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  • 1.
    Fedorov, Aleksey
    et al.
    Russian Quantum Ctr, Russia; QRate, Russia; QApp, Russia.
    Gerhardt, Ilja
    Univ Stuttgart, Germany; Inst Quantum Sci and Technol, Germany; Max Planck Inst Solid State Res, Germany.
    Huang, Anqi
    Natl Univ Def Technol, Peoples R China; Natl Univ Def Technol, Peoples R China.
    Jogenfors, Jonathan
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Kurochkin, Yury
    Russian Quantum Ctr, Russia; QRate, Russia.
    Lamas-Linares, Antia
    Univ Texas Austin, TX 78712 USA.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Leuchs, Gerd
    Max Planck Inst Sci Light, Germany; Univ Erlangen Nurnberg, Germany.
    Lydersen, Lars
    Kringsjavegen 3E, Norway.
    Makarov, Vadim
    Russian Quantum Ctr, Russia; Natl Univ Sci and Technol MISIS, Russia.
    Skaar, Johannes
    Univ Oslo, Norway.
    Comment on Inherent security of phase coding quantum key distribution systems against detector blinding attacks (vol 15, 095203, 2018)2019In: Laser Physics Letters, ISSN 1612-2011, E-ISSN 1612-202X, Vol. 16, no 1, article id 019401Article in journal (Other academic)
    Abstract [en]

    n/a

  • 2.
    Johansson, Niklas
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Quantum Simulation Logic, Oracles, and the Quantum Advantage2019In: Entropy, ISSN 1099-4300, E-ISSN 1099-4300, Vol. 21, no 8, article id 800Article in journal (Refereed)
    Abstract [en]

    Query complexity is a common tool for comparing quantum and classical computation, and it has produced many examples of how quantum algorithms differ from classical ones. Here we investigate in detail the role that oracles play for the advantage of quantum algorithms. We do so by using a simulation framework, Quantum Simulation Logic (QSL), to construct oracles and algorithms that solve some problems with the same success probability and number of queries as the quantum algorithms. The framework can be simulated using only classical resources at a constant overhead as compared to the quantum resources used in quantum computation. Our results clarify the assumptions made and the conditions needed when using quantum oracles. Using the same assumptions on oracles within the simulation framework we show that for some specific algorithms, such as the Deutsch-Jozsa and Simons algorithms, there simply is no advantage in terms of query complexity. This does not detract from the fact that quantum query complexity provides examples of how a quantum computer can be expected to behave, which in turn has proved useful for finding new quantum algorithms outside of the oracle paradigm, where the most prominent example is Shors algorithm for integer factorization.

  • 3.
    Abellán, C.
    et al.
    ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Barcelona, Spain.
    Acín, A.
    ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Barcelona, Spain / ICREA - Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain.
    Alarcón, A.
    Millennium Institute for Research in Optics, Universidad de Concepción, Universidad de Concepción, Concepción, Chile / Departamento de Ingeniería Eléctrica, Universidad de Concepción, Concepción, Chile.
    Alibart, O.
    Université Côte d’Azur, CNRS UMR 7010, Institut de Physique de Nice (INPHYNI), Nice, France.
    Andersen, C. K.
    Department of Physics, ETH Zurich,, Zurich, Switzerland.
    Andreoli, F.
    Dipartimento di Fisica, Sapienza Università di Roma, Rome, Italy.
    Beckert, A.
    Department of Physics, ETH Zurich,, Zurich, Switzerland.
    Beduini, F. A.
    ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Barcelona, Spain.
    Bendersky, A.
    Departamento de Computación, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires and Instituto de Investigación en Ciencias de la Comunicación (ICC), CONICET, Buenos Aires, Argentina.
    Bentivegna, M.
    Dipartimento di Fisica, Sapienza Università di Roma, Rome, Italy.
    Bierhorst, P.
    National Institute of Standards and Technology, Boulder, CO, USA.
    Burchardt, D.
    Ludwig-Maximilians-Universität, Munich, Germany.
    Cabello, A.
    Departamento de Física Aplicada II, Universidad de Sevilla, Seville, Spain.
    Cariñe, J.
    Millennium Institute for Research in Optics, Universidad de Concepción, Universidad de Concepción, Concepción, Chile.
    Carrasco, S.
    ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Barcelona, Spain.
    Carvacho, G.
    Dipartimento di Fisica, Sapienza Università di Roma, Rome, Italy.
    Cavalcanti, D.
    Chaves, R.
    Cortés-Vega, J.
    Cuevas, A.
    Delgado, A.
    de Riedmatten, H.
    Eichler, C.
    Farrera, P.
    Fuenzalida, J.
    García-Matos, M.
    Garthoff, R.
    Gasparinetti, S.
    Gerrits, T.
    Ghafari Jouneghani, F.
    Glancy, S.
    Gómez, E. S.
    González, P.
    Guan, J. -Y.
    Handsteiner, J.
    Heinsoo, J.
    Heintze, G.
    Hirschmann, A.
    Jiménez, O.
    Kaiser, F.
    Knill, E.
    Knoll, L. T.
    Krinner, S.
    Kurpiers, P.
    Larotonda, M. A.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Lenhard, A.
    Li, H.
    Li, M. -H.
    Lima, G.
    Liu, B.
    Liu, Y.
    López Grande, I. H.
    Lunghi, T.
    Ma, X.
    Magaña-Loaiza, O. S.
    Magnard, P.
    Magnoni, A.
    Martí­-Prieto, M.
    Martínez, D.
    Mataloni, P.
    Mattar, A.
    Mazzera, M.
    Mirin, R. P.
    Mitchell, M. W.
    Nam, S.
    Oppliger, M.
    Pan, J. -W.
    Patel, R. B.
    Pryde, G. J.
    Rauch, D.
    Redeker, K.
    Rieländer, D.
    Ringbauer, M.
    Roberson, T.
    Rosenfeld, W.
    Salathé, Y.
    Santodonato, L.
    Sauder, G.
    Scheidl, T.
    Schmiegelow, C. T.
    Sciarrino, F.
    Seri, A.
    Shalm, L. K.
    Shi, S. -C
    Slussarenko, S.
    Stevens, M. J.
    Tanzilli, S.
    Toledo, F.
    Tura, J.
    Ursin, R.
    Vergyris, P.
    Verma, V. B.
    Walter, T.
    Wallraff, A.
    Wang, Z.
    Weinfurter, H.
    Weston, M. M.
    White, A. G.
    Wu, C.
    Xavier, Guilherme B.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    You, L.
    Yuan, X.
    Zeilinger, A.
    Zhang, Q.
    Zhang, W.
    Zhong, J.
    Challenging Local Realism with Human Choices2018In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 557, p. 212-216Article in journal (Refereed)
    Abstract [en]

    A Bell test is a randomized trial that compares experimental observations against the philosophical worldview of local realism , in which the properties of the physical world are independent of our observation of them and no signal travels faster than light. A Bell test requires spatially distributed entanglement, fast and high-efficiency detection and unpredictable measurement settings. Although technology can satisfy the first two of these requirements, the use of physical devices to choose settings in a Bell test involves making assumptions about the physics that one aims to test. Bell himself noted this weakness in using physical setting choices and argued that human 'free will' could be used rigorously to ensure unpredictability in Bell tests. Here we report a set of local-realism tests using human choices, which avoids assumptions about predictability in physics. We recruited about 100,000 human participants to play an online video game that incentivizes fast, sustained input of unpredictable selections and illustrates Bell-test methodology. The participants generated 97,347,490 binary choices, which were directed via a scalable web platform to 12 laboratories on five continents, where 13 experiments tested local realism using photons, single atoms, atomic ensembles and superconducting devices. Over a 12-hour period on 30 November 2016, participants worldwide provided a sustained data flow of over 1,000 bits per second to the experiments, which used different human-generated data to choose each measurement setting. The observed correlations strongly contradict local realism and other realistic positions in bi-partite and tri-partite 12 scenarios. Project outcomes include closing the 'freedom-of-choice loophole' (the possibility that the setting choices are influenced by 'hidden variables' to correlate with the particle properties), the utilization of video-game methods for rapid collection of human-generated randomness, and the use of networking techniques for global participation in experimental science.

  • 4.
    Vedovato, Francesco
    et al.
    Univ Padua, Italy.
    Agnesi, Costantino
    Univ Padua, Italy.
    Tomasin, Marco
    Univ Padua, Italy.
    Avesani, Marco
    Univ Padua, Italy.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Vallone, Giuseppe
    Univ Padua, Italy; CNR, Italy.
    Villoresi, Paolo
    Univ Padua, Italy; CNR, Italy.
    Postselection-Loophole-Free Bell Violation with Genuine Time-Bin Entanglement2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 19, article id 190401Article in journal (Refereed)
    Abstract [en]

    Entanglement is an invaluable resource for fundamental tests of physics and the implementation of quantum information protocols such as device-independent secure communications. In particular, time-bin entanglement is widely exploited to reach these purposes both in free space and optical fiber propagation, due to the robustness and simplicity of its implementation. However, all existing realizations of time-bin entanglement suffer from an intrinsic postselection loophole, which undermines their usefulness. Here, we report the first experimental violation of Bells inequality with "genuine" time-bin entanglement, free of the postselection loophole. We introduced a novel function of the interferometers at the two measurement stations, that operate as fast synchronized optical switches. This scheme allowed us to obtain a postselection-loophole-free Bell violation of more than 9 standard deviations. Since our scheme is fully implementable using standard fiber-based components and is compatible with modem integrated photonics, our results pave the way for the distribution of genuine time-bin entanglement over long distances.

  • 5.
    Giustina, Marissa
    et al.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Versteegh, Marijn A. M.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Wengerowsky, Soeren
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Handsteiner, Johannes
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Hochrainer, Armin
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Phelan, Kevin
    Austrian Academic Science, Austria.
    Steinlechner, Fabian
    Austrian Academic Science, Austria.
    Kofler, Johannes
    Barcelona Institute Science and Technology, Spain.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Abellan, Carlos
    Barcelona Institute Science and Technology, Spain.
    Amaya, Waldimar
    Barcelona Institute Science and Technology, Spain.
    Mitchell, Morgan W.
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Beyer, Joern
    Phys Technical Bundesanstalt, Germany.
    Gerrits, Thomas
    NIST, CO 80305 USA.
    Lita, Adriana E.
    NIST, CO 80305 USA.
    Shalm, Lynden K.
    NIST, CO 80305 USA.
    Woo Nam, Sae
    NIST, CO 80305 USA.
    Scheidl, Thomas
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Ursin, Rupert
    Austrian Academic Science, Austria.
    Wittmann, Bernhard
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Zeilinger, Anton
    Austrian Academic Science, Austria; University of Vienna, Austria.
    A Significant-Loophole-Free Test of Bells Theorem with Entangled Photons2017In: QUANTUM INFORMATION SCIENCE AND TECHNOLOGY III, SPIE-INT SOC OPTICAL ENGINEERING , 2017, Vol. 10442, article id UNSP 1044204Conference paper (Refereed)
    Abstract [en]

    John Bells theorem of 1964 states that local elements of physical reality, existing independent of measurement, are inconsistent with the predictions of quantum mechanics (Bell, J. S. (1964), Physics (College. Park. Md). 1 (3), 195). Specifically, correlations between measurement results from distant entangled systems would be smaller than predicted by quantum physics. This is expressed in Bells inequalities. Employing modifications of Bells inequalities, many experiments have been performed that convincingly support the quantum predictions. Yet, all experiments rely on assumptions, which provide loopholes for a local realist explanation of the measurement. Here we report an experiment with polarization-entangled photons that simultaneously closes the most significant of these loopholes. We use a highly efficient source of entangled photons, distributed these over a distance of 58.5 meters, and implemented rapid random setting generation and high-efficiency detection to observe a violation of a Bell inequality with high statistical significance. The merely statistical probability of our results to occur under local realism is less than 3.74 . 10(-31), corresponding to an 11.5 standard deviation effect.

  • 6.
    Jogenfors, Jonathan
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Adán, Cabello
    Departamento de Física Aplicada II, Universidad de Sevilla, Sevilla, Spain.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Comment on "Franson Interference Generated by a Two-Level System"2017Other (Other academic)
    Abstract [en]

    In a recent Letter [Phys. Rev. Lett. 118, 030501 (2017)], Peiris, Konthasinghe, and Muller report a Franson interferometry experiment using pairs of photons generated from a two-level semiconductor quantum dot. The authors report a visibility of 66% and claim that this visibility “goes beyond the classical limit of 50% and approaches the limit of violation of Bell’s inequalities (70.7%).” We explain why we do not agree with this last statement and how to fix the problem.

  • 7.
    Johansson, Niklas
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Efficient classical simulation of the Deutsch-Jozsa and Simons algorithms2017In: Quantum Information Processing, ISSN 1570-0755, E-ISSN 1573-1332, Vol. 16, no 9, article id UNSP 233Article in journal (Refereed)
    Abstract [en]

    Along-standing aim of quantum information research is to understand what gives quantum computers their advantage. This requires separating problems that need genuinely quantum resources from those for which classical resources are enough. Two examples of quantum speed-up are the Deutsch-Jozsa and Simons problem, both efficiently solvable on a quantum Turing machine, and both believed to lack efficient classical solutions. Here we present a framework that can simulate both quantum algorithms efficiently, solving the Deutsch-Jozsa problem with probability 1 using only one oracle query, and Simons problem using linearly many oracle queries, just as expected of an ideal quantum computer. The presented simulation framework is in turn efficiently simulatable in a classical probabilistic Turing machine. This shows that the Deutsch-Jozsa and Simons problem do not require any genuinely quantum resources, and that the quantum algorithms show no speed-up when compared with their corresponding classical simulation. Finally, this gives insight into what properties are needed in the two algorithms and calls for further study of oracle separation between quantum and classical computation.

  • 8.
    Tomasin, Marco
    et al.
    University of Padua, Italy; UOS Padova, Italy.
    Mantoan, Elia
    University of Padua, Italy; UOS Padova, Italy.
    Jogenfors, Jonathan
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Vallone, Giuseppe
    University of Padua, Italy; UOS Padova, Italy.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Villoresi, Paolo
    University of Padua, Italy; UOS Padova, Italy.
    High-visibility time-bin entanglement for testing chained Bell inequalities2017In: Physical Review A, ISSN 2469-9926, Vol. 95, no 3, article id 032107Article in journal (Refereed)
    Abstract [en]

    The violation of Bells inequality requires a well-designed experiment to validate the result. In experiments using energy-time and time-bin entanglement, initially proposed by Franson in 1989, there is an intrinsic loophole due to the high postselection. To obtain a violation in this type of experiment, a chained Bell inequality must be used. However, the local realism bound requires a high visibility in excess of 94.63% in the time-bin entangled state. In this work, we show how such a high visibility can be reached in order to violate a chained Bell inequality with six, eight, and ten terms.

  • 9.
    Jogenfors, Jonathan
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Tight bounds for the Pearle-Braunstein-Caves chained inequality without the fair-coincidence assumption2017In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 96, no 2, article id 022102Article in journal (Refereed)
    Abstract [en]

    In any Bell test, loopholes can cause issues in the interpretation of the results, since an apparent violation of the inequality may not correspond to a violation of local realism. An important example is the coincidence-time loophole that arises when detector settings might influence the time when detection will occur. This effect can be observed in many experiments where measurement outcomes are to be compared between remote stations because the interpretation of an ostensible Bell violation strongly depends on the method used to decide coincidence. The coincidence-time loophole has previously been studied for the Clauser-Horne-Shimony-Holt and Clauser-Horne inequalities, but recent experiments have shown the need for a generalization. Here, we study the generalized chained inequality by Pearle, Braunstein, and Caves (PBC) with N amp;gt;= 2 settings per observer. This inequality has applications in, for instance, quantum key distribution where it has been used to reestablish security. In this paper we give the minimum coincidence probability for the PBC inequality for all N amp;gt;= 2 and show that this bound is tight for a violation free of the fair-coincidence assumption. Thus, if an experiment has a coincidence probability exceeding the critical value derived here, the coincidence-time loophole is eliminated.

  • 10.
    Pacher, Christoph
    et al.
    Department of Safety & Security, AIT Austrian Institute of Technology, Austria.
    Abidin, Aysajan
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Lorünser, Thomas
    Department of Safety & Security, AIT Austrian Institute of Technology, Austria.
    Peev, Momtchil
    Department of Safety & Security, AIT Austrian Institute of Technology, Austria.
    Ursin, Rupert
    Institut für Experimentalphysik, Universität Wien, Austria and Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Austria.
    Zeilinger, Anton
    Institut für Experimentalphysik, Universität Wien, Austria and Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Austria.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Attacks on quantum key distribution protocols that employ non-ITS authentication2016In: Quantum Information Processing, ISSN 1570-0755, E-ISSN 1573-1332, Vol. 15, no 1, p. 327-362Article in journal (Refereed)
    Abstract [en]

    We demonstrate how adversaries with unbounded computing resources can break Quantum Key Distribution (QKD) protocols which employ a particular message authentication code suggested previously. This authentication code, featuring low key consumption, is not Information-Theoretically Secure (ITS) since for each message the eavesdropper has intercepted she is able to send a different message from a set of messages that she can calculate by finding collisions of a cryptographic hash function. However, when this authentication code was introduced it was shown to prevent straightforward Man-In-The-Middle (MITM) attacks against QKD protocols.

    In this paper, we prove that the set of messages that collide with any given message under this authentication code contains with high probability a message that has small Hamming distance to any other given message. Based on this fact we present extended MITM attacks against different versions of BB84 QKD protocols using the addressed authentication code; for three protocols we describe every single action taken by the adversary. For all protocols the adversary can obtain complete knowledge of the key, and for most protocols her success probability in doing so approaches unity.

    Since the attacks work against all authentication methods which allow to calculate colliding messages, the underlying building blocks of the presented attacks expose the potential pitfalls arising as a consequence of non-ITS authentication in QKDpostprocessing. We propose countermeasures, increasing the eavesdroppers demand for computational power, and also prove necessary and sufficient conditions for upgrading the discussed authentication code to the ITS level.

  • 11.
    Hiesmayr, Beatrix C.
    et al.
    Faculty of Physics, University of Vienna, Austria.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Contextuality and nonlocality in decaying multipartite systems2016In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 93, no 2, p. 020106(R)-Article in journal (Refereed)
    Abstract [en]

    Everyday experience supports the existence of physical properties independent of observation in strong contrast to the predictions of quantum theory. In particular, the existence of physical properties that are independent of the measurement context is prohibited for certain quantum systems. This property is known as contextuality. This Rapid Communication studies whether the process of decay in space-time generally destroys the ability of revealing contextuality. We find that in the most general situation the decay property does not diminish this ability. However, applying certain constraints due to the space-time structure either on the time evolution of the decaying system or on the measurement procedure, the criteria revealing contextuality become inherently dependent on the decay property or an impossibility. In particular, we derive how the context-revealing setup known as Bells nonlocality tests changes for decaying quantum systems. Our findings illustrate the interdependence between hidden and local hidden parameter theories and the role of time.

  • 12.
    Kofler, Johannes
    et al.
    Max Planck Institute Quantum Opt MPQ, Germany.
    Giustina, Marissa
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Mitchell, Morgan W.
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Requirements for a loophole-free photonic Bell test using imperfect setting generators2016In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 93, no 3, p. 032115-Article in journal (Refereed)
    Abstract [en]

    Experimental violations of Bell inequalities are in general vulnerable to so-called loopholes. In this work, we analyze the characteristics of a loophole-free Bell test with photons, closing simultaneously the locality, freedom-of-choice, fair-sampling (i.e., detection), coincidence-time, and memory loopholes. We pay special attention to the effect of excess predictability in the setting choices due to nonideal random-number generators. We discuss necessary adaptations of the Clauser-Horne and Eberhard inequality when using such imperfect devices and-using Hoeffdings inequality and Doobs optional stopping theorem-the statistical analysis in such Bell tests.

  • 13.
    Giustina, M.
    et al.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Versteegh, M. A. M.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Wengerowsky, S.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Handsteiner, J.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Hochrainer, A.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Phelan, K.
    Austrian Academic Science, Austria.
    Steinlechner, F.
    Austrian Academic Science, Austria.
    Kofler, J.
    Max Planck Institute Quantum Opt MPQ, Germany.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Abellan, C.
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Amaya, W.
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Pruneri, V.
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Mitchell, M. W. M.
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Beyer, J.
    Phys Technical Bundesanstalt, Germany.
    Gerrits, T.
    NIST, CO 80305 USA.
    Lita, A.
    NIST, CO 80305 USA.
    Shalm, L. K.
    NIST, CO 80305 USA.
    Nam, S. W.
    NIST, CO 80305 USA.
    Scheidl, T.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Ursin, R.
    Austrian Academic Science, Austria.
    Wittmann, B.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Zeilinger, A.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Significant-Loophole-Free Test of Local Realism with Entangled Photons2016In: 2016 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO), IEEE , 2016Conference paper (Refereed)
    Abstract [en]

    We report an experimental violation of a Bell inequality with strong statistical significance. Our experiment employs polarization measurements on entangled single photons and closes the locality, freedom-of-choice, fair-sampling, coincidence-time, and memory loopholes simultaneously.

  • 14.
    Cabello, Adan
    et al.
    University of Seville, Spain.
    Gu, Mile
    Nanyang Technology University, Singapore; National University of Singapore, Singapore.
    Guehne, Otfried
    University of Siegen, Germany.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Wiesner, Karoline
    University of Bristol, England.
    Thermodynamical cost of some interpretations of quantum theory2016In: PHYSICAL REVIEW A, ISSN 2469-9926, Vol. 94, no 5, article id 052127Article in journal (Refereed)
    Abstract [en]

    The interpretation of quantum theory is one of the longest-standing debates in physics. Type I interpretations see quantum probabilities as determined by intrinsic properties of the observed system. Type II see them as relational experiences between an observer and the system. It is usually believed that a decision between these two options cannot be made simply on purely physical grounds but requires an act of metaphysical judgment. Here we show that, under some assumptions, the problem is decidable using thermodynamics. We prove that type I interpretations are incompatible with the following assumptions: (i) The choice of which measurement is performed can be made randomly and independently of the system under observation, (ii) the system has limited memory, and (iii) Landauers erasure principle holds.

  • 15.
    Dzhafarov, Ehtibar N.
    et al.
    Purdue University, IN 47907 USA.
    Kujala, Janne V.
    University of Jyvaskyla, Finland.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Contextuality in Three Types of Quantum-Mechanical Systems2015In: Foundations of physics, ISSN 0015-9018, E-ISSN 1572-9516, Vol. 45, no 7, p. 762-782Article in journal (Refereed)
    Abstract [en]

    We present a formal theory of contextuality for a set of random variables grouped into different subsets (contexts) corresponding to different, mutually incompatible conditions. Within each context the random variables are jointly distributed, but across different contexts they are stochastically unrelated. The theory of contextuality is based on the analysis of the extent to which some of these random variables can be viewed as preserving their identity across different contexts when one considers all possible joint distributions imposed on the entire set of the random variables. We illustrate the theory on three systems of traditional interest in quantum physics (and also in non-physical, e.g., behavioral studies). These are systems of the Klyachko-Can-Binicioglu-Shumovsky-type, Einstein-Podolsky-Rosen-Bell-type, and Suppes-Zanotti-Leggett-Garg-type. Listed in this order, each of them is formally a special case of the previous one. For each of them we derive necessary and sufficient conditions for contextuality while allowing for experimental errors and contextual biases or signaling. Based on the same principles that underly these derivations we also propose a measure for the degree of contextuality and compute it for the three systems in question.

  • 16.
    Jogenfors, Jonathan
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Elhassan, Ashraf M
    Physics Department, Stockholm University, Stockholm, Sweden.
    Ahrens, Johan
    Physics Department, Stockholm University, Stockholm, Sweden.
    Bourennane, Mohamed
    Physics Department, Stockholm University, Stockholm, Sweden.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Hacking the Bell test using classical light in energy-time entanglement–based quantum key distribution2015In: Science Advances, ISSN 2375-2548, Vol. 1, no 11, p. 1-7, 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.

  • 17.
    Kujala, Janne V.
    et al.
    University of Jyvaskyla, Finland.
    Dzhafarov, Ehtibar N.
    Purdue University, IN 47907 USA.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Necessary and Sufficient Conditions for an Extended Noncontextuality in a Broad Class of Quantum Mechanical Systems2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 15, p. 150401-Article in journal (Refereed)
    Abstract [en]

    The notion of (non) contextuality pertains to sets of properties measured one subset (context) at a time. We extend this notion to include so-called inconsistently connected systems, in which the measurements of a given property in different contexts may have different distributions, due to contextual biases in experimental design or physical interactions (signaling): a system of measurements has a maximally noncontextual description if they can be imposed a joint distribution on in which the measurements of any one property in different contexts are equal to each other with the maximal probability allowed by their different distributions. We derive necessary and sufficient conditions for the existence of such a description in a broad class of systems including Klyachko-Can-Binicioglu-Shumvosky-type (KCBS), EPR-Bell-type, and Leggett-Garg-type systems. Because these conditions allow for inconsistent connectedness, they are applicable to real experiments. We illustrate this by analyzing an experiment by Lapkiewicz and colleagues aimed at testing contextuality in a KCBS-type system.

  • 18.
    Carvacho, Gonzalo
    et al.
    University of Concepcion, Chile; University of Concepcion, Chile; University of Concepcion, Chile.
    Carine, Jaime
    University of Concepcion, Chile; University of Concepcion, Chile; University of Concepcion, Chile.
    Saavedra, Gabriel
    University of Concepcion, Chile; University of Concepcion, Chile; University of Concepcion, Chile.
    Cuevas, Alvaro
    University of Concepcion, Chile; University of Concepcion, Chile; University of Concepcion, Chile.
    Fuenzalida, Jorge
    University of Concepcion, Chile; University of Concepcion, Chile; University of Concepcion, Chile.
    Toledo, Felipe
    University of Concepcion, Chile; University of Concepcion, Chile; University of Concepcion, Chile.
    Figueroa, Miguel
    University of Concepcion, Chile; University of Concepcion, Chile.
    Cabello, Adan
    University of Seville, Spain.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Mataloni, Paolo
    University of Roma La Sapienza, Italy.
    Lima, Gustavo
    University of Concepcion, Chile; University of Concepcion, Chile; University of Concepcion, Chile.
    Xavier, Guilherme B.
    University of Concepcion, Chile; University of Concepcion, Chile; University of Concepcion, Chile.
    Postselection-Loophole-Free Bell Test Over an Installed Optical Fiber Network2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 3, p. 030503-Article in journal (Refereed)
    Abstract [en]

    Device-independent quantum communication will require a loophole-free violation of Bell inequalities. In typical scenarios where line of sight between the communicating parties is not available, it is convenient to use energy-time entangled photons due to intrinsic robustness while propagating over optical fibers. Here we show an energy-time Clauser-Horne-Shimony-Holt Bell inequality violation with two parties separated by 3.7 km over the deployed optical fiber network belonging to the University of Concepcion in Chile. Remarkably, this is the first Bell violation with spatially separated parties that is free of the postselection loophole, which affected all previous in-field long-distance energy-time experiments. Our work takes a further step towards a fiber-based loophole-free Bell test, which is highly desired for secure quantum communication due to the widespread existing telecommunication infrastructure.

  • 19.
    Giustina, Marissa
    et al.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Versteegh, Marijn A. M.
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Wengerowsky, Soeren
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Handsteiner, Johannes
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Hochrainer, Armin
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Phelan, Kevin
    Austrian Academic Science, Austria.
    Steinlechner, Fabian
    Austrian Academic Science, Austria.
    Kofler, Johannes
    Max Planck Institute Quantum Opt, Germany.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Abellan, Carlos
    Barcelona Institute Science and Technology, Spain.
    Amaya, Waldimar
    Barcelona Institute Science and Technology, Spain.
    Pruneri, Valerio
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Mitchell, Morgan W.
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Beyer, Joern
    Phys Technical Bundesanstalt, Germany.
    Gerrits, Thomas
    NIST, CO 80305 USA.
    Lita, Adriana E.
    NIST, CO 80305 USA.
    Shalm, Lynden K.
    NIST, CO 80305 USA.
    Woo Nam, Sae
    NIST, CO 80305 USA.
    Scheidl, Thomas
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Ursin, Rupert
    Austrian Academic Science, Austria.
    Wittmann, Bernhard
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Zeilinger, Anton
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Significant-Loophole-Free Test of Bells Theorem with Entangled Photons2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 25, p. 250401-Article in journal (Refereed)
    Abstract [en]

    Local realism is the worldview in which physical properties of objects exist independently of measurement and where physical influences cannot travel faster than the speed of light. Bells theorem states that this worldview is incompatible with the predictions of quantum mechanics, as is expressed in Bells inequalities. Previous experiments convincingly supported the quantum predictions. Yet, every experiment requires assumptions that provide loopholes for a local realist explanation. Here, we report a Bell test that closes the most significant of these loopholes simultaneously. Using a well-optimized source of entangled photons, rapid setting generation, and highly efficient superconducting detectors, we observe a violation of a Bell inequality with high statistical significance. The purely statistical probability of our results to occur under local realism does not exceed 3.74 x 10(-31), corresponding to an 11.5 standard deviation effect.

  • 20.
    Vallone, Giuseppe
    et al.
    University of Padua, Italy University of Roma La Sapienza, Italy .
    Lima, Gustavo
    University of Concepcion, Chile .
    Gomez, Esteban S.
    University of Concepcion, Chile .
    Canas, Gustavo
    University of Concepcion, Chile .
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Mataloni, Paolo
    University of Roma La Sapienza, Italy Ist Nazl Ott INO CNR, Italy .
    Cabello, Adan
    University of Seville, Spain .
    Bell scenarios in which nonlocality and entanglement are inversely related2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, no 1, p. 012102-Article in journal (Refereed)
    Abstract [en]

    We show that for two-qubit chained Bell inequalities with an arbitrary number of measurement settings, nonlocality and entanglement are not only different properties but are inversely related. Specifically, we analytically prove that in absence of noise, robustness of nonlocality, defined as the maximum fraction of detection events that can be lost such that the remaining ones still do not admit a local model, and concurrence are inversely related for any chained Bell inequality with an arbitrary number of settings. The closer quantum states are to product states, the harder it is to reproduce quantum correlations with local models. We also show that, in presence of noise, nonlocality and entanglement are simultaneously maximized only when the noise level is equal to the maximum level tolerated by the inequality; in any other case, a more nonlocal state is always obtained by reducing the entanglement. In addition, we observed that robustness of nonlocality and concurrence are also inversely related for the Bell scenarios defined by the tight two-qubit three-setting I-3322 inequality, and the tight two-qutrit inequality I-3.

  • 21.
    Larsson, Jan-Åke
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Giustina, Marissa
    Austrian Academic Science, Austria.
    Kofler, Johannes
    Max Planck Institute Quantum Opt MPQ, Germany.
    Wittmann, Bernhard
    Austrian Academic Science, Austria.
    Ursin, Rupert
    Austrian Academic Science, Austria.
    Ramelow, Sven
    Austrian Academic Science, Austria; University of Vienna, Austria; Cornell University, NY USA.
    Bell-inequality violation with entangled photons, free of the coincidence-time loophole2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 90, no 3, p. 032107-Article in journal (Refereed)
    Abstract [en]

    In a local realist model, physical properties are defined prior to and independent of measurement and no physical influence can propagate faster than the speed of light. Proper experimental violation of a Bell inequality would show that the world cannot be described with such a model. Experiments intended to demonstrate a violation usually require additional assumptions that make them vulnerable to a number of "loopholes." In both pulsed and continuously pumped photonic experiments, an experimenter needs to identify which detected photons belong to the same pair, giving rise to the coincidence-time loophole. Here, via two different methods, we derive Clauser-Horne- and Eberhard-type inequalities that are not only free of the fair-sampling assumption (thus not being vulnerable to the detection loophole), but also free of the fair-coincidence assumption (thus not being vulnerable to the coincidence-time loophole). Both approaches can be used for pulsed as well as for continuously pumped experiments. Moreover, as they can also be applied to already existing experimental data, we finally show that a recent experiment [Giustina et al., Nature (London) 497, 227 (2013)] violated local realism without requiring the fair-coincidence assumption.

  • 22.
    Guehne, Otfried
    et al.
    University of Siegen, Germany .
    Budroni, Costantino
    University of Siegen, Germany .
    Cabello, Adan
    University of Seville, Spain .
    Kleinmann, Matthias
    University of Siegen, Germany .
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Bounding the quantum dimension with contextuality2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, no 6Article in journal (Refereed)
    Abstract [en]

    We show that the phenomenon of quantum contextuality can be used to certify lower bounds on the dimension accessed by the measurement devices. To prove this, we derive bounds for different dimensions and scenarios of the simplest noncontextuality inequalities. Some of the resulting dimension witnesses work independently of the prepared quantum state. Our constructions are robust against noise and imperfections, and we show that a recent experiment can be viewed as an implementation of a state-independent quantum dimension witness.

  • 23.
    Abidin, Aysajan
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Direct proof of security of Wegman-Carter authentication with partially known key2014In: Quantum Information Processing, ISSN 1570-0755, E-ISSN 1573-1332, Vol. 13, no 10, p. 2155-2170Article in journal (Refereed)
    Abstract [en]

    Information-theoretically secure (ITS) authentication is needed in Quantum Key Distribution (QKD). In this paper, we study security of an ITS authentication scheme proposed by Wegman& Carter, in the case of partially known authentication key. This scheme uses a new authentication key in each authentication attempt, to select a hash function from an Almost Strongly Universal2 hash function family. The partial knowledge of the attacker is measured as the trace distance between the authentication key distribution and the uniform distribution; this is the usual measure in QKD. We provide direct proofs of security of the scheme, when using partially known key, first in the information-theoretic setting and then in terms of witness indistinguishability as used in the Universal Composability (UC) framework. We find that if the authentication procedure has a failure probability ε and the authentication key has an ε´ trace distance to the uniform, then under ITS, the adversary’s success probability conditioned on an authentic message-tag pair is only bounded by ε +|Ƭ|ε´, where |Ƭ| is the size of the set of tags. Furthermore, the trace distance between the authentication key distribution and the uniform increases to |Ƭ|ε´ after having seen an authentic message-tag pair. Despite this, we are able to prove directly that the authenticated channel is indistinguishable from an (ideal) authentic channel (the desired functionality), except with probability less than ε + ε´. This proves that the scheme is (ε + ε´)-UC-secure, without using the composability theorem.

  • 24.
    Jogenfors, Jonathan
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Energy-time entanglement, elements of reality, and local realism2014In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 47, no 42, p. 424032-Article in journal (Refereed)
    Abstract [en]

    The Franson interferometer, proposed in 1989 (Franson 1989 Phys. Rev. Lett. 62 2205-08), beautifully shows the counter-intuitive nature of light. The quantum description predicts sinusoidal interference for specific outcomes of the experiment, and these predictions can be verified in experiment. In the spirit of Einstein, Podolsky, and Rosen it is possible to ask if the quantum-mechanical description (of this setup) can be considered complete. This question will be answered in detail in this paper, by delineating the quite complicated relation between energy-time entanglement experiments and Einstein-Podolsky-Rosen (EPR) elements of reality. The mentioned sinusoidal interference pattern is the same as that giving a violation in the usual Bell experiment. Even so, depending on the precise requirements made on the local realist model, this can imply (a) no violation, (b) smaller violation than usual, or (c) full violation of the appropriate statistical bound. Alternatives include (a) using only the measurement outcomes as EPR elements of reality, (b) using the emission time as EPR element of reality, (c) using path realism, or (d) using a modified setup. This paper discusses the nature of these alternatives and how to choose between them. The subtleties of this discussion needs to be taken into account when designing and setting up experiments intended to test local realism. Furthermore, these considerations are also important for quantum communication, for example in Bell-inequality-based quantum cryptography, especially when aiming for device independence.

  • 25.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Loopholes in Bell inequality tests of local realism2014In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 47, no 42, p. 424003-Article, review/survey (Refereed)
    Abstract [en]

    Bell inequalities are intended to show that local realist theories cannot describe the world. A local realist theory is one where physical properties are defined prior to and independent of measurement, and no physical influence can propagate faster than the speed of light. Quantum-mechanical predictions for certain experiments violate the Bell inequality while a local realist theory cannot, and this shows that a local realist theory cannot give those quantum-mechanical predictions. However, because of unexpected circumstances or loopholes in available experiment tests, local realist theories can reproduce the data from these experiments. This paper reviews such loopholes, what effect they have on Bell inequality tests, and how to avoid them in experiment. Avoiding all these simultaneously in one experiment, usually called a loophole-free or definitive Bell test, remains an open task, but is very important for technological tasks such as device-independent security of quantum cryptography, and ultimately for our understanding of the world.

  • 26.
    Appleby, D.M.
    et al.
    Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada.
    Bengtsson, Ingemar
    Stockholms Universitet, AlbaNova, Fysikum, Stockholm, Sweden.
    Brierley, Stephen
    Heilbronn Institute for Mathematical Research, Department of Mathematics, University of Bristol, United Kingdom.
    Ericsson, Åsa
    Linköping University, Department of Mathematics, Mathematics and Applied Mathematics. Linköping University, The Institute of Technology.
    Grassl, Markus
    Centre for Quantum Technologies, National University of Singapore, Singapore.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Systems of Imprimitivity for the Clifford Group2014In: Quantum information & computation, ISSN 1533-7146, Vol. 14, no 3-4, p. 339-360Article in journal (Refereed)
    Abstract [en]

    It is known that if the dimension is a perfect square the Clifford group can be represented by monomial matrices. Another way of expressing this result is to say that when the dimension is a perfect square the standard representation of the Clifford group has a system of imprimitivity consisting of one dimensional subspaces. We generalize this result to the case of an arbitrary dimension. Let k be the square-free part of the dimension. Then we show that the standard representation of the Clifford group has a system of imprimitivity consisting of k-dimensional subspaces. To illustrate the use of this result we apply it to the calculation of SIC-POVMs (symmetric informationally complete positive operator valued measures), constructing exact solutions in dimensions 8 (hand-calculation) as well as 12 and 28 (machine-calculation).

  • 27.
    Amselem, E.
    et al.
    Department of Physics, Stockholm University, Sweden .
    Bourennane, M.
    Department of Physics, Stockholm University, Stockholm University, Sweden .
    Budroni, C.
    Naturwissenschaftlich-Technische Fakultät, Universität Siegen, Germany .
    Cabello, A.
    Departamento de Física Aplicada II, Universidad de Sevilla, Spain .
    Guehne, O.
    Naturwissenschaftlich-Technische Fakultät, Universität Siegen, Germany .
    Kleinmann, M.
    Naturwissenschaftlich-Technische Fakultät, Universität Siegen, Germany .
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Wiesniak, M.
    Institute of Theoretical Physics and Astrophysics, University of Gdańsk, Poland .
    Editorial Material: Comment on "State-Independent Experimental Test of Quantum Contextuality"2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 7, p. 1-1Article in journal (Other academic)
    Abstract [en]

    In this Comment we argue that the experiment describedin the recent Letter does not allow one to make con-clusions about contextuality. Our main criticism is that themeasurement of the observables as well as the preparationof the state manifestly depend on the chosen context.Contrary to that, contextuality is about the behavior ofthesamemeasurement device in different experimentalcontexts.

  • 28.
    Pettersson, Sofia
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Forchheimer, Robert
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
    Meta-Boolean models of asymmetric division patterns in the C. elegans intestinal lineage: Implications for the posterior boundary of intestinal twist2013In: Worm, ISSN 2162-4054, Vol. 2, article id e23701Article in journal (Refereed)
    Abstract [en]

    The intestine of Caenorhabditis elegans is derived from 20 cells that are organized into nine intestinal rings. During embryogenesis, three of the rings rotate approximately 90 degrees in a process known as intestinal twist. The underlying mechanisms for this morphological event are not fully known, but it has been demonstrated that both left-right and anterior-posterior asymmetry is required for intestinal twist to occur. We have recently presented a rule-based meta-Boolean tree model intended to describe complex lineages. In this report we apply this model to the E lineage of C. elegans, specifically targeting the asymmetric anterior-posterior division patterns within the lineage. The resulting model indicates that cells with the same factor concentration are located next to each other in the intestine regardless of lineage origin. In addition, the shift in factor concentrations coincides with the boundary for intestinal twist. When modeling lit-1 mutant data according to the same principle, the factor distributions in each cell are altered, yet the concurrence between the shift in concentration and intestinal twist remains. This pattern suggests that intestinal twist is controlled by a threshold mechanism. In the current paper we present the factor concentrations for all possible combinations of symmetric and asymmetric divisions in the E lineage and relate these to the potential threshold by studying existing data for wild-type and mutant embryos. Finally, we discuss how the resulting models can serve as a basis for experimental design in order to reveal the underlying mechanisms of intestinal twist.

  • 29.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology. Linköping University, Faculty of Science & Engineering.
    A contextual extension of Spekkens' toy model2012In: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1424, p. 211-220Article in journal (Refereed)
    Abstract [en]

    Quantum systems show contextuality. More precisely, it is impossible to reproduce the quantum-mechanical predictions using a non-contextual realist model, i.e., a model where the outcome of one measurement is independent of the choice of compatible measurements performed in the measurement context. There has been several attempts to quantify the amount of contextuality for specific quantum systems, for example, in the number of rays needed in a KS proof, or the number of terms in certain inequalities, or in the violation, noise sensitivity, and other measures. This paper is about another approach: to use a simple contextual model that reproduces the quantum-mechanical contextual behaviour, but not necessarily all quantum predictions. The amount of contextuality can then be quantified in terms of additional resources needed as compared with a similar model without contextuality. In this case the contextual model needs to keep track of the context used, so the appropriate measure would be memory. Another way to view this is as a memory requirement to be able to reproduce quantum contextuality in a realist model. The model we will use can be viewed as an extension of Spekkens toy model [Phys. Rev. A 75, 032110 (2007)], and the relation is studied in some detail. To reproduce the quantum predictions for the Peres-Mermin square, the memory requirement is more than one bit in addition to the memory used for the individual outcomes in the corresponding noncontextual model.

  • 30.
    Cirkic, Mirsad
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Persson, Daniel
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Approximating the LLR Distribution for a Class of Soft-Output MIMO Detectors2012In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 60, no 12, p. 6421-6434Article in journal (Refereed)
    Abstract [en]

    We present approximations of the LLR distribution for a class of fixed-complexity soft-output MIMO detectors, such as the optimal soft detector and the soft-output via partial marginalization detector. More specifically, in a MIMO AWGN setting, we approximate the LLR distribution conditioned on the transmitted signal and the channel matrix with a Gaussian mixture model (GMM). Our main results consist of an analytical expression of the GMM model (including the number of modes and their corresponding parameters) and a proof that, in the limit of high SNR, this LLR distribution converges in probability towards a unique Gaussian distribution.

  • 31.
    Larsson, Jan-Åke
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Kleinmann, Matthias
    Universität Siegen, Germany.
    Budroni, Constantino
    Universidad de Sevilla, Spain.
    Guehne, Otfried
    Universität Siegen, Germany.
    Cabello, Adán
    Universidad de Sevilla, Spain.
    Maximal violation of state-independent contextuality inequalities2012In: Quantum Theory: Reconsideration of Foundations - 6, American Institute of Physics (AIP), 2012, Vol. 1508, p. 265-274Conference paper (Refereed)
    Abstract [en]

    The discussion on noncontextual hidden variable models as an underlying description for the quantum-mechanical predictions started in ernest with 1967 paper by Kochen and Specker. There, it was shown that no noncontextual hidden-variable model can give these predictions. The proof used in that paper is complicated, but recently, a paper by Yu and Oh [PRL, 2012] proposes a simpler statistical proof that can also be the basis of an experimental test. Here we report on a sharper version of that statistical proof, and also explain why the algebraic upper bound to the expressions used are not reachable, even with a reasonable contextual hidden variable model. Specifically, we show that the quantum mechanical predictions reach the maximal possible value for a contextual model that keeps the expectation value of the measurement outcomes constant.

  • 32.
    Abidin, Aysajan
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding.
    New Universal Hash Functions2012In: Lecture Notes in Computer Science, Vol. 7242 / [ed] Frederik Armknecht and Stefan Lucks, Springer Berlin Heidelberg , 2012, p. 99-108Conference paper (Refereed)
    Abstract [en]

    Universal hash functions are important building blocks for unconditionally secure message authentication codes. In this paper, we present a new construction of a class of Almost Strongly Universal hash functions with much smaller description (or key) length than the Wegman-Carter construction. Unlike some other constructions, our new construction has a very short key length and a security parameter that is independent of the message length, which makes it suitable for authentication in practical applications such as Quantum Cryptography.

  • 33.
    Kleinmann, Matthias
    et al.
    University of Siegen, Germany .
    Budroni, Costantino
    University of Siegen, Germany .
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Guehne, Otfried
    University of Siegen, Germany .
    Cabello, Adan
    University of Seville, Spain .
    Optimal Inequalities for State-Independent Contextuality2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 25, p. 250402-Article in journal (Refereed)
    Abstract [en]

    Contextuality is a natural generalization of nonlocality which does not need composite systems or spacelike separation and offers a wider spectrum of interesting phenomena. Most notably, in quantum mechanics there exist scenarios where the contextual behavior is independent of the quantum state. We show that the quest for an optimal inequality separating quantum from classical noncontextual correlations in a state-independent manner admits an exact solution, as it can be formulated as a linear program. We introduce the noncontextuality polytope as a generalization of the locality polytope and apply our method to identify two different tight optimal inequalities for the most fundamental quantum scenario with state-independent contextuality.

  • 34.
    Lima, G
    et al.
    University of Concepcion.
    Inostroza, E B
    University of Concepcion.
    Vianna, R O
    University of Federal Minas Gerais.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Saavedra, C
    University of Concepcion.
    Optimal measurement bases for Bell tests based on the Clauser-Horne inequality2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 1, p. 012105-Article in journal (Refereed)
    Abstract [en]

    The Hardy test of nonlocality can be seen as a particular case of the Bell tests based on the Clauser-Horne (CH) inequality. Here we stress this connection when we analyze the relation between the CH-inequality violation, its threshold detection efficiency, and the measurement settings adopted in the test. It is well known that the threshold efficiencies decrease when one considers partially entangled states and that the use of these states, unfortunately, generates a reduction in the CH violation. Nevertheless, these quantities are both dependent on the measurement settings considered, and in this paper we show that there are measurement bases which allow for an optimal situation in this trade-off relation. These bases are given as a generalization of the Hardy measurement bases, and they will be relevant for future Bell tests relying on pairs of entangled qubits.

  • 35.
    Hiesmayr, Beatrix C
    et al.
    Faculty of Physics, University of Vienna, Vienna, Austria.
    Di Domenico, Antonio
    Sapienza Università di Roma and INFN Sezione di Roma, Rome, Italy.
    Curceanu, Catalina
    Laboratori Nazionali di Frascati dell’INFN, Frascati, Italy.
    Gabriel, Andreas
    Faculty of Physics, University of Vienna, Vienna, Austria.
    Huber, Marcus
    Faculty of Physics, University of Vienna, Vienna, Austria.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Moskal, Pawel
    Institute of Physics, Jagiellonian University, Cracow, Poland.
    Revealing Bells nonlocality for unstable systems in high energy physics2012In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, no 1, article id 1856Article in journal (Refereed)
    Abstract [en]

    Entanglement and its consequences—in particular the violation of Bell inequalities, which defies our concepts of realism and locality—have been proven to play key roles in Nature by many experiments for various quantum systems. Entanglement can also be found in systems not consisting of ordinary matter and light, i.e. in massive meson–antimeson systems. Bell inequalities have been discussed for these systems, but up to date no direct experimental test to conclusively exclude local realism was found. This mainly stems from the fact that one only has access to a restricted class of observables and that these systems are also decaying. In this Letter we put forward a Bell inequality for unstable systems which can be tested at accelerator facilities with current technology. Herewith, the long awaited proof that such systems at different energy scales can reveal the sophisticated “dynamical” nonlocal feature of Nature in a direct experiment gets feasible. Moreover, the role of entanglement and  violation, an asymmetry between matter and antimatter, is explored, a special feature offered only by these meson–antimeson systems

  • 36.
    Appleby, D. M.
    et al.
    Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada.
    Bengtsson, Ingemar
    Stockholms Universitet, AlbaNova, Fysikum, Stockholm, Sweden.
    Brierley, Stephen
    Department of Mathematics, University of Bristol, UK.
    Grassl, Markus
    Centre for Quantum Technologies, National University of Singapore, Singapore.
    Gross, David
    Institute for Theoretical Physics, ETH Zürich, 8093 Züurich, Switzerland.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    The monomial representations of the Clifford group2012In: Quantum information & computation, ISSN 1533-7146, Vol. 12, no 5-6, p. 404-431Article in journal (Refereed)
    Abstract [en]

    We show that the Clifford group-the normaliser of the Weyl-Heisenberg group-can be represented by monomial phase-permutation matrices if and only if the dimension is a square number. This simplifies expressions for SIC vectors, and has other applications to SICs and to Mutually Unbiased Bases. Exact solutions for SICs in dimension 16 are presented for the first time.

  • 37.
    Čirkić, Mirsad
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Persson, Daniel
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Gaussian Approximation of the LLR Distribution for the ML and Partial Marginalization MIMO detectors2011In: Proceedings of the IEEE International Conference on Acoustics, Speech and SignalProcessing (ICASSP), IEEE conference proceedings, 2011, p. 3232-3235Conference paper (Refereed)
    Abstract [en]

    We derive a Gaussian approximation of the LLR distribution  conditioned on the transmitted signal and the channel matrix for the  soft-output via partial marginalization MIMO detector. This detector  performs exact ML as a special case. Our main results consist of  discussing the operational meaning of this approximation and a proof  that, in the limit of high SNR, the LLR distribution of interest  converges in probability towards a Gaussian distribution.

  • 38.
    Kleinmann, Matthias
    et al.
    University of Siegen.
    Guehne, Otfried
    University of Siegen.
    Portillo, Jose R.
    University of Seville.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering. Linköping University, The Institute of Technology.
    Cabello, Adan
    University of Seville.
    Memory cost of quantum contextuality2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, no 113011Article in journal (Refereed)
    Abstract [en]

    The simulation of quantum effects requires certain classical resources, and quantifying them is an important step to characterize the difference between quantum and classical physics. For a simulation of the phenomenon of state-independent quantum contextuality, we show that the minimum amount of memory used by the simulation is the critical resource. We derive optimal simulation strategies for important cases and prove that reproducing the results of sequential measurements on a two-qubit system requires more memory than the information-carrying capacity of the system.

  • 39.
    Larsson, Jan-Åke
    et al.
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Wadströmer, Niclas
    Linköping University, Department of Electrical Engineering. Linköping University, The Institute of Technology.
    Hermanson, Ola
    Karolinska Institutet, Stockholm, Sweden.
    Lendahl, Urban
    Karolinska Institutet, Stockholm, Sweden.
    Forchheimer, Robert
    Linköping University, Department of Electrical Engineering. Linköping University, The Institute of Technology.
    Modelling cell lineage using a meta-Boolean tree model with a relation to gene regulatory networks2011In: Journal of Theoretical Biology, ISSN 0022-5193, Vol. 268, no 1, p. 62-76Article in journal (Refereed)
    Abstract [en]

    A cell lineage is the ancestral relationship between a group of cells that originate from a single founder cell. For example, in the embryo of the nematode Caenorhabditis elegans an invariant cell lineage has been traced, and with this information at hand it is possible to theoretically model the emergence of different cell types in the lineage, starting from the single fertilized egg. In this report we outline a modelling technique for cell lineage trees, which can be used for the C. elegans embryonic cell lineage but also extended to other lineages. The model takes into account both cell-intrinsic (transcription factor-based) and -extrinsic (extracellular) factors as well as synergies within and between these two types of factors. The model can faithfully recapitulate the entire C. elegans cell lineage, but is also general, i.e., it can be applied to describe any cell lineage. We show that synergy between factors, as well as the use of extrinsic factors, drastically reduce the number of regulatory factors needed for recapitulating the lineage. The model gives indications regarding co-variation of factors, number of involved genes and where in the cell lineage tree that asymmetry might be controlled by external influence. Furthermore, the model is able to emulate other (Boolean, discrete and differential-equation-based) models. As an example, we show that the model can be translated to the language of a previous linear sigmoid-limited concentration-based model (Geard and Wiles, 2005). This means that this latter model also can exhibit synergy effects, and also that the cumbersome iterative technique for parameter estimation previously used is no longer needed. In conclusion, the proposed model is general and simple to use, can be mapped onto other models to extend and simplify their use, and can also be used to indicate where synergy and external influence would reduce the complexity of the regulatory process.

  • 40.
    Badziag, Piotr
    et al.
    Stockholms University.
    Bengtsson, Ingemar
    Stockholms University.
    Cabello, Adan
    University of Seville.
    Granstrom, Helena
    Stockholms University.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Pentagrams and Paradoxes2011In: FOUNDATIONS OF PHYSICS, ISSN 0015-9018, Vol. 41, no 3, p. 414-423Article in journal (Refereed)
    Abstract [en]

    Klyachko and coworkers consider an orthogonality graph in the form of a pentagram, and in this way derive a Kochen-Specker inequality for spin 1 systems. In some low-dimensional situations Hilbert spaces are naturally organised, by a magical choice of basis, into SO(N) orbits. Combining these ideas some very elegant results emerge. We give a careful discussion of the pentagram operator, and then show how the pentagram underlies a number of other quantum "paradoxes", such as that of Hardy.

  • 41.
    Abidin, Aysajan
    et al.
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Pacher, Christoph
    Austrian Institute of Technology, Austria.
    Lorünser, Thomas
    Austrian Institute of Technology, Austria.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Peev, Momtchil
    Austrian Institute of Technology, Austria.
    Quantum cryptography and authentication with low key-consumption2011In: Proceedings of SPIE - The International Society for Optical Engineering, 2011, p. 818916-Conference paper (Refereed)
    Abstract [en]

    Quantum Key Distribution (QKD - also referred to as Quantum Cryptography) is a technique for secret key agreement. It has been shown that QKD rigged with Information-Theoretic Secure (ITS) authentication (using secret key) of the classical messages transmitted during the key distribution protocol is also ITS. Note, QKD without any authentication can trivially be broken by man-in-the-middle attacks. Here, we study an authentication method that was originally proposed because of its low key consumption; a two-step authentication that uses a publicly known hash function, followed by a secret strongly universal2 hash function, which is exchanged each round. This two-step authentication is not information-theoretically secure but it was argued that nevertheless it does not compromise the security of QKD. In the current contribution we study intrinsic weaknesses of this approach under the common assumption that the QKD adversary has access to unlimited resources including quantum memories. We consider one implementation of Quantum Cryptographic protocols that use such authentication and demonstrate an attack that fully extract the secret key. Even including the final key from the protocol in the authentication does not rule out the possibility of these attacks. To rectify the situation, we propose a countermeasure that, while not informationtheoretically secure, restores the need for very large computing power for the attack to work. Finally, we specify conditions that must be satisfied by the two-step authentication in order to restore informationtheoretic security.

  • 42.
    Löfvenberg, Jacob
    et al.
    Linköping University, Department of Electrical Engineering, Information Theory. Linköping University, The Institute of Technology.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Comments on "New Results on Frame-Proof Codes and Traceability Schemes"2010In: IEEE Transactions on Information Theory, ISSN 0018-9448, E-ISSN 1557-9654, Vol. 56, no 11, p. 5888-5889Article in journal (Other academic)
    Abstract [en]

    n/a

  • 43.
    Guehne, Otfried
    et al.
    Austrian Acadamy of Science.
    Kleinmann, Matthias
    Austrian Acadamy of Science.
    Cabello, Adan
    University of Seville.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Kirchmair, Gerhard
    Austrian Acadamy of Science.
    Zaehringer, Florian
    Austrian Acadamy of Science.
    Gerritsma, Rene
    Austrian Acadamy of Science.
    Roos, Christian F
    Austrian Acadamy of Science.
    Compatibility and noncontextuality for sequential measurements2010In: PHYSICAL REVIEW A, ISSN 1050-2947, Vol. 81, no 2, p. 022121-Article in journal (Refereed)
    Abstract [en]

    A basic assumption behind the inequalities used for testing noncontextual hidden variable models is that the observables measured on the same individual system are perfectly compatible. However, compatibility is not perfect in actual experiments using sequential measurements. We discuss the resulting "compatibility loophole" and present several methods to rule out certain hidden variable models that obey a kind of extended noncontextuality. Finally, we present a detailed analysis of experimental imperfections in a recent trapped-ion experiment and apply our analysis to that case.

  • 44.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Energy-time entanglement, Elements of Reality, and Local Realism2010In: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1232, p. 115-127Article in journal (Refereed)
    Abstract [en]

    This paper discusses energy-time entanglement experiments and their relation to Einstein-Podolsky-Rosen (EPR) elements of reality. The interferometric experiment proposed by J. D. Franson in 1989 provides the background, and the main issue here is a detailed discussion on whether a Local Realist model can give the Quantum-Mechanical predictions for this setup. The Franson interferometer gives the same interference pattern as the usual Bell experiment (modulo postselection). Even so, depending on the precise requirements made on the Local Realist model, this can imply a) no violation, b) smaller violation than usual, or c) full violation of the appropriate statistical bound. This paper discusses what requirements are necessary on the model to reach a violation, and the motivation for making these requirements. The alternatives include using a) only the measurement outcomes as EPR elements of reality, b) the emission time as EPR element of reality, and c) path realism. The subtleties of this discussion needs to be taken into account when designing and setting up future experiments of this kind, intended to test Local Realism.

  • 45.
    Cabello, Adan
    et al.
    University Seville.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Quantum contextuality for rational vectors2010In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 375, no 2, p. 99-99Article in journal (Refereed)
    Abstract [en]

    The Kochen-Specker theorem states that noncontextual hidden variable models are inconsistent with the quantum predictions for every yes-no question on a qutrit, corresponding to every projector in three dimensions. It has been suggested [D.A. Meyer, Phys. Rev. Lett. 83 (1999) 3751] that the inconsistency would disappear when restricting to projectors on unit vectors with rational components; that noncontextual hidden variables could reproduce the quantum predictions for rational vectors. Here we show that a qutrit state with rational components violates an inequality valid for noncontextual hidden-variable models [A.A. Klyachko et al., Phys. Rev. Lett. 101 (2008) 020403] using rational projectors. This shows that the inconsistency remains even when using only rational vectors.

  • 46.
    Cabello, Adan
    et al.
    University of Seville.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Rodriguez, David
    University of Seville.
    Eficiencia crítica para las desigualdades encadenadas de Braunstein y Caves2009In: XXXII Bienal de la Real Sociedad Española de Física (Ciudad Real, 7 al 11 de Septiembre de 2009), Real Sociedad Española de Física, Madrid, 2009, 529 / [ed] M. A. López de la Torre, J. A. de Toro and J. González, 2009Conference paper (Refereed)
  • 47.
    Fuchs, Christopher
    et al.
    Perimeter Institute for Theoretical Physics, Waterloo, Ontario Canada.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    Foreword: Unperformed experiments have no results2009In: FOUNDATIONSOF PROBABILITYAND PHYSICS - 5 / [ed] Luigi Accardi, Guillaume Adenier, Christopher A. Fuchs, Gregg Jaeger, Andrei Khrennikov, Jan-Åke Larsson, Stig Stenholm, Melville, New York: American Institute of Physics (AIP), 2009, p. 221-222Conference paper (Other academic)
    Abstract [en]

    This year in Växjö we thought we would try an experiment—it felt high timefor a new result. Much of the foundations discussion ofprevious years has focussed on EPR-style arguments and the meaningand experimental validity of various Bell inequality violations. Yet, thereis another pillar of the quantum foundations puzzle that hashardly received any attention in our great series of meetings:It is the phenomenon first demonstrated by Kochen and Specker,quantum contextuality. Recently there has been a rapid growth ofactivity aimed toward better understanding this aspect of quantum mechanics,which Asher Peres sloganized by the phrase, “unperformed experiments haveno results.” Below is a sampling of some important paperson the topic for the reader not yet familiar withthe subject. What is the source of this phenomenon? Doesit depend only on high level features of quantum mechanics,or is it deep in the conceptual framework on whichthe theory rests? Might it, for instance, arise from theway quantum mechanics amends the classic laws of probability? Whatare the mathematically simplest ways contextuality can be demonstrated? Howmight the known results be made amenable to experimental tests?These were the sorts of discussions we hoped the sessionwould foster.

  • 48.
    Cabello, Adan
    et al.
    Departamento de Física Aplicada II, Universidad de Sevilla,, Spain.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Rodriguez, David
    Departamento de Física Aplicada II, Universidad de Sevilla, Spain.
    Minimum detection efficiency required for a loophole-free violation of the Braunstein-Caves chained Bell inequalities2009In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 79, no 6, p. 062109-1-062109-7Article in journal (Refereed)
    Abstract [en]

    The chained Bell inequalities of Braunstein and Caves involving N settings per observer have some interesting applications. Here we obtain the minimum detection efficiency required for a loophole-free violation of the Braunstein-Caves inequalities for any N greater than= 2. We discuss both the case in which both particles are detected with the same efficiency and the case in which the particles are detected with different efficiencies.

  • 49.
    Axell, Erik
    et al.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Erik G.
    Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
    Larsson, Jan-Åke
    Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
    On the Optimal K-term Approximation of a Sparse Parameter Vector MMSE Estimate2009In: Proceedings of the 2009 IEEE Workshop on Statistical Signal Processing (SSP'09), IEEE , 2009, p. 245-248Conference paper (Refereed)
    Abstract [en]

    This paper considers approximations of marginalization sums thatarise in Bayesian inference problems. Optimal approximations ofsuch marginalization sums, using a fixed number of terms, are analyzedfor a simple model. The model under study is motivated byrecent studies of linear regression problems with sparse parametervectors, and of the problem of discriminating signal-plus-noise samplesfrom noise-only samples. It is shown that for the model understudy, if only one term is retained in the marginalization sum, thenthis term should be the one with the largest a posteriori probability.By contrast, if more than one (but not all) terms are to be retained,then these should generally not be the ones corresponding tothe components with largest a posteriori probabilities.

  • 50.
    Abidin, Aysajan
    et al.
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Larsson, Jan-Åke
    Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
    Special Properties of Strongly Universal2 Hash Functions Important in Quantum Cryptography2009In: AIP Conference Proceedings, ISSN 0094-243X, Foundations of Probability and Physics—5, Växjö, augusti 2008, New York: American Institute of Physics , 2009, Vol. 1101, p. 289-293Conference paper (Refereed)
    Abstract [en]

    Secure message authentication is an important part of Quantum Key Distribution. In this paper we analyze special properties of a Strongly Universal2 hash function family, an understanding of which is important in the security analysis of the authentication used in Quantum Cryptography. We answer the following question: How much of Alices message does Eve need to influence so that the message along with its tag will give her enough information to create the correct tag for her message?

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