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• 51.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Bell Inequalities for Position Measurements2003In: Joint MaPhySto and QUANTOP Workshop on Quantum Measurements and Quantum Stochastics,2003, 2003Conference paper (Other academic)
• 52.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Bell Inequalities for Position Measurements2003In: Mysteries, Puzzles and Paradoxes in Quantum Mechanics,2003, 2003Conference paper (Other academic)
• 53.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Bell Inequalities for Position Measurements2003In: Quantum Theory-reconsideration of foundations-2,2003, 2003Conference paper (Other academic)
• 54.
Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
Bell inequalities for position measurements2004In: Physical Review A, ISSN 1050-2947, Vol. 70, no 2, p. 022102-1-022102-5Article in journal (Refereed)

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.

• 55.
Bell’s inequality and detector inefficiency1998In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 57, no 5, p. 3304-3308Article in journal (Refereed)

In this paper, a method of generalizing the Bell inequality is presented that makes it possible to include detector inefficiency directly in the original Bell inequality. To enable this, the concept of “change of ensemble” will be presented, providing both qualitative and quantitative information on the nature of the “loophole” in the proof of the original Bell inequality. In a local hidden-variable model lacking change of ensemble, the generalized inequality reduces to an inequality that quantum mechanics violates as strongly as the original Bell inequality, irrespective of the level of efficiency of the detectors. A model that contains change of ensemble lowers the violation, and a bound for the level of change is obtained. The derivation of the bound in this paper is not dependent upon any symmetry assumptions such as constant efficiency, or even the assumption of independent errors.

• 56.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Bell's inequality and the coincidence-time loophole2004In: Workshop i tillämpad matematik,2004, 2004Conference paper (Other academic)
• 57.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Book review: Quantum Mechanics at the Crossroads, James Evans, Alan S. Thorndike. Springer, Berlin (2007)2008In: Studies in history and philosophy of modern physics, ISSN 1355-2198, E-ISSN 1879-2502, Vol. 39, p. 229-230Article in journal (Other academic)
• 58.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Communication of Mathematics' as a tool to improve students' general communicative skills2007In: The 3rd International CDIO Conference,,2007, Cambridge, MA: MIT , 2007Conference paper (Refereed)

This is an account of an attempt to improve students- communicative skills, with a focus on mathematics. The intent is to give the students skill and experience in communicating in an environment where precision is important, both in mathematics and science in general, but also in engineering. The first part of the course is intended to improve the students- ability to follow a logical argument, especially long (even infinite) chains of logical arguments. Later parts of the course focus more on the practice of presentation of, discussion of, and writing mathematics. Examination is not by a written exam, the examination consists of students' participation in oral presentations and the ensuing discussions, a one-page handwritten hand-in at the start of the course, and finally a short typed piece on a suitable mathematical topic. Experiences from this first attempt are discussed, and the most striking effect is the visibly improving oral communication skills of the students as the course proceeds. There are also indications that participation in this course is beneficial to later mathematics courses, but only for the able students. We do expect an improved overall performance of the students but there is no clear effect as yet, partly because there has not passed enough (read -any-) time after the finished course, but perhaps also because the sample is small.

• 59.
Detector efficiency in the Greenberger-Horne-Zeilinger paradox: independent errors1999In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 59, no 6, p. 4801-4804Article in journal (Refereed)

The Greenberger-Horne-Zeilinger (GHZ) paradox is subject to the detector-efficiency “loophole” in a similar manner as the Bell inequality. In a paper by J.-Å. Larsson [Phys. Rev. A 57, R3145 (1998)], the issue is investigated for very general assumptions. Here, the assumptions of constant efficiency and independent errors will be imposed, and it will be shown that the necessary and sufficient efficiency bound is not lowered, but remains at 75%. An explicit local-variable model is constructed in this paper to show the necessity of this bound. In other words, it is not possible to use the independence of experimental nondetection errors to rule out local realism in the GHZ paradox below 75% efficiency.

• 60.
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)

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.

• 61.
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)

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.

• 62.
Modeling the singlet state with local variables1999In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 256, no 4, p. 245-252Article in journal (Refereed)

A local-variable model yielding the statistics from the singlet state is presented for the case of inefficient detectors and/or lowered visibility. It has independent errors and the highest efficiency at perfect visibility is 77.80%, while the highest visibility at perfect detector-efficiency is 63.66%. The model cannot be refuted by measurements made to date.

• 63.
Necessary and sufficient detector-efficiency conditions for the Greenberger-Horne-Zeilinger paradox1998In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 57, no 5, p. R3145-R3149Article in journal (Refereed)

In this paper detector efficiency conditions are derived for the Greenberger-Horne-Zeilinger (GHZ) paradox. The conditions will be necessary and sufficient, i.e., the GHZ paradox is explicable in terms of a local-variable model if the efficiency is below the bounds, and the GHZ prerequisites are inconsistent at higher efficiencies. The derivation does not make use of any of the symmetry assumptions usually made in the literature, most notably the assumption of independent errors. The errors in local-hidden-variable models are governed by the “hidden variable” and, therefore, one cannot in general assume that the errors are independent. It will be shown that this assumption is not necessary. Moreover, bounds are presented that do not need the emission rate of particle triples to be known. An example of such a bound is the ratio of the triple coincidence rate and the double coincidence rate at two detectors, which needs to be higher than 75% to yield a contradiction.

• 64.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
No information flow using statistical fluctuations and quantum cryptography2004In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 69, no 4, p. 042317-1-042317-8, article id 42317Article in journal (Refereed)

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.

• 65.
Quantum paradoxes, probability theory, and change of ensemble2000Doctoral thesis, comprehensive summary (Other academic)

In this thesis, the question "What kind of models can be used to describe microcosmos?" will be discussed. Being difficult and very large in scope, the question has here been restricted to whether or not Local Realistic models can be used to describe Quantum-Mechanical processes, one of a collection of questions often referred to as Quantum Paradoxes. Two such paradoxes will be investigated using techniques from probability theory: the Bell inequality and the Greenberger-Horne-Zeilinger (GHZ) paradox.

A problem with the two mentioned paradoxes is that they are only valid when the detectors are 100% efficient, whereas present experimental efficiency is much lower than that. Here, an approach is presented which enables a generalization of both the Bell inequality and the GHZ paradox to the inefficient case. This is done by introducing the concept of change of ensemble, which provides both qualitative and quantitative information on the nature of the "loophole" in the 100% efficiency prerequisite, and is more fundamental in this regard than the efficiency concept. Efficiency estimates are presented which are easy to obtain from experimental coincidence data, and a connection is established between these estimates and the concept of change of ensemble.

The concept is also studied in the context of Franson interferometry, where the Bell inequality cannot immediately be used. Unexpected subtleties occur when trying to establish whether or not a Local Realistic model of the data is possible even in the ideal case. A Local Realistic model of the experiment is presented, but nevertheless, by introducing an additional requirement on the experimental setup it is possible to refute the mentioned model and show that no other Local Realistic model exists.

1. Bell’s inequality and detector inefficiency
Open this publication in new window or tab >>Bell’s inequality and detector inefficiency
1998 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 57, no 5, p. 3304-3308Article in journal (Refereed) Published
##### Abstract [en]

In this paper, a method of generalizing the Bell inequality is presented that makes it possible to include detector inefficiency directly in the original Bell inequality. To enable this, the concept of “change of ensemble” will be presented, providing both qualitative and quantitative information on the nature of the “loophole” in the proof of the original Bell inequality. In a local hidden-variable model lacking change of ensemble, the generalized inequality reduces to an inequality that quantum mechanics violates as strongly as the original Bell inequality, irrespective of the level of efficiency of the detectors. A model that contains change of ensemble lowers the violation, and a bound for the level of change is obtained. The derivation of the bound in this paper is not dependent upon any symmetry assumptions such as constant efficiency, or even the assumption of independent errors.

##### Place, publisher, year, edition, pages
American Physical Society, 1998
##### National Category
Engineering and Technology
##### Identifiers
urn:nbn:se:liu:diva-87169 (URN)10.1103/PhysRevA.57.3304 (DOI)
Available from: 2013-01-11 Created: 2013-01-11 Last updated: 2019-08-16Bibliographically approved
2. Modeling the singlet state with local variables
Open this publication in new window or tab >>Modeling the singlet state with local variables
1999 (English)In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 256, no 4, p. 245-252Article in journal (Refereed) Published
##### Abstract [en]

A local-variable model yielding the statistics from the singlet state is presented for the case of inefficient detectors and/or lowered visibility. It has independent errors and the highest efficiency at perfect visibility is 77.80%, while the highest visibility at perfect detector-efficiency is 63.66%. The model cannot be refuted by measurements made to date.

##### Place, publisher, year, edition, pages
Elsevier, 1999
Mathematics
##### Identifiers
urn:nbn:se:liu:diva-38610 (URN)10.1016/S0375-9601(99)00236-4 (DOI)44987 (Local ID)44987 (Archive number)44987 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2018-09-07Bibliographically approved
3. Necessary and sufficient detector-efficiency conditions for the Greenberger-Horne-Zeilinger paradox
Open this publication in new window or tab >>Necessary and sufficient detector-efficiency conditions for the Greenberger-Horne-Zeilinger paradox
1998 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 57, no 5, p. R3145-R3149Article in journal (Refereed) Published
##### Abstract [en]

In this paper detector efficiency conditions are derived for the Greenberger-Horne-Zeilinger (GHZ) paradox. The conditions will be necessary and sufficient, i.e., the GHZ paradox is explicable in terms of a local-variable model if the efficiency is below the bounds, and the GHZ prerequisites are inconsistent at higher efficiencies. The derivation does not make use of any of the symmetry assumptions usually made in the literature, most notably the assumption of independent errors. The errors in local-hidden-variable models are governed by the “hidden variable” and, therefore, one cannot in general assume that the errors are independent. It will be shown that this assumption is not necessary. Moreover, bounds are presented that do not need the emission rate of particle triples to be known. An example of such a bound is the ratio of the triple coincidence rate and the double coincidence rate at two detectors, which needs to be higher than 75% to yield a contradiction.

##### Place, publisher, year, edition, pages
American Physical Society, 1998
Mathematics
##### Identifiers
urn:nbn:se:liu:diva-38612 (URN)10.1103/PhysRevA.57.R3145 (DOI)44989 (Local ID)44989 (Archive number)44989 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2019-08-16Bibliographically approved
4. Detector efficiency in the Greenberger-Horne-Zeilinger paradox: independent errors
Open this publication in new window or tab >>Detector efficiency in the Greenberger-Horne-Zeilinger paradox: independent errors
1999 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 59, no 6, p. 4801-4804Article in journal (Refereed) Published
##### Abstract [en]

The Greenberger-Horne-Zeilinger (GHZ) paradox is subject to the detector-efficiency “loophole” in a similar manner as the Bell inequality. In a paper by J.-Å. Larsson [Phys. Rev. A 57, R3145 (1998)], the issue is investigated for very general assumptions. Here, the assumptions of constant efficiency and independent errors will be imposed, and it will be shown that the necessary and sufficient efficiency bound is not lowered, but remains at 75%. An explicit local-variable model is constructed in this paper to show the necessity of this bound. In other words, it is not possible to use the independence of experimental nondetection errors to rule out local realism in the GHZ paradox below 75% efficiency.

##### Place, publisher, year, edition, pages
American Physical Society, 1999
Mathematics
##### Identifiers
urn:nbn:se:liu:diva-38611 (URN)10.1103/PhysRevA.59.4801 (DOI)44988 (Local ID)44988 (Archive number)44988 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2019-08-16Bibliographically approved
5. Two-photon Franson-type experiments and local realism
Open this publication in new window or tab >>Two-photon Franson-type experiments and local realism
1999 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 83, no 15, p. 2872-2876Article in journal (Refereed) Published
##### Abstract [en]

The two-photon interferometric experiment proposed by J. D. Franson [Phys. Rev. Lett. 62, 2205 (1989)] is often treated as a “Bell test of local realism.” However, it has been suggested that this is incorrect due to the 50% postselection performed even in the ideal gedanken version of the experiment. Here we present a simple local hidden variable model of the experiment that successfully explains the results obtained in usual realizations of the experiment, even with perfect detectors. Furthermore, we also show that there is no such model if the switching of the local phase settings is done at a rate determined by the internal geometry of the interferometers.

##### Place, publisher, year, edition, pages
American Physical Society, 1999
Mathematics
##### Identifiers
urn:nbn:se:liu:diva-38609 (URN)10.1103/PhysRevLett.83.2872 (DOI)44986 (Local ID)44986 (Archive number)44986 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2019-08-16Bibliographically approved
6. A possible unification of the Copenhagen and the Bohm interpretations using local realism
Open this publication in new window or tab >>A possible unification of the Copenhagen and the Bohm interpretations using local realism
2000 (English)In: Foundations of physics letters, ISSN 0894-9875, E-ISSN 1572-9524, Vol. 13, no 5, p. 477-486Article in journal (Refereed) Published
##### Abstract [en]

It is well-known in the physics community that the Copenhagen interpretation of quantum mechanics is very different from the Bohm interpretation. Usually, a local realistic model is thought to be even further from these two, as in its purest form it cannot even yield the probabilities from quantum mechanics by the Bell theorem. Nevertheless, by utilizing the “efficiency loophole” such a model can mimic the quantum probabilities, and more importantly, in this paper it is shown that it is possible to interpret this latter kind of local realistic model such that it contains elements of reality as found in the Bohm interpretation, while retaining the complementarity present in the Copenhagen interpretation.

##### Place, publisher, year, edition, pages
Springer Netherlands, 2000
Mathematics
##### Identifiers
urn:nbn:se:liu:diva-38607 (URN)10.1023/A:1007884816456 (DOI)44945 (Local ID)44945 (Archive number)44945 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2019-08-16Bibliographically approved
• 66.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Qubits from number states and Bell inequalities for number measurements2003In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 67, p. 022108-1-022108-8, article id 022108Article in journal (Refereed)

Bell inequalities for number measurements are derived via the observation that the bits of the number indexing a number state are proper qubits. Violations of these inequalities are obtained from the output state of the nondegenerate optical parametric amplifier.

• 67.
Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
The Kochen-Specker Paradox and Great-Circle Descents2009In: Foundations of Probability and Physics—5 / [ed] Accardi, L; Adenier, G; Fuchs, CA; Jaeger, G; Khrennikov, A; Larsson, JA; Stenholm, S, Melville, NY, USA: American Institute of Physics (AIP), 2009, Vol. 1101, p. 280-286Conference paper (Other academic)

The Kochen-Specker paradox has recently been subject to experimental interest, and in this situation the number of steps in the proof in question is important. The fewer number of steps there are in the proof, the more imperfections can be tolerated in the experimental setup. In the spin-1 version of the Kochen-Specker paradox, when the settings used are directions in three-dimensional space, the proofs can be easily visualized and the steps can easily be counted. In particular, the original Kochen-Specker paradox makes use of so-called great-circle descents. Here, we will examine such descents in detail and also some other versions of the proof for spin-1 systems. We will see that, perhaps contrary to intuition, the proofs that use a small number of steps do not in general use only great-circle descents, and examine the reasons for this and possible extensions. At least one new proof will also be presented for the spin-1 case.

• 68.
Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
The quantum and the random: Similarities, differences, and "contradictions"2006In: Quantum Theory: Reconsideration of Foundations,2005, Melville, NY, USA: American Institute of Physics (AIP), 2006, p. 353-359Conference paper (Other academic)

This paper makes an attempt to present Quantum Mechanics in simple terms, but without oversimplification. It will be in essence, a comparison of the usual understanding of randomness to the more difficult notion of "quantumness." The restricted format of this paper will unfortunately force the presentation to be very terse. But perhaps this paper can be seen as a synopsis of the approach I have in mind.

• 69.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Security aspects of the authentication used in quantum key growing2006In: Foundations of Probability and Physics,2006, 2006Conference paper (Other academic)
• 70.
Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
Security aspects of the authentication used in quantum key growing2006In: Advanced Free-Space Optical Communication Techniques/Applications III,2006 / [ed] Sjöqvist, LJ; Wilson, RA; Merlet, TJ, Bellingham, WA ,USA: SPIE - International Society for Optical Engineering, 2006, p. 63990H-Conference paper (Other academic)
• 71.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Bell's inequality and the coincidence-time loophole2005In: Foundations of probability and physics,2004, New York: American Institute of Physics , 2005, p. 228-Conference paper (Refereed)
• 72.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
Department of Mathematics, University of Utrecht, Utrecht, The Netherlands / EURANDOM, Eindhoven, The Netherlands.
Bell's inequality and the coincidence-time loophole2004In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 67, no 5, p. 707-713Article in journal (Refereed)

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".

• 73.
Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
Austrian Academic Science, Austria. Max Planck Institute Quantum Opt MPQ, Germany. Austrian Academic Science, Austria. Austrian Academic Science, Austria. 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)

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.

• 74.
Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
Universität Siegen, Germany. Universidad de Sevilla, Spain. Universität Siegen, Germany. 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)

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.

• 75.
Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
Centre for Quantum Computation, Clarendon Laboratory University of Oxford, UK.
Strict detector-efficiency bounds for n-site Clauser-Horne inequalities2001In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 63, p. 022117-1-022117-5, article id 022117Article in journal (Refereed)

An analysis of detector-efficiency in many-site Clauser-Horne inequalities is presented for the case of perfect visibility. It is shown that there is a violation of the presented n-site Clauser-Horne inequalities if and only if the efficiency is greater than n/(2n−1). Thus, for a two-site two-setting experiment there are no quantum-mechanical predictions that violate local realism unless the efficiency is greater than $\tiny\frac 23$. Second, there are n-site experiments for which the quantum-mechanical predictions violate local realism whenever the efficiency exceeds $\tiny\frac 12$.

• 76.
Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology.
Linköping University, Department of Electrical Engineering. Linköping University, The Institute of Technology. Karolinska Institutet, Stockholm, Sweden. Karolinska Institutet, Stockholm, Sweden. 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)

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.

• 77.
University of Concepcion.
University of Concepcion. University of Federal Minas Gerais. Linköping University, Department of Mathematics, Applied Mathematics. Linköping University, The Institute of Technology. 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)

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.

• 78.
Linköping University, Department of Electrical Engineering, Information Theory. Linköping University, The Institute of Technology.
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)

n/a

• 79.
Department of Safety & Security, AIT Austrian Institute of Technology, Austria.
Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology. Department of Safety & Security, AIT Austrian Institute of Technology, Austria. Department of Safety & Security, AIT Austrian Institute of Technology, Austria. Institut für Experimentalphysik, Universität Wien, Austria and Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Austria. Institut für Experimentalphysik, Universität Wien, Austria and Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Austria. 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)

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.

• 80.
Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering. 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)

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.

• 81.
Institute for History and Foundations of Science, Utrecht University, The Netherlands.
Linköping University, The Institute of Technology. Linköping University, Department of Mathematics, Applied Mathematics.
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. Miyashita2007In: The European Physical Journal B, ISSN 1434-6028, Vol. 58, no 1, p. 51-53Article in journal (Refereed)

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.

• 82.
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)

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.

• 83.
University of Padua, Italy University of Roma La Sapienza, Italy .
University of Concepcion, Chile . University of Concepcion, Chile . University of Concepcion, Chile . Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology. University of Roma La Sapienza, Italy Ist Nazl Ott INO CNR, Italy . 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)

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.

• 84.
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)

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.

• 85.
Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology.
Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology. Linköping University, Department of Electrical Engineering, Communication Systems. Linköping University, The Institute of Technology. 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)

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.

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