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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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Kurochkin, Yury
    Russian Quantum Ctr, Russia; QRate, Russia.
    Lamas-Linares, Antia
    Univ Texas Austin, TX 78712 USA.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    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)2019Ingår i: Laser Physics Letters, ISSN 1612-2011, E-ISSN 1612-202X, Vol. 16, nr 1, artikel-id 019401Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    n/a

  • 2.
    Johansson, Niklas
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Quantum Simulation Logic, Oracles, and the Quantum Advantage2019Ingår i: Entropy, ISSN 1099-4300, E-ISSN 1099-4300, Vol. 21, nr 8, artikel-id 800Artikel i tidskrift (Refereegranskat)
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    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.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    You, L.
    Yuan, X.
    Zeilinger, A.
    Zhang, Q.
    Zhang, W.
    Zhong, J.
    Challenging Local Realism with Human Choices2018Ingår i: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 557, s. 212-216Artikel i tidskrift (Refereegranskat)
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Vallone, Giuseppe
    Univ Padua, Italy; CNR, Italy.
    Villoresi, Paolo
    Univ Padua, Italy; CNR, Italy.
    Postselection-Loophole-Free Bell Violation with Genuine Time-Bin Entanglement2018Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, nr 19, artikel-id 190401Artikel i tidskrift (Refereegranskat)
    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.
    Jogenfors, Jonathan
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Adán, Cabello
    Departamento de Física Aplicada II, Universidad de Sevilla, Sevilla, Spain.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Comment on "Franson Interference Generated by a Two-Level System"2017Övrigt (Övrigt vetenskapligt)
    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.

  • 6.
    Johansson, Niklas
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Efficient classical simulation of the Deutsch-Jozsa and Simons algorithms2017Ingår i: Quantum Information Processing, ISSN 1570-0755, E-ISSN 1573-1332, Vol. 16, nr 9, artikel-id UNSP 233Artikel i tidskrift (Refereegranskat)
    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.

  • 7.
    Tomasin, Marco
    et al.
    University of Padua, Italy; UOS Padova, Italy.
    Mantoan, Elia
    University of Padua, Italy; UOS Padova, Italy.
    Jogenfors, Jonathan
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Vallone, Giuseppe
    University of Padua, Italy; UOS Padova, Italy.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Villoresi, Paolo
    University of Padua, Italy; UOS Padova, Italy.
    High-visibility time-bin entanglement for testing chained Bell inequalities2017Ingår i: Physical Review A, ISSN 2469-9926, Vol. 95, nr 3, artikel-id 032107Artikel i tidskrift (Refereegranskat)
    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.

  • 8.
    Jogenfors, Jonathan
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Tight bounds for the Pearle-Braunstein-Caves chained inequality without the fair-coincidence assumption2017Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 96, nr 2, artikel-id 022102Artikel i tidskrift (Refereegranskat)
    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.

  • 9.
    Pacher, Christoph
    et al.
    Department of Safety & Security, AIT Austrian Institute of Technology, Austria.
    Abidin, Aysajan
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Attacks on quantum key distribution protocols that employ non-ITS authentication2016Ingår i: Quantum Information Processing, ISSN 1570-0755, E-ISSN 1573-1332, Vol. 15, nr 1, s. 327-362Artikel i tidskrift (Refereegranskat)
    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.

  • 10.
    Hiesmayr, Beatrix C.
    et al.
    Faculty of Physics, University of Vienna, Austria.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Contextuality and nonlocality in decaying multipartite systems2016Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 93, nr 2, s. 020106(R)-Artikel i tidskrift (Refereegranskat)
    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.

  • 11.
    Kofler, Johannes
    et al.
    Max Planck Institute Quantum Opt MPQ, Germany.
    Giustina, Marissa
    Austrian Academic Science, Austria; University of Vienna, Austria.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Mitchell, Morgan W.
    Barcelona Institute Science and Technology, Spain; ICREA, Spain.
    Requirements for a loophole-free photonic Bell test using imperfect setting generators2016Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 93, nr 3, s. 032115-Artikel i tidskrift (Refereegranskat)
    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.

  • 12.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Wiesner, Karoline
    University of Bristol, England.
    Thermodynamical cost of some interpretations of quantum theory2016Ingår i: PHYSICAL REVIEW A, ISSN 2469-9926, Vol. 94, nr 5, artikel-id 052127Artikel i tidskrift (Refereegranskat)
    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.

  • 13.
    Dzhafarov, Ehtibar N.
    et al.
    Purdue University, IN 47907 USA.
    Kujala, Janne V.
    University of Jyvaskyla, Finland.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Contextuality in Three Types of Quantum-Mechanical Systems2015Ingår i: Foundations of physics, ISSN 0015-9018, E-ISSN 1572-9516, Vol. 45, nr 7, s. 762-782Artikel i tidskrift (Refereegranskat)
    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.

  • 14.
    Jogenfors, Jonathan
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Hacking the Bell test using classical light in energy-time entanglement–based quantum key distribution2015Ingår i: Science Advances, ISSN 2375-2548, Vol. 1, nr 11, s. 1-7, artikel-id e1500793Artikel i tidskrift (Refereegranskat)
    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.

  • 15.
    Kujala, Janne V.
    et al.
    University of Jyvaskyla, Finland.
    Dzhafarov, Ehtibar N.
    Purdue University, IN 47907 USA.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Necessary and Sufficient Conditions for an Extended Noncontextuality in a Broad Class of Quantum Mechanical Systems2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, nr 15, s. 150401-Artikel i tidskrift (Refereegranskat)
    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.

  • 16.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    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 Network2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, nr 3, s. 030503-Artikel i tidskrift (Refereegranskat)
    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.

  • 17.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    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 Photons2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, nr 25, s. 250401-Artikel i tidskrift (Refereegranskat)
    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.

  • 18.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    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 related2014Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, nr 1, s. 012102-Artikel i tidskrift (Refereegranskat)
    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.

  • 19.
    Larsson, Jan-Åke
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    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 loophole2014Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 90, nr 3, s. 032107-Artikel i tidskrift (Refereegranskat)
    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.

  • 20.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Bounding the quantum dimension with contextuality2014Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, nr 6Artikel i tidskrift (Refereegranskat)
    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.

  • 21.
    Abidin, Aysajan
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Direct proof of security of Wegman-Carter authentication with partially known key2014Ingår i: Quantum Information Processing, ISSN 1570-0755, E-ISSN 1573-1332, Vol. 13, nr 10, s. 2155-2170Artikel i tidskrift (Refereegranskat)
    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.

  • 22.
    Jogenfors, Jonathan
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Energy-time entanglement, elements of reality, and local realism2014Ingår i: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 47, nr 42, s. 424032-Artikel i tidskrift (Refereegranskat)
    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.

  • 23.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Loopholes in Bell inequality tests of local realism2014Ingår i: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 47, nr 42, s. 424003-Artikel, forskningsöversikt (Refereegranskat)
    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.

  • 24.
    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
    Linköpings universitet, Matematiska institutionen, Matematik och tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Grassl, Markus
    Centre for Quantum Technologies, National University of Singapore, Singapore.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Systems of Imprimitivity for the Clifford Group2014Ingår i: Quantum information & computation, ISSN 1533-7146, Vol. 14, nr 3-4, s. 339-360Artikel i tidskrift (Refereegranskat)
    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).

  • 25.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Wiesniak, M.
    Institute of Theoretical Physics and Astrophysics, University of Gdańsk, Poland .
    Editorial Material: Comment on "State-Independent Experimental Test of Quantum Contextuality"2013Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, nr 7, s. 1-1Artikel i tidskrift (Övrigt vetenskapligt)
    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.

  • 26.
    Pettersson, Sofia
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Forchheimer, Robert
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska fakulteten.
    Meta-Boolean models of asymmetric division patterns in the C. elegans intestinal lineage: Implications for the posterior boundary of intestinal twist2013Ingår i: Worm, ISSN 2162-4054, Vol. 2, artikel-id e23701Artikel i tidskrift (Refereegranskat)
    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.

  • 27.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan. Linköpings universitet, Tekniska fakulteten.
    A contextual extension of Spekkens' toy model2012Ingår i: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1424, s. 211-220Artikel i tidskrift (Refereegranskat)
    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.

  • 28.
    Cirkic, Mirsad
    et al.
    Linköpings universitet, Institutionen för systemteknik, Kommunikationssystem. Linköpings universitet, Tekniska högskolan.
    Persson, Daniel
    Linköpings universitet, Institutionen för systemteknik, Kommunikationssystem. Linköpings universitet, Tekniska högskolan.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Larsson, Erik G.
    Linköpings universitet, Institutionen för systemteknik, Kommunikationssystem. Linköpings universitet, Tekniska högskolan.
    Approximating the LLR Distribution for a Class of Soft-Output MIMO Detectors2012Ingår i: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 60, nr 12, s. 6421-6434Artikel i tidskrift (Refereegranskat)
    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.

  • 29.
    Kleinmann, Matthias
    et al.
    University of Siegen, Germany .
    Budroni, Costantino
    University of Siegen, Germany .
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Guehne, Otfried
    University of Siegen, Germany .
    Cabello, Adan
    University of Seville, Spain .
    Optimal Inequalities for State-Independent Contextuality2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 25, s. 250402-Artikel i tidskrift (Refereegranskat)
    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.

  • 30.
    Lima, G
    et al.
    University of Concepcion.
    Inostroza, E B
    University of Concepcion.
    Vianna, R O
    University of Federal Minas Gerais.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Saavedra, C
    University of Concepcion.
    Optimal measurement bases for Bell tests based on the Clauser-Horne inequality2012Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, nr 1, s. 012105-Artikel i tidskrift (Refereegranskat)
    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.

  • 31.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    Moskal, Pawel
    Institute of Physics, Jagiellonian University, Cracow, Poland.
    Revealing Bells nonlocality for unstable systems in high energy physics2012Ingår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 1, artikel-id 1856Artikel i tidskrift (Refereegranskat)
    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

  • 32.
    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
    Linköpings universitet, Institutionen för systemteknik, Informationskodning. Linköpings universitet, Tekniska högskolan.
    The monomial representations of the Clifford group2012Ingår i: Quantum information & computation, ISSN 1533-7146, Vol. 12, nr 5-6, s. 404-431Artikel i tidskrift (Refereegranskat)
    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.

  • 33.
    Kleinmann, Matthias
    et al.
    University of Siegen.
    Guehne, Otfried
    University of Siegen.
    Portillo, Jose R.
    University of Seville.
    Larsson, Jan-Åke
    Linköpings universitet, Institutionen för systemteknik. Linköpings universitet, Tekniska högskolan.
    Cabello, Adan
    University of Seville.
    Memory cost of quantum contextuality2011Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, nr 113011Artikel i tidskrift (Refereegranskat)
    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.

  • 34.
    Larsson, Jan-Åke
    et al.
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Wadströmer, Niclas
    Linköpings universitet, Institutionen för systemteknik. Linköpings universitet, Tekniska högskolan.
    Hermanson, Ola
    Karolinska Institutet, Stockholm, Sweden.
    Lendahl, Urban
    Karolinska Institutet, Stockholm, Sweden.
    Forchheimer, Robert
    Linköpings universitet, Institutionen för systemteknik. Linköpings universitet, Tekniska högskolan.
    Modelling cell lineage using a meta-Boolean tree model with a relation to gene regulatory networks2011Ingår i: Journal of Theoretical Biology, ISSN 0022-5193, Vol. 268, nr 1, s. 62-76Artikel i tidskrift (Refereegranskat)
    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.

  • 35.
    Badziag, Piotr
    et al.
    Stockholms University.
    Bengtsson, Ingemar
    Stockholms University.
    Cabello, Adan
    University of Seville.
    Granstrom, Helena
    Stockholms University.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Pentagrams and Paradoxes2011Ingår i: FOUNDATIONS OF PHYSICS, ISSN 0015-9018, Vol. 41, nr 3, s. 414-423Artikel i tidskrift (Refereegranskat)
    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.

  • 36.
    Löfvenberg, Jacob
    et al.
    Linköpings universitet, Institutionen för systemteknik, Informationsteori. Linköpings universitet, Tekniska högskolan.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Comments on "New Results on Frame-Proof Codes and Traceability Schemes"2010Ingår i: IEEE Transactions on Information Theory, ISSN 0018-9448, E-ISSN 1557-9654, Vol. 56, nr 11, s. 5888-5889Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    n/a

  • 37.
    Guehne, Otfried
    et al.
    Austrian Acadamy of Science.
    Kleinmann, Matthias
    Austrian Acadamy of Science.
    Cabello, Adan
    University of Seville.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    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 measurements2010Ingår i: PHYSICAL REVIEW A, ISSN 1050-2947, Vol. 81, nr 2, s. 022121-Artikel i tidskrift (Refereegranskat)
    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.

  • 38.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Energy-time entanglement, Elements of Reality, and Local Realism2010Ingår i: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1232, s. 115-127Artikel i tidskrift (Refereegranskat)
    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.

  • 39.
    Cabello, Adan
    et al.
    University Seville.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Quantum contextuality for rational vectors2010Ingår i: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 375, nr 2, s. 99-99Artikel i tidskrift (Refereegranskat)
    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.

  • 40.
    Cabello, Adan
    et al.
    Departamento de Física Aplicada II, Universidad de Sevilla,, Spain.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    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 inequalities2009Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 79, nr 6, s. 062109-1-062109-7Artikel i tidskrift (Refereegranskat)
    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.

  • 41.
    Abidin, Aysajan
    et al.
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Vulnerability of "A Novel Protocol-Authentication Algorithm Ruling out a Man-in-the-Middle Attack in Quantum Cryptography"2009Ingår i: International Journal of Quantum Information, ISSN 0219-7499, Vol. 7, nr 5, s. 1047-1052Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper, we review and comment on "A novel protocol-authentication algorithm ruling out a man-in-the-middle attack in quantum cryptography" [M. Peev et al., Int. J. Quant. Inf. 3 (2005) 225]. In particular, we point out that the proposed primitive is not secure when used in a generic protocol, and needs additional authenticating properties of the surrounding quantum-cryptographic protocol.

  • 42.
    Larsson, Jan-Åke
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Matematiska institutionen, Tillämpad matematik.
    Book review: Quantum Mechanics at the Crossroads, James Evans, Alan S. Thorndike. Springer, Berlin (2007)2008Ingår i: Studies in history and philosophy of modern physics, ISSN 1355-2198, E-ISSN 1879-2502, Vol. 39, s. 229-230Artikel i tidskrift (Övrigt vetenskapligt)
  • 43.
    Cederlöf, Jörgen
    et al.
    Linköpings universitet, Matematiska institutionen. Linköpings universitet, Tekniska högskolan.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Security aspects of the Authentication used in Quantum Cryptography2008Ingår i: IEEE Transactions on Information Theory, ISSN 0018-9448, E-ISSN 1557-9654, Vol. 54, nr 4, s. 1735-1741Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Unconditionally secure message authentication is an important part of Quantum Cryptography (QC). We analyze security effects of using a key obtained from QC for authentication purposes in later rounds of QC. In particular, the eavesdropper gains partial knowledge on the key in QC that may have an effect on the security of the authentication in the later round. Our initial analysis indicates that this partial knowledge has little effect on the authentication part of the system, in agreement with previous results on the issue. However, when taking the full QC protocol into account, the picture is different. By accessing the quantum channel used in QC, the attacker can change the message to be authenticated. This together with partial knowledge of the key does incur a security weakness of the authentication. The underlying reason for this is that the authentication used, which is insensitive to such message changes when the key is unknown, becomes sensitive when used with a partially known key. We suggest a simple solution to this problem, and stress usage of this or an equivalent extra security measure in QC.

  • 44.
    Seevinck, Michael P.
    et al.
    Institute for History and Foundations of Science, Utrecht University, The Netherlands.
    Larsson, Jan-Åke
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Matematiska institutionen, Tillämpad matematik.
    Comment on "A local realist model for correlations of the singlet state" by K. De Raedt, K. Keimpema, H. De Raedt, K. Michielsen and S. Miyashita2007Ingår i: The European Physical Journal B, ISSN 1434-6028, Vol. 58, nr 1, s. 51-53Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    De Raedt et al. [Eur. Phys. J. B 53, 139 (2006)] have claimed to provide a local realist model for correlations of the singlet state in the familiar Einstein-Podolsky-Rosen-Bohm (EPRB) experiment when time-coincidence is used to decide which detection events should count in the analysis, and furthermore that this suggests that it is possible to construct local realistic models that can reproduce the quantum mechanical expectation values. In this letter we show that these conclusions cannot be upheld since their model exploits the so-called coincidence-time loophole. When this is properly taken into account no startling conclusions can be drawn about local realist modelling of quantum mechanics.

  • 45.
    Cabello, Adan
    et al.
    Departamento de Física Aplicada II, Universidad de Sevilla, Spain.
    Larsson, Jan-Åke
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Matematiska institutionen, Tillämpad matematik.
    Minimum Detection Efficiency for a Loophole-Free Atom-Photon Bell Experiment2007Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 98, s. 220402-1-220402-4, artikel-id 220402Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In Bell experiments, one problem is to achieve high enough photodetection to ensure that there is no possibility of describing the results via a local hidden-variable model. Using the Clauser-Horne inequality and a two-photon nonmaximally entangled state, a photodetection efficiency higher than 0.67 is necessary. Here we discuss atom-photon Bell experiments. We show that, assuming perfect detection efficiency of the atom, it is possible to perform a loophole-free atom-photon Bell experiment whenever the photodetection efficiency exceeds 0.50.

  • 46.
    Bengtsson, Ingemar
    et al.
    Stockholms Universitet, AlbaNova, Fysikum, Stockholm, Sweden.
    Bruzda, Wojciech
    Instytut Fizyki im. Smoluchowskiego, Uniwersytet Jagielloński, Kraków, Poland.
    Ericsson, Åsa
    Stockholms Universitet, AlbaNova, Fysikum, Stockholm, Sweden.
    Larsson, Jan-Åke
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Matematiska institutionen, Tillämpad matematik.
    Tadej, Wojciech
    Wydział Matematyczno-Przyrodniczy, Szkoła Nauk Ścisłych, Universytet Kardynała Stefana Wyszyńskiego, Warszawa, Poland.
    Zyczkowski, Karol
    Instytut Fizyki im. Smoluchowskiego, Uniwersytet Jagielloński, Kraków, Poland and Centrum Fizyki Teoretycznej, Polska Akademia Nauk, Warszawa, Poland.
    Mutually unbiased bases and Hadamard matrices of order six2007Ingår i: Journal of Mathematical Physics, ISSN 0022-2488, E-ISSN 1089-7658, Vol. 48, nr 5, s. 052106-1-052106-21Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on a search for mutually unbiased bases (MUBs) in six dimensions. We find only triplets of MUBs, and thus do not come close to the theoretical upper bound 7. However, we point out that the natural habitat for sets of MUBs is the set of all complex Hadamard matrices of the given order, and we introduce a natural notion of distance between bases in Hilbert space. This allows us to draw a detailed map of where in the landscape the MUB triplets are situated. We use available tools, such as the theory of the discrete Fourier transform, to organize our results. Finally, we present some evidence for the conjecture that there exists a four dimensional family of complex Hadamard matrices of order 6. If this conjecture is true the landscape in which one may search for MUBs is much larger than previously thought.

  • 47.
    Larsson, Jan-Åke
    Linköpings universitet, Matematiska institutionen, Tillämpad matematik. Linköpings universitet, Tekniska högskolan.
    Bell inequalities for position measurements2004Ingår i: Physical Review A, ISSN 1050-2947, Vol. 70, nr 2, s. 022102-1-022102-5Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The bits of the binary expansion of position measurement results were used to derive Bell inequalities for position measurements. The output state of the nondegenerate optical parametric amplifier (NOPA) was used to obtain violations of these inequalities. It was shown that the position operator itself, together with other suitable operators, also can be used to violate the Bell inequality, deriving a Bell inequality more suited to the original Einstein-Podolsky-Rosen (EPR) setting. It was concluded that the NOPA state cannot be described by a local realist model, despite having a strictly positive Wigner function.

  • 48.
    Larsson, Jan-Åke
    et al.
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Matematiska institutionen, Tillämpad matematik.
    Gill, Richard D
    Department of Mathematics, University of Utrecht, Utrecht, The Netherlands / EURANDOM, Eindhoven, The Netherlands.
    Bell's inequality and the coincidence-time loophole2004Ingår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 67, nr 5, s. 707-713Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper analyzes effects of time dependence in the Bell inequality. A generalized inequality is derived for the case when coincidence and non-coincidence (and hence whether or not a pair contributes to the actual data) is controlled by timing that depends on the detector settings. Needless to say, this inequality is violated by quantum mechanics and could be violated by experimental data provided that the loss of measurement pairs through failure of coincidence is small enough, but the quantitative bound is more restrictive in this case than in the previously analyzed "efficiency loophole".

  • 49.
    Larsson, Jan-Åke
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Matematiska institutionen, Tillämpad matematik.
    No information flow using statistical fluctuations and quantum cryptography2004Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 69, nr 4, s. 042317-1-042317-8, artikel-id 42317Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The use of entanglement by quantum-cryptographic protocol to transfer the data was discussed. The detection of individual eavesdropping attack on each qubit was detected by the security test where the qubits provides the key, and there exists a coherent attack internal to these groups, which goes unnoticed in security tests. The result shows that the level of the individual qubits also detect the coherent attack by testing equality for the measurements. A modified test was proposed to ensure security against a coherent attack.

  • 50.
    Gill, Richard D.
    et al.
    Mathematical Institute, Utrecht University, Netherlands.
    Larsson, Jan-Åke
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Matematiska institutionen, Tillämpad matematik.
    Accardi contra Bell (cum mundi): The Impossible Coupling2003Ingår i: Mathematical Statistics and Applications: Festschrift for Constance van Eeden / [ed] Marc Moore, Sorana Froda & Christian Léger, Hayward, CA: Institute of Mathematical Statistic , 2003, s. 133-154Kapitel i bok, del av antologi (Övrigt vetenskapligt)
    Abstract [en]

    An experimeotally observed violation of Bell's inequality i.s suppooed to show the failuro of local realism to deal with quantum roality. However, finite statistics and the Lime sequential nature of real experiments still allows a loophole for local roalism. We show that the raodomised design of the Aspect experiment closes this loophole. Our main tool is van de Geer's (1995, 2000) martingale version of the classical Bernstein (1924) incquality guaranteeing, at the  root n scale, a not-beavier-than-Gaussian tail of the distribution of a sum of bouoded supermartingale dilferences. The results are used to specify a protocol for a public bet between the author and L. Accardi, who in recont papers (Aocardi and Regal.i, 2000a, b, 2001; Accardi, lmafuku and Regoli, 2002) has claimed to have produced a suite of computer programmes, to be run on a network of computers, wbich will simulate a violation of Bell's inequalites. At a sarnple size of twenty five thousand, botb error probabilities are guaranteed smaller than about one in a million, provided we adhere to tho sequential randomized design while Accardi aims for tbe greatest possible violation allowed by quantum mechanics.

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