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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.
    Ahrens, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Badziag, Piotr
    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.
    Experimental device independent tests of classical and quantum dimensions2012In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 8, no 8, p. 592-595Article in journal (Refereed)
    Abstract [en]

    A fundamental resource in any communication and computation task is the amount of information that can be transmitted and processed. The classical information encoded in a set of states is limited by the number of distinguishable states or classical dimension d(c) of the set. The sets used in quantum communication and information processing contain states that are neither identical nor distinguishable, and the quantum dimension d(q) of the set is the dimension of the Hilbert space spanned by these states. An important challenge is to assess the (classical or quantum) dimension of a set of states in a device-independent way, that is, without referring to the internal working of the device generating the states. Here we experimentally test dimension witnesses designed to efficiently determine the minimum dimension of sets of (three or four) photonic states from the correlations originated from measurements on them, and distinguish between classical and quantum sets of states.

  • 3.
    Ahrens, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Badziag, Piotr
    Stockholm University, Faculty of Science, Department of Physics.
    Pawlowski, Marcin
    Zukowski, Marek
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental Tests of Classical and Quantum Dimensionality2014In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, no 14, article id 140401Article in journal (Refereed)
    Abstract [en]

    We report on an experimental test of classical and quantum dimension. We have used a dimension witness that can distinguish between quantum and classical systems of dimensions two, three, and four and performed the experiment for all five cases. The witness we have chosen is a base of semi-device-independent cryptographic and randomness expansion protocols. Therefore, the part of the experiment in which qubits were used is a realization of these protocols. In our work we also present an analytic method for finding the maximum quantum value of the witness along with corresponding measurements and preparations. This method is quite general and can be applied to any linear dimension witness.

  • 4.
    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.

  • 5.
    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.))
  • 6.
    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.

  • 7.
    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.

  • 8.
    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.

  • 9.
    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.

  • 10.
    Anwer, Hammad
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Nawareg, Mohamed
    Stockholm University, Faculty of Science, Department of Physics. Damietta University, Egypt.
    Cabello, Adán
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental test of maximal tripartite nonlocality using an entangled state and local measurements that are maximally incompatible2019In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 100, no 2, article id 022104Article in journal (Refereed)
    Abstract [en]

    The only known qubit states that produce maximal quantum violation of a tight Bell inequality with maximally incompatible local measurements are Bell states, Greenberger-Horne-Zeilinger states, and a recently identified three-qubit state called vertical bar S >. Here we report the results of an experiment for preparing vertical bar S > and testing the maximum quantum violation of the corresponding tripartite Bell inequality. Using a heralded source of three entangled photons and three tunable polarization-dependent filters, we experimentally prepare vertical bar S > with 0.924 fidelity. Using maximally incompatible measurements for the three parties, we observe a value of T-26 = 7.30 +/- 0.06, which clearly violates the tight Bell inequality T-26 <= 5. In addition, we show that our setup allows preparing multipartite states that were inaccessible with previous methods and are useful for quantum information and metrology.

  • 11.
    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.

  • 12. Cabello, Adan
    et al.
    Badziag, Piotr
    Stockholm University, Faculty of Science, Department of Physics.
    Cunha, Marcelo Terra
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Simple Hardy-Like Proof of Quantum Contextuality2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 18, p. 180404-Article in journal (Refereed)
    Abstract [en]

    Contextuality and nonlocality are two fundamental properties of nature. Hardy's proof is considered the simplest proof of nonlocality and can also be seen as a particular violation of the simplest Bell inequality. A fundamental question is: Which is the simplest proof of contextuality? We show that there is a Hardy-like proof of contextuality that can also be seen as a particular violation of the simplest noncontextuality inequality. Interestingly, this new proof connects this inequality with the proof of the Kochen-Specker theorem, providing the missing link between these two fundamental results, and can be extended to an arbitrary odd number n of settings, an extension that can be seen as a particular violation of the n-cycle inequality.

  • 13. 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

  • 14.
    Davidson, Ian A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Azzouz, Hatim
    Stockholm University, Faculty of Science, Department of Physics.
    Hueck, Klaus
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    A highly versatile optical fiber vacuum feed-throughManuscript (preprint) (Other academic)
    Abstract [en]

    Coupling light into a vacuum system is a non-trivial problem, requiring the use of a specialized feed-through. This feed-through must be both leak tight and offer a low optical loss if it is to be suitable for general use. In this paper, we report on the development of an extremely simple yet versatile, low cost, demountable optical fiber vacuum feed-through based on the modification of a standard optical fiber bulkhead connector. The modified connector was found to have a leak rate of 6.6 ±2.1 x 10-6 mbar.L/s, and an optical loss of -0.41 ± 0.28 dB, making it suitable for use in high vacuum applications.

  • 15.
    Davidson, Ian A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Azzouz, Hatim
    Stockholm University, Faculty of Science, Department of Physics.
    Hueck, Klaus
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    A highly versatile optical fiber vacuum feed-throughManuscript (preprint) (Other academic)
    Abstract [en]

    Coupling light into a vacuum system is a non-trivial problem, requiring the use of a specialized feed-through. This feed-through must be both leak tight and offer a low optical loss if it is to be suitable for general use. In this paper, we report on the development of an extremely simple yet versatile, low cost, demountable optical fiber vacuum feedthrough based on the modification of a standard optical fiber bulkhead connector. The modified connector was found to have a leak rate of 6.6 ±2.1 x 10-6 mbar.L/s, and an optical loss of -0.41 ± 0.28 dB, making it suitable for use in high vacuum applications. 

  • 16.
    Davidson, Ian A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. KTH, Sweden.
    Azzouz, Hatim
    Stockholm University, Faculty of Science, Department of Physics. ASML, Netherlands.
    Hueck, Klaus
    Stockholm University, Faculty of Science, Department of Physics. Universität Hamburg, Germany.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    A highly versatile optical fibre vacuum feed-through2016In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 87, no 5, article id 053104Article in journal (Refereed)
    Abstract [en]

    Coupling light into a vacuum system is a non-trivial problem, requiring the use of a specialized feed-through. This feed-through must be both leak tight and offer a low optical loss if it is to be suitable for general use. In this paper, we report on the development of an extremely simple yet versatile, low cost, demountable optical fiber vacuum feed-through based on the modification of a standard optical fiber bulkhead connector. The modified connector was found to have a leak rate of 6.6 +/- 2.1 x 10(-6) mbar l/s and an optical loss of -0.41 +/- 0.28 dB, making it suitable for use in high vacuum applications.

  • 17.
    El Hassan, Ashraf
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kunst, Flore K.
    Stockholm University, Faculty of Science, Department of Physics.
    Moritz, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Andler, Guillermo
    Stockholm University, Faculty of Science, Department of Physics.
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Corner states of light in photonic waveguides2019In: Nature Photonics, ISSN 1749-4885, E-ISSN 1749-4893, Vol. 13, no 10, p. 697-700Article in journal (Refereed)
    Abstract [en]

    The recently established paradigm of higher-order topological states of matter has shown that not only edge and surface states(1,2) but also states localized to corners, can have robust and exotic properties(3-9). Here we report on the experimental realization of novel corner states made out of visible light in three-dimensional photonic structures inscribed in glass samples using femtosecond laser technology(10,11). By creating and analysing waveguide arrays, which form two-dimensional breathing kagome lattices in various sample geometries, we establish this as a platform for corner states exhibiting a remarkable degree of flexibility and control. In each sample geometry we measure eigenmodes that are localized at the corners in a finite frequency range, in complete analogy with a theoretical model of the breathing kagome(7-9,12-14). Here, measurements reveal that light can be 'fractionalized,' corresponding to simultaneous localization to each corner of a triangular sample, even in the presence of defects.

  • 18. Gaertner, S.
    et al.
    Kurtsiefer, C.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Weinfurter, H.
    Experimental demonstration of four-party quantum secret sharing2007In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 98, no 2, p. 20503-Article in journal (Refereed)
    Abstract [en]

    Secret sharing is a multiparty cryptographic task in which some secret information is split into several pieces which are distributed among the participants such that only an authorized set of participants can reconstruct the original secret. Similar to quantum key distribution, in quantum secret sharing, the secrecy of the shared information relies not on computational assumptions, but on laws of quantum physics. Here, we present an experimental demonstration of four-party quantum secret sharing via the resource of four-photon entanglement.

  • 19.
    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.
    Mironowicz, Piotr
    Saha, Debashis
    Pawlowski, Marcin
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental test of nonclassicality with arbitrary low detection efficiencyManuscript (preprint) (Other academic)
    Abstract [en]

    We theoretically introduce and experimentally demonstrate the realization of a nonclassicality test that allows for arbitrarily low detection efficiency without invoking any extra assumptions as independence of the devices. Our test and its implementation is set in a prepare-and-measure scenario with an upper limit on the communication capacity of the channel through which the systems are communicated. The essence for our novel test is the use of two preparation and two measurement devices, which are randomly paired in each round. Our work opens up the possibility of experimental realizations of device independent protocols with current off-the-shelf technology.

  • 20.
    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.

  • 21.
    Hameedi, Alley
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Saha, Debashis
    Mironowicz, Piotr
    Pawlowski, Marcin
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Complementarity between entanglement-assisted and quantum distributed random access code2017In: Physical Review A, ISSN 2469-9926, Vol. 95, no 5, article id 052345Article in journal (Refereed)
    Abstract [en]

    Collaborative communication tasks such as random access codes (RACs) employing quantum resources have manifested great potential in enhancing information processing capabilities beyond the classical limitations. The two quantum variants of RACs, namely, quantum random access code (QRAC) and the entanglement-assisted random access code (EARAC), have demonstrated equal prowess for a number of tasks. However, there do exist specific cases where one outperforms the other. In this article, we study a family of 3 -> 1 distributed RACs [J. Bowles, N. Brunner, and M. Pawlowski, Phys. Rev. A 92, 022351 (2015)] and present its general construction of both the QRAC and the EARAC. We demonstrate that, depending on the function of inputs that is sought, if QRAC achieves the maximal success probability then EARAC fails to do so and vice versa. Moreover, a tripartite Bell-type inequality associated with the EARAC variants reveals the genuine multipartite nonlocality exhibited by our protocol. We conclude with an experimental realization of the 3 -> 1 distributed QRAC that achieves higher success probabilities than the maximum possible with EARACs for a number of tasks.

  • 22.
    Hameedi, Alley
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Saha, Debashis
    Mironowicz, Piotr
    Pawlowski, Marcin
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Distributed random access code with quantum resourcesManuscript (preprint) (Other academic)
    Abstract [en]

    Collaborative communication tasks as random access codes (RACs) employing quantum resources have manifested great potential in enhancing information processing capabilities beyond the classical limitations. The two quantum variants of RACs, namely quantum random access code (QRAC) and the entanglement assisted random access code (EARAC), have demonstrated equal prowess for a number of tasks. However, there do exist specific cases where one outperforms the other. In this letter, we study a family of 3 to 1 distributed RACs, which are the simplest communication network of that type. We present its construction of both the QRAC and the EARAC. We demonstrate that, depending on the task, if QRAC achieves the maximal success probability then the EARAC fails to do so and vice versa. Moreover, a tripartite Bell-type inequality associated with the EARAC variants reveals the genuine multipartite nonlocality exhibited by our protocol. We conclude with an experimental realization of the 3 to 1 distributed QRAC that achieves higher success probabilities than the maximum possible with EARACs for a number of tasks.

  • 23.
    Hameedi, Alley
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Tavakoli, Armin
    Stockholm University, Faculty of Science, Department of Physics. Université de Genève, Switzerland.
    Marques, Breno
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Communication Games Reveal Preparation Contextuality2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 22, article id 220402Article in journal (Refereed)
    Abstract [en]

    A communication game consists of distributed parties attempting to jointly complete a task with restricted communication. Such games are useful tools for studying limitations of physical theories. A theory exhibits preparation contextuality whenever its predictions cannot be explained by a preparation noncontextual model. Here, we show that communication games performed in operational theories reveal the preparation contextuality of that theory. For statistics obtained in a particular family of communication games, we show a direct correspondence with correlations in spacelike separated events obeying the no-signaling principle. Using this, we prove that all mixed quantum states of any finite dimension are preparation contextual. We report on an experimental realization of a communication game involving three-level quantum systems from which we observe a strong violation of the constraints of preparation noncontextuality.

  • 24. Jogenfors, Jonathan
    et al.
    Elhassan, Ashraf Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Ahrens, Johan
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Jan-Åke
    Hacking the Bell test using classical light in energy-time entanglement-based quantum key distribution2015In: Science Advances, E-ISSN 2375-2548, Vol. 1, no 11, article id e1500793Article in journal (Refereed)
    Abstract [en]

    Photonic systems based on energy-time entanglement have been proposed to test local realism using the Bell inequality. A violation of this inequality normally also certifies security of device-independent quantum key distribution (QKD) so that an attacker cannot eavesdrop or control the system. We show how this security test can be circumvented in energy-time entangled systems when using standard avalanche photodetectors, allowing an attacker to compromise the system without leaving a trace. We reach Bell values up to 3.63 at 97.6% faked detector efficiency using tailored pulses of classical light, which exceeds even the quantum prediction. This is the first demonstration of a violation-faking source that gives both tunable violation and high faked detector efficiency. The implications are severe: the standard Clauser-Horne-Shimony-Holt inequality cannot be used to show device-independent security for energy-time entanglement setups based on Franson's configuration. However, device-independent security can be reestablished, and we conclude by listing a number of improved tests and experimental setups that would protect against all current and future attacks of this type.

  • 25. Joshi, Siddarth Koduru
    et al.
    Pienaar, Jacques
    Ralph, Timothy C.
    Cacciapuoti, Luigi
    McCutcheon, Will
    Rarity, John
    Giggenbach, Dirk
    Lim, Jin Gyu
    Makarov, Vadim
    Fuentes, Ivette
    Scheidl, Thomas
    Beckert, Erik
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Bruschi, David Edward
    Cabello, Adam
    Capmany, Jose
    Carrasco-Casado, Alberto
    Diamanti, Eleni
    Dusek, Miloslav
    Elser, Dominique
    Gulinatti, Angelo
    Hadfield, Robert H.
    Jennewein, Thomas
    Kaltenbaek, Rainer
    Krainak, Michael A.
    Lo, Hoi-Kwong
    Marquardt, Christoph
    Milburn, Gerard
    Peev, Momtchil
    Poppe, Andreas
    Pruneri, Valerio
    Renner, Renato
    Salomon, Christophe
    Skaar, Johannes
    Solomos, Nikolaos
    Stipcevic, Mario
    Torres, Juan P.
    Toyoshima, Morio
    Villoresi, Paolo
    Walrnsley, Ian
    Weihs, Gregor
    Weinfurter, Harald
    Zeilinger, Anton
    Zukowski, Marek
    Ursin, Rupert
    Space QUEST mission proposal: experimentally testing decoherence due to gravity2018In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 20, article id 063016Article in journal (Refereed)
    Abstract [en]

    Models of quantum systems on curved space-times lack sufficient experimental verification. Some speculative theories suggest that quantum correlations, such as entanglement, may exhibit different behavior to purely classical correlations in curved space. By measuring this effect or lack thereof, we can test the hypotheses behind several such models. For instance, as predicted by Ralph et al [5] and Ralph and Pienaar [1], a bipartite entangled system could decohere if each particle traversed through a different gravitational field gradient. We propose to study this effect in a ground to space uplink scenario. We extend the above theoretical predictions of Ralph and coworkers and discuss the scientific consequences of detecting/failing to detect the predicted gravitational decoherence. We present a detailed mission design of the European Space Agency's Space QUEST (Space-Quantum Entanglement Space Test) mission, and study the feasibility of the mission scheme.

  • 26.
    Kothe, Christian
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bjork, Gunnar
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Arbitrarily high super-resolving phase measurements at telecommunication wavelengths2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, no 6, p. 63836-Article in journal (Refereed)
    Abstract [en]

    We present two experiments that achieve phase super-resolution at telecommunication wavelengths. One of the experiments is realized in the space domain and the other is realized in the time domain. Both experiments show high visibility and are performed with standard lasers and single-photon detectors. The first experiment uses six-photon coincidences, whereas the latter experiment needs no coincidence measurements, is easy to perform, and achieves, in principle, arbitrarily high phase super-resolution. Here, we demonstrate a 30-fold increase of the resolution. We stress that neither entanglement nor joint detection is needed in these experiments, which demonstrates that neither is necessary to achieve phase super-resolution.

  • 27.
    Kothe, Christian
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Björk, Gunnar
    Inoue, Shuichiro
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    On the efficiency of quantum lithography2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, p. 043028-Article in journal (Refereed)
    Abstract [en]

    Quantum lithography promises, in principle, unlimited feature resolution, independent of wavelength. However, in the literature, at least two different theoretical descriptions of quantum lithography exist. They differ in the extent to which they predict that the photons retain spatial correlation from generation to absorption, and although both predict the same feature size, they vastly differ in predicting how efficiently a quantum lithographic pattern can be exposed. Until recently, essentially all quantum lithography experiments have been performed in such a way that it is difficult to distinguish between the two theoretical explanations. However, last year an experiment was performed that gives different outcomes for the two theories. We comment on the experiment and show that the model that fits the data unfortunately indicates that the trade-off between resolution and efficiency in quantum lithography is very unfavourable.

  • 28. Laskowski, Wieslaw
    et al.
    Wiesniak, Marcin
    Zukowski, Marek
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Weinfurter, Harald
    Interference contrast in multisource few-photon optics2009In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 42, no 11, p. 114004-Article in journal (Refereed)
    Abstract [en]

    Many recent experiments employ several parametric down-conversion (PDC) sources to get multiphoton interference. Such interference has applications in quantum information. We study here how effects due to photon statistics, misalignment and partial distinguishability of the PDC pairs originating from different sources may lower the interference contrast in multiphoton experiments.

  • 29.
    Marques, Breno
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ahrens, Johan
    Stockholm University, Faculty of Science, Department of Physics.
    Nawareg, Mohamed
    Stockholm University, Faculty of Science, Department of Physics. University of Gdansk, Poland.
    Cabello, Adan
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental Observation of Hardy-Like Quantum Contextuality2014In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, no 25, article id 250403Article in journal (Refereed)
    Abstract [en]

    Contextuality is a fundamental property of quantum theory and a critical resource for quantum computation. Here, we experimentally observe the arguably cleanest form of contextuality in quantum theory [A. Cabello et al., Phys. Rev. Lett. 111, 180404 (2013)] by implementing a novel method for performing two sequential measurements on heralded photons. This method opens the door to a variety of fundamental experiments and applications.

  • 30. Mironowicz, Piotr
    et al.
    Tavakoli, Armin
    Stockholm University, Faculty of Science, Department of Physics. National Quantum Information Centre in Gdańsk, Poland.
    Hameedi, Alley
    Stockholm University, Faculty of Science, Department of Physics.
    Marques, Breno
    Stockholm University, Faculty of Science, Department of Physics. Universidade de São Paulo, Brazil.
    Pawlowski, Marcin
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Increased certification of semi-device independent random numbers using many inputs and more post-processing2016In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 18, article id 065004Article in journal (Refereed)
    Abstract [en]

    Quantum communication with systems of dimension larger than two provides advantages in information processing tasks. Examples include higher rates of key distribution and random number generation. The main disadvantage of using such multi-dimensional quantum systems is the increased complexity of the experimental setup. Here, we analyze a not-so-obvious problem: the relation between randomness certification and computational requirements of the post-processing of experimental data. In particular, we consider semi-device independent randomness certification from an experiment using a four dimensional quantum system to violate the classical bound of a random access code. Using state-of-the-art techniques, a smaller quantum violation requires more computational power to demonstrate randomness, which at some point becomes impossible with today's computers although the randomness is (probably) still there. We show that by dedicating more input settings of the experiment to randomness certification, then by more computational postprocessing of the experimental data which corresponds to a quantum violation, one may increase the amount of certified randomness. Furthermore, we introduce a method that significantly lowers the computational complexity of randomness certification. Our results show how more randomness can be generated without altering the hardware and indicate a path for future semi-device independent protocols to follow.

  • 31.
    Mohamed El Hassan, Ashraf
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Andler, Guillermo
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Tunable integrated devices for quantum optics experiments, based on fs-laser writing of optical waveguides in glassManuscript (preprint) (Other academic)
  • 32.
    Mohamed El Hassan, Ashraf
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kunst, Flore
    Stockholm University, Faculty of Science, Department of Physics.
    Moritz, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Andler, Guillermo
    Stockholm University, Faculty of Science, Department of Physics.
    Bergholtz, Emil
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Corner state of light in photonic waveguides2018In: Article in journal (Refereed)
  • 33.
    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.

  • 34.
    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.

  • 35.
    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.

  • 36.
    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.

  • 37.
    Radmark, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wiesniak, Marcin
    Zukowski, Marek
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental multilocation remote state preparation2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, no 3, p. 032304-Article in journal (Refereed)
    Abstract [en]

    Transmission of quantum states is a central task in quantum information science. Remote state preparation (RSP) has the same goal as teleportation, i.e., transferring quantum information without sending physically the information carrier, but in RSP the sender knows the state which is to be transmitted. We present experimental demonstrations of RSP for two and three locations. In our experimental scheme Alice (the preparer) and her three partners share four and six photon polarization entangled singlets. This allows us to perform RSP of two or three copies of a single-qubit state, a two-qubit Bell state, and a three-qubit W, or (W) over bar state. A possibility to prepare two-qubit nonmaximally entangled and GHZ states is also discussed. The ability to remotely prepare an entangled states by local projections at Alice is a distinguishing feature of our scheme.

  • 38. 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.

  • 39.
    Rådmark, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wieśniak, Marcin
    Institute for Theoretical Physics and Astrophysics, Uniwersytet Gdański.
    Żukowski, Marek
    Institute for Theoretical Physics and Astrophysics, Uniwersytet Gdański.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental filtering of two-, four-, and six-photon singlets from a single parametric down-conversion source2009In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 80, no 040302Article in journal (Refereed)
    Abstract [en]

    Invariant entangled states remain unchanged under simultaneous identical unitary transformations of all their subsystems. We experimentally generate and characterize such invariant two-, four-, and six-photon polarization entangled states. This is done only with a suitable filtering procedure of multiple emissions of entangled photon pairs from a single source without any interferometric overlaps. We get the desired states utilizing bosonic emission enhancement due to indistinguishability. The setup is very stable and gives high interference contrasts. Thus, the process is a very likely candidate for various photonic demonstrations of quantum information protocols.

  • 40.
    Rådmark, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Żukowski, Marek
    Institute for Theoretical Physics and Astrophysics, Uniwersytet Gdański.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental high fidelity six-photon entangled state for telecloning protocols2009In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 11, no 103016Article in journal (Refereed)
    Abstract [en]

    We experimentally generate and characterize a six-photon polarizationentangled state, which is usually called ‘9+6 ’. This is realized with a filteringprocedure of triple emissions of entangled photon pairs from a single source,which does not use any interferometric overlaps. The setup is very stable and weobserve the six-photon state with high fidelity. The observed state can be usedfor demonstrations of telecloning and secret sharing protocols.

  • 41.
    Rådmark, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Żukowski, Marek
    Institute for Theoretical Physics and Astrophysics, Uniwersytet Gdański.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental Test of Fidelity Limits in Six-Photon Interferometry and of Rotational Invariance Properties of the Photonic Six-Qubit Entanglement Singlet State2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 103, no 150501Article in journal (Refereed)
    Abstract [en]

    Quantum multiphoton interferometry has now reached the six-photon stage. Thus far, the observed fidelities of entangled states never reached 2/3. We report a high fidelity (estimated at 88%) experiment in which six-qubit singlet correlations were observed. With such a high fidelity we are able to demonstrate the central property of these “singlet” correlations, their “rotational invariance,” by performing a full set of measurements in three complementary polarization bases. The patterns are almost indistinguishable. The data reveal genuine six-photon entanglement. We also study several five-photon states, which result upon detection of one of the photons. Multiphoton singlet states survive some types of depolarization and are thus important in quantum communication schemes.

  • 42.
    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

  • 43.
    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.

  • 44.
    Smania, Massimiliano
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Elhassan, Ashraf M.
    Stockholm University, Faculty of Science, Department of Physics.
    Tavakoli, Armin
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental quantum multiparty communication protocols2016In: npj Quantum Information, ISSN 2056-6387, Vol. 2, article id 16010Article in journal (Refereed)
    Abstract [en]

    Quantum information science breaks limitations of conventional information transfer, cryptography and computation by using quantum superpositions or entanglement as resources for information processing. Here we report on the experimental realisation of three-party quantum communication protocols using single three-level quantum system (qutrit) communication: secret-sharing, detectable Byzantine agreement and communication complexity reduction for a three-valued function. We have implemented these three schemes using the same optical fibre interferometric setup. Our realisation is easily scalable without compromising on detection efficiency or generating extremely complex many-particle entangled states.

  • 45.
    Tavakoli, Armin
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Anwer, Hammad
    Stockholm University, Faculty of Science, Department of Physics.
    Hameedi, Alley
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Quantum communication complexity using the quantum Zeno effect2015In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 92, no 1, article id 012303Article in journal (Refereed)
    Abstract [en]

    The quantum Zeno effect (QZE) is the phenomenon in which the unitary evolution of a quantum state is suppressed, e.g., due to frequent measurements. Here, we investigate the use of the QZE in a class of communication complexity problems (CCPs). Quantum entanglement is known to solve certain CCPs beyond classical constraints. However, recent developments have yielded CCPs for which superclassical results can be obtained using only communication of a single d-level quantum state (qudit) as a resource. In the class of CCPs considered here, we show quantum reduction of complexity in three ways: using (i) entanglement and the QZE, (ii) a single qudit and the QZE, and (iii) a single qudit. We have performed a proof of concept experimental demonstrations of three party CCP protocol based on single-qubit communication with and without QZE.

  • 46.
    Tavakoli, Armin
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Cabello, Adan
    Zukowski, Marek
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Quantum Clock Synchronization with a Single Qudit2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 7982Article in journal (Refereed)
    Abstract [en]

    Clock synchronization for nonfaulty processes in multiprocess networks is indispensable for a variety of technologies. A reliable system must be able to resynchronize the nonfaulty processes upon some components failing causing the distribution of incorrect or conflicting information in the network. The task of synchronizing such networks is related to Byzantine agreement (BA), which can classically be solved using recursive algorithms if and only if less than one-third of the processes are faulty. Here we introduce a nonrecursive quantum algorithm, based on a quantum solution of the detectable BA, which achieves clock synchronization in the presence of arbitrary many faulty processes by using only a single quantum system.

  • 47.
    Tavakoli, Armin
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Hameedi, Alley
    Stockholm University, Faculty of Science, Department of Physics.
    Marques, Breno
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Quantum Random Access Codes Using Single d-Level Systems2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 17, article id 170502Article in journal (Refereed)
    Abstract [en]

    Random access codes (RACs) are used by a party to, with limited communication, access an arbitrary subset of information held by another party. Quantum resources are known to enable RACs that break classical limitations. Here, we study quantum and classical RACs with high-level communication. We derive average performances of classical RACs and present families of high-level quantum RACs. Our results show that high-level quantum systems can significantly increase the advantage of quantum RACs over their classical counterparts. We demonstrate our findings in an experimental realization of a quantum RAC with four-level communication.

  • 48.
    Tavakoli, Armin
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Herbauts, Isabelle
    Stockholm University, Faculty of Science, Department of Physics.
    Zukowski, Marek
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Secret sharing with a single d-level quantum system2015In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 92, no 3, article id 030302Article in journal (Refereed)
    Abstract [en]

    We give an example of a wide class of problems for which quantum-information protocols based on multisystem entanglement can be mapped into much simpler ones involving one system. Secret sharing is a cryptographic primitive which plays a central role in various secure multiparty computation tasks and management of keys in cryptography. In secret sharing protocols, a classical message is divided into shares given to recipient parties in such a way that some number of parties need to collaborate in order to reconstruct the message. Quantum protocols for the task commonly rely on multipartite GHZ entanglement. We present a multiparty secret sharing protocol which requires only sequential communication of a single quantum d-level system (for any prime d). It has huge advantages in scalability and can be realized with state-of-the-art technology.

  • 49.
    Tavakoli, Armin
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Uniwersytet Gda´nski, Poland.
    Marques, Breno
    Stockholm University, Faculty of Science, Department of Physics. Universidade de S˜ao Paulo, Brazil.
    Pawlowski, Marcin
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Spatial versus sequential correlations for random access coding2016In: Physical Review A, ISSN 2469-9926, Vol. 93, no 3, article id 032336Article in journal (Refereed)
    Abstract [en]

    Random access codes are important for a wide range of applications in quantum information. However, their implementation with quantum theory can be made in two very different ways: (i) by distributing data with strong spatial correlations violating a Bell inequality or (ii) using quantum communication channels to create stronger-than-classical sequential correlations between state preparation and measurement outcome. Here we study this duality of the quantum realization. We present a family of Bell inequalities tailored to the task at hand and study their quantum violations. Remarkably, we show that the use of spatial and sequential quantum correlations imposes different limitations on the performance of quantum random access codes: Sequential correlations can outperform spatial correlations. We discuss the physics behind the observed discrepancy between spatial and sequential quantum correlations.

  • 50.
    Tavakoli, Armin
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Uniwersytet Gdański, Poland.
    Pawlowski, Marcin
    Zukowski, Marek
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Dimensional discontinuity in quantum communication complexity at dimension seven2017In: Physical Review A, ISSN 2469-9926, Vol. 95, no 2, article id 020302Article in journal (Refereed)
    Abstract [en]

    Entanglement-assisted classical communication and transmission of a quantum system are the two quantum resources for information processing. Many information tasks can be performed using either quantum resource. However, this equivalence is not always present since entanglement-assisted classical communication is sometimes known to be the better performing resource. Here, we show not only the opposite phenomenon, that there exist tasks for which transmission of a quantum system is a more powerful resource than entanglement-assisted classical communication, but also that such phenomena can have a surprisingly strong dependence on the dimension of Hilbert space. We introduce a family of communication complexity problems parametrized by the dimension of Hilbert space and study the performance of each quantum resource. Under an additional assumption of a linear strategy for the receiving party, we find that for low dimensions the two resources perform equally well, whereas for dimension seven and above the equivalence is suddenly broken and transmission of a quantum system becomes more powerful than entanglement-assisted classical communication. Moreover, we find that transmission of a quantum system may even outperform classical communication assisted by the stronger-than-quantum correlations obtained from the principle of macroscopic locality.

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