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  • 1.
    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.

  • 2.
    Hameedi, Alley
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Marques, Breno
    Stockholm University, Faculty of Science, Department of Physics. Universidade de São Paulo, Brazil.
    Sadiq, Muhammad
    Stockholm University, Faculty of Science, Department of Physics.
    Wiesniak, Marcin
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental quantum solution to the Dining Cryptographers ProblemManuscript (preprint) (Other academic)
    Abstract [en]

    Quantum resources such as superposition and entanglement have been used to provide unconditional key distribution, secret sharing and communication complexity reduction. In this letter we present a novel quantum information protocol for dining cryptographers problem and anonymous vote casting by a group of voters. We successfully demonstrate the experimental realization of the protocol using single photon transmission. Our implementation employs a flying particle scheme where a photon passes by the voters who perform a sequence of actions (unitary transformations) on the photonic state at their local stations.

  • 3.
    Muhammad, Sadiq
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Tavakoli, Armin
    Stockholm University, Faculty of Science, Department of Physics.
    Kurant, Maciej
    Pawlowski, Marcin
    Zukowski, Marek
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Quantum Bidding in Bridge2014In: Physical Review X, ISSN 2160-3308, E-ISSN 2160-3308, Vol. 4, no 2, article id 021047Article in journal (Refereed)
    Abstract [en]

    Quantum methods allow us to reduce communication complexity of some computational tasks, with several separated partners, beyond classical constraints. Nevertheless, experimental demonstrations of this have thus far been limited to some abstract problems, far away from real-life tasks. We show here, and demonstrate experimentally, that the power of reduction of communication complexity can be harnessed to gain an advantage in a famous, immensely popular, card game-bridge. The essence of a winning strategy in bridge is efficient communication between the partners. The rules of the game allow only a specific form of communication, of very low complexity (effectively, one has strong limitations on the number of exchanged bits). Surprisingly, our quantum technique does not violate the existing rules of the game (as there is no increase in information flow). We show that our quantum bridge auction corresponds to a biased nonlocal Clauser-Horne-Shimony-Holt game, which is equivalent to a 2 -> 1 quantum random access code. Thus, our experiment is also a realization of such protocols. However, this correspondence is not complete, which enables the bridge players to have efficient strategies regardless of the quality of their detectors.

  • 4.
    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.

  • 5.
    Nawareg, Mohamed
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Gdańsk, Poland.
    Muhammad, Sadiq
    Stockholm University, Faculty of Science, Department of Physics.
    Horodecki, Pawel
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Superadditivity of two quantum information resources2017In: Science Advances, ISSN 0036-8156, E-ISSN 2375-2548, Vol. 3, no 9, article id e1602485Article in journal (Refereed)
    Abstract [en]

    Entanglement is one of the most puzzling features of quantum theory and a principal resource for quantum information processing. It is well known that in classical information theory, the addition of two classical information resources will not lead to any extra advantages. On the contrary, in quantuminformation, a spectacular phenomenon of the superadditivity of two quantum information resources emerges. It shows that quantum entanglement, which was completely absent in any of the two resources separately, emerges as a result of combining them together. We present the first experimental demonstration of this quantum phenomenon with two photonic three-partite nondistillable entangled states shared between three parties Alice, Bob, and Charlie, where the entanglement was completely absent between Bob and Charlie.

  • 6.
    Nawareg, Mohamed
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Sadiq, Muhammad
    Stockholm University, Faculty of Science, Department of Physics.
    Horodecki, Pawel
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Superadditivity of two quantum information resourcesIn: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114Article in journal (Refereed)
    Abstract [en]

    Entanglement is one of the most puzzling features of quantum theory and a principal resource forquantum information processing. It is well known that in classical information theory, the additionof two classical information resources will not lead to any extra advantages. On the contrary, inquantum information a spectacular phenomenon of the super-additivity of two quantum informationresources emerges. It clearly shows that quantum entanglement, which was completely absent inany of the two resources separately, emerges as a result of combining them together. We presentthe rst experimental demonstration of this quantum phenomenon with photonic three-partite nondistillableentangled states.

  • 7. Ramanathan, Ravishankar
    et al.
    Goyeneche, Dardo
    Muhammad, Sadiq
    Stockholm University, Faculty of Science, Department of Physics.
    Mironowicz, Piotr
    Grünfeld, Marcus
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Horodecki, Paweł
    Steering is an essential feature of non-locality in quantum theory2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 4244Article in journal (Refereed)
    Abstract [en]

    A physical theory is called non-local when observers can produce instantaneous effects over distant systems. Non-local theories rely on two fundamental effects: local uncertainty relations and steering of physical states at a distance. In quantum mechanics, the former one dominates the other in a well-known class of non-local games known as XOR games. In particular, optimal quantum strategies for XOR games are completely determined by the uncertainty principle alone. This breakthrough result has yielded the fundamental open question whether optimal quantum strategies are always restricted by local uncertainty principles, with entanglement-based steering playing no role. In this work, we provide a negative answer to the question, showing that both steering and uncertainty relations play a fundamental role in determining optimal quantum strategies for non-local games. Our theoretical findings are supported by an experimental implementation with entangled photons.

  • 8.
    Sadiq, Muhammad
    Stockholm University, Faculty of Science, Department of Physics.
    Experiments with Entangled Photons: Bell Inequalities, Non-local Games and Bound Entanglement2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Quantum mechanics is undoubtedly a weird field of science, which violates many deep conceptual tenets of classical physics, requiring reconsideration of the concepts on which classical physics is based. For instance, it permits persistent correlations between classically separated systems, that are termed as entanglement. To circumvent these problems and explain entanglement, hidden variables theories--based on undiscovered parameters--have been devised. However, John S. Bell and others invented inequalities that can distinguish between the predictions of local hidden variable (LHV) theories and quantum mechanics. The CHSH-inequality (formulated by J. Clauser, M. Horne, A. Shimony and R. A. Holt), is one of the most famous among these inequalities. In the present work, we found that this inequality actually contains an even simpler logical structure, which can itself be described by an inequality and will be violated by quantum mechanics. We found 3 simpler inequalities and were able to violate them experimentally.

    Furthermore, the CHSH inequality can be used to devise games that can outperform classical strategies. We explore CHSH-games for biased and unbiased cases and present their experimental realizations. We also found a remarkable application of CHSH-games in real life, namely in the card game of duplicate Bridge.  In this thesis, we have described this application along with its experimental realization. Moreover, non-local games with quantum inputs can be used to certify entanglement in a measurement device independent manner. We implemented this method and detected entanglement in a set of two-photon Werner states. Our results are in good agreement with theory.

    A peculiar form of entanglement that is not distillable through local operations and classical communication (LOCC) is known as bound entanglement (BE). In the present work, we produced and studied BE in four-partite Smolin states and present an experimental violation of a Bell inequality by such states. Moreover we produced a three-qubit BE state, which is also the first experimental realization of a tripartite BE state. We also present its activation, where we experimentally demonstrate super additivity of quantum information resources.

  • 9.
    Sadiq, Muhammad
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Badziag, Piotr
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Cabello, Adán
    Stockholm University, Faculty of Science, Department of Physics. Universidad de Sevilla, Spain.
    Bell inequalities for the simplest exclusivity graph2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 87, no 1, article id 012128Article in journal (Refereed)
    Abstract [en]

    Which is the simplest logical structure for which there is quantum nonlocality? We show that there are only three bipartite Bell inequalities with quantum violation associated with the simplest graph of relationships of exclusivitywith a quantum-classical gap. These are the most elementary logical Bell inequalities. We showthat the quantum violation of some well-known Bell inequalities is related to them. We test the three Bell inequalities with pairs of polarization-entangled photons and report violations in good agreement with the quantum predictions. Unlike other experiments testing noncontextuality inequalities with pentagonal exclusivity, the ones reported here are free of the compatibility loophole. DOI: 10.1103/PhysRevA.87.012128

  • 10.
    Sadiq, Muhammad
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Nawareg, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Horodecki, Pawel
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental Three-Qubit Bound EntanglementIn: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114Article in journal (Refereed)
    Abstract [en]

    Entanglement is one of the most puzzling features of quantum theory and of great importancefor the new eld of quantum information. A peculiar form of entanglement, which emerges inmixed quantum states, is recognized as Bound entanglement since this kind of entanglement is nondistillableby local operations and classical communication. Here we report on the rst experimentaldemonstration of a three qubits bound entangled state and fully characterize its entanglement properties.Remarkably, this is the rst experimental realization of a bound entangled state that can beused for generation of multipartite bound information.

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