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  • 1.
    Ahrens, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Amselem, Elias
    Stockholm University, Faculty of Science, Department of Physics.
    Cabello, Adan
    Stockholm University, Faculty of Science, Department of Physics. University of Sevilla, Spain.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Two Fundamental Experimental Tests of Nonclassicality with Qutrits2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, article id 2170Article in journal (Refereed)
    Abstract [en]

    We report two fundamental experiments on three-level quantum systems (qutrits). The first one tests the simplest task for which quantum mechanics provides an advantage with respect to classical physics. The quantum advantage is certified by the violation of Wright's inequality, the simplest classical inequality violated by quantum mechanics. In the second experiment, we obtain contextual correlations by sequentially measuring pairs of compatible observables on a qutrit, and show the violation of Klyachko et al.'s inequality, the most fundamental noncontextuality inequality violated by qutrits. Our experiment tests exactly Klyachko et al.'s inequality, uses the same measurement procedure for each observable in every context, and implements the sequential measurements in any possible order.

  • 2.
    Amselem, Elias
    Stockholm University, Faculty of Science, Department of Physics.
    Dynamics of Quantum Correlations with Photons: Experiments on bound entanglement and contextuality for application in quantum information2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The rapidly developing interdisciplinary field of quantum information, which merges quantum and information science, studies non-classical aspects of quantum systems. These studies are motivated by the promise that the non-classicality can be used to solve tasks more efficiently than classical methods would allow. In many quantum informational studies, non-classical behaviour is attributed to the notion of entanglement.

    In this thesis we use photons to experimentally investigate fundamental questions such as: What happens to the entanglement in a system when it is affected by noise? In our study of noisy entanglement we pursue the challenging task of creating bound entanglement. Bound entangled states are created through an irreversible process that requires entanglement. Once in the bound regime, entanglement cannot be distilled out through local operations assisted by classical communication. We show that it is possible to experimentally produce four-photon bound entangled states and that a violation of a Bell inequality can be achieved. Moreover, we demonstrate an entanglement-unlocking protocol by relaxing the condition of local operations.

    We also explore the non-classical nature of quantum mechanics in several single-photon experiments. In these experiments, we show the violation of various inequalities that were derived under the assumption of non-contextuality. Using qutrits we construct and demonstrate the simplest possible test that offers a discrepancy between classical and quantum theory. Furthermore, we perform an experiment in the spirit of the Kochen-Specker theorem to illustrate the state-independence of this theorem. Here, we investigate whether or not measurement outcomes exhibit fully contextual correlations. That is, no part of the correlations can be attributed to the non-contextual theory. Our results show that only a small part of the experimental generated correlations are amenable to a non-contextual interpretation.

  • 3.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental four-qubit bound entanglement2009In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 5, no 10, p. 748-752Article in journal (Refereed)
    Abstract [en]

    Entanglement is one of the most puzzling features of quantum theory and of great importance for the new field of quantum information. Being a peculiar form of entanglement, bound entanglement emerges in certain mixed quantum states. This form of entanglement is not distillable by local operators and classical communication. Bound-entangled states are different from both the free entangled (distillable) and separable states. Here we report on the first experimental demonstration of a four-qubit polarization bound-entangled state, the so-called Smolin state. We have fully characterized its entanglement properties. Moreover, we have realized unlocking of the entanglement protocol for this state. The special properties of the Smolin state constitute a useful quantum resource for new multiparty communication schemes.

  • 4.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Reply to 'Experimental bound entanglement?'2010In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 6, p. 827-827Article in journal (Other (popular science, discussion, etc.))
  • 5.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Cabello, Adán
    Departamento de Física Aplicada II, Universidad de Sevilla.
    Danielsen, Lars Eirik
    Department of Informatics, University of Bergen.
    López-Tarrida, Antonio
    Departamento de Física Aplicada II, Universidad de Sevilla.
    Portillo, José
    Departamento de Matemática Aplicada I, Universidad de Sevilla.
    Experimental fully contextual correlationsArticle in journal (Refereed)
    Abstract [en]

    Quantum correlations are contextual yet, in general, nothing prevents the existence of even morecontextual correlations. We identify and test a simple noncontextual inequality in which the quantumviolation cannot be improved by any hypothetical post-quantum resource, and use it to experimentally obtain correlations in which the maximum noncontextual content, defined as the maximumfraction of noncontextual correlations, is less than 0.06. Our correlations are experimentally generated from the outcomes of sequential compatible measurements on a four-state quantum system encoded in the polarization and path of a single photon.

  • 6.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Danielsen, Lars Eirik
    Lopez-Tarrida, Antonio J.
    Portillo, Jose R.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Cabello, Adan
    Stockholm University, Faculty of Science, Department of Physics. University of Sevilla, Spain .
    Experimental fully contextual correlations2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 20, p. 200405-Article in journal (Refereed)
    Abstract [en]

    Quantum correlations are contextual yet, in general, nothing prevents the existence of even more contextual correlations. We identify and test a noncontextuality inequality in which the quantum violation cannot be improved by any hypothetical postquantum theory, and use it to experimentally obtain correlations in which the fraction of noncontextual correlations is less than 0.06. Our correlations are experimentally generated from the results of sequential compatible tests on a four-state quantum system encoded in the polarization and path of a single photon.

  • 7.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Rådmark, Magnus
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Cabello, Adán
    Departamento de Física Aplicada II, Universidad de Sevilla.
    State-Independent Quantum Contextuality with Single Photons2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 103, no 160405Article in journal (Refereed)
    Abstract [en]

    Wepresent an experimental state-independent violation of an inequality for noncontextualtheories on single particles. We show that 20 different single-photonstates violate an inequality which involves correlations between results ofsequential compatible measurements by at least 419 standard deviations. Ourresults show that, for any physical system, even for asingle system, and independent of its state, there is auniversal set of tests whose results do not admit anoncontextual interpretation. This sheds new light on the role ofquantum mechanics in quantum information processing.

  • 8.
    Amselem, Elias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Sadiq, Muhamad
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental bound entanglement through a Pauli channel2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, article id 1966Article in journal (Refereed)
    Abstract [en]

    Understanding the characteristics of a quantum systems when affected by noise is one of the biggest challenges for quantum technologies. The general Pauli error channel is an important lossless channel for quantum communication. In this work we consider the effects of a Pauli channel on a pure four-qubit state and simulate the Pauli channel experimentally by studying the action on polarization encoded entangled photons. When the noise channel acting on the photons is correlated, a set spanned by four orthogonal bound entangled states can be generated. We study this interesting case experimentally and demonstrate that products of Bell states can be brought into a bound entangled regime. We find states in the set of bound entangled states which experimentally violate the CHSH inequality while still possessing a positive partial transpose.

  • 9.
    Cabello, Adan
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Sevilla, Spain.
    Amselem, Elias
    Stockholm University, Faculty of Science, Department of Physics.
    Blanchfield, Kate
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Bengtsson, Ingemar
    Stockholm University, Faculty of Science, Department of Physics.
    Proposed experiments of qutrit state-independent contextuality and two-qutrit contextuality-based nonlocality2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 3, p. 032108-Article in journal (Refereed)
    Abstract [en]

    Recent experiments have demonstrated ququart state-independent quantum contextuality and qutrit state-dependent quantum contextuality. So far, the most basic form of quantum contextuality pointed out by Kochen and Specker, and Bell, has eluded experimental confirmation. Here we present an experimentally feasible test to observe qutrit state-independent quantum contextuality using single photons in a three-path setup. In addition, we show that if the same measurements are performed on two entangled qutrits, rather than sequentially on the same qutrit, then the noncontextual inequality becomes a Bell inequality. We show that this connection also applies to other recently introduced noncontextual inequalities.

  • 10. D'Ambrosio, Vincenzo
    et al.
    Herbauts, Isabelle
    Stockholm University, Faculty of Science, Department of Physics.
    Amselem, Elias
    Stockholm University, Faculty of Science, Department of Physics.
    Nagali, Eleonora
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Sciarrino, Fabio
    Cabello, Adan
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental Implementation of a Kochen-Specker Set of Quantum Tests2013In: Physical Review X, ISSN 2160-3308, E-ISSN 2160-3308, Vol. 3, no 1, p. 011012-Article in journal (Refereed)
    Abstract [en]

    The conflict between classical and quantum physics can be identified through a series of yes-no tests on quantum systems, without it being necessary that these systems be in special quantum states. Kochen-Specker (KS) sets of yes-no tests have this property and provide a quantum-versus-classical advantage that is free of the initialization problem that affects some quantum computers. Here, we report the first experimental implementation of a complete KS set that consists of 18 yes-no tests on four-dimensional quantum systems and show how to use the KS set to obtain a state-independent quantum advantage. We first demonstrate the unique power of this KS set for solving a task while avoiding the problem of state initialization. Such a demonstration is done by showing that, for 28 different quantum states encoded in the orbital-angular-momentum and polarization degrees of freedom of single photons, the KS set provides an impossible-to-beat solution. In a second experiment, we generate maximally contextual quantum correlations by performing compatible sequential measurements of the polarization and path of single photons. In this case, state independence is demonstrated for 15 different initial states. Maximum contextuality and state independence follow from the fact that the sequences of measurements project any initial quantum state onto one of the KS set's eigenstates. Our results show that KS sets can be used for quantum-information processing and quantum computation and pave the way for future developments. DOI: 10.1103/PhysRevX.3.011012

  • 11.
    Nawareg, Mohamed
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Gdansk, Poland.
    Muhammad, Sadiq
    Stockholm University, Faculty of Science, Department of Physics.
    Amselem, Elias
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental Measurement-Device-Independent Entanglement Detection2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 8048Article in journal (Refereed)
    Abstract [en]

    Entanglement is one of the most puzzling features of quantum theory and of great importance for the new field of quantum information. The determination whether a given state is entangled or not is one of the most challenging open problems of the field. Here we report on the experimental demonstration of measurement-device-independent (MDI) entanglement detection using witness method for general two qubits photon polarization systems. In the MDI settings, there is no requirement to assume perfect implementations or neither to trust the measurement devices. This experimental demonstration can be generalized for the investigation of properties of quantum systems and for the realization of cryptography and communication protocols.

1 - 11 of 11
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