort has been dedicated. However, from a practical point of view, a crucial problem is to characterize the steerability of quantum states under restrictive measurement scenarios. Here we address the problem of determining whether a quantum state can demonstrate steering when subjected to

Quantum bounds on multiplayer linear games and device-independent witness of genuine tripartite entanglement

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