| 1. |
- Lindehammer, Sabina, et al.
(författare)
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Temporal trends of HLA genotype frequencies of type 1 diabetes patients in Sweden from 1986 to 2005 suggest altered risk
- 2008
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Ingår i: Acta Diabetologica. - : Springer Science and Business Media LLC. - 0940-5429 .- 1432-5233. ; 45:4, s. 231-5
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to compare the frequency of human leukocyte antigen (HLA) genotypes in 1-18-year-old patients with type 1 diabetes newly diagnosed in 1986-1987 (n = 430), 1996-2000 (n = 342) and in 2003-2005 (n = 171). We tested the hypothesis that the HLA DQ genotype distribution changes over time. Swedish type 1 diabetes patients and controls were typed for HLA using polymerase chain reaction amplification and allele specific probes for DQ A1* and B1* alleles. The most common type 1 diabetes HLA DQA1*-B1*genotype 0501-0201/0301-0302 was 36% (153/430) in 1986-1987 and 37% (127/342) in 1996-2000, but decreased to 19% (33/171) in 2003-2005 (P \ 0.0001). The 0501-0201/0501-0201 genotype increased from 1% in 1986-1987 to 7% in 1996-2000 (P = 0.0047) and to 5% in 2003-2005 (P > 0.05). This study in 1-18-year-old Swedish type 1 diabetes patients supports the notion that there is a temporal change in HLA risk.
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| 2. |
- Johansson, Niklas, 1987-
(författare)
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A Resource for Quantum Computation
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we address the question, what is the resource, or property, that enables the advantage of quantum computers? The theory of quantum computers dates back to the eighties, so one would think there already is an answer to this question. There are several proposed solutions, but to this date, there is no consensus on an answer. Primarily, the advantage of quantum computers is characterized by a speedup for certain computational problems. This speedup is measured by comparing quantum algorithms with the best-known classical algorithms. For some algorithms we assume access to an object called oracle. The oracle computes a function, and the complexity of the oracle is of no concern. Instead, we count the number of queries to the oracle needed to solve the problem. Informally, the question we ask using an oracle is: if we can compute this function efficiently, what else could we then compute. However, using oracles while measuring a quantum speedup, we assume access to vastly different oracles residing in different models of computation.For our investigation of the speedup, we introduce a classical simulation framework that imitates quantum algorithms. The simulation suggests that the property enabling the potential quantum speedup is the ability to store, process, and retrieve information in an additional degree of freedom. We then theoretically verified that this is true for all problems that can be efficiently solved with a quantum computer.In parallel to this, we also see that quantum oracles sharply specify the information we can retrieve from the additional degree of freedom, while regular oracles do not. A regular oracle does not even allow for an extra degree of freedom. We conclude that comparing quantum with classical oracle query complexity bounds does not provide conclusive evidence for a quantum advantage.
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| 3. |
- Johansson, Niklas, 1987-
(författare)
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On the Power of Quantum Computation: Oracles
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Quantum computation solve some computational problems faster than the best-known alternative in classical computation. The evidence for this consists of examples where a quantum algorithm outperforms the best-known classical algorithm. A large body of these examples relies on oracle query complexity, where the performance (complexity) of the algorithms is measured by the number of times they need to access an oracle. Here, an oracle is usually considered to be a black box that computes a specific function at unit cost.However, the quantum algorithm is given access to an oracle with more structure than the classical algorithm. This thesis argues that the two oracles are so vastly different that comparing quantum and classical query complexity should not be considered evidence, but merely hints for a quantum advantage.The approach used is based on a model that can be seen as an approximation of quantum theory, but can be efficiently simulated on a classical computer. This model solves several oracular problems with the same performance as their quantum counterparts, showing that there is no genuine quantum advantage for these problems. This approach also clarifies the assumptions made in quantum computation, and which properties that can be seen as resources in these algorithms.
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| 4. |
- Abellán, C., et al.
(författare)
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Challenging Local Realism with Human Choices
- 2018
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Ingår i: Nature. - : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 557, s. 212-216
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
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| 6. |
- Aerts, Sven, et al.
(författare)
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Two-photon Franson-type experiments and local realism
- 1999
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 83:15, s. 2872-2876
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Alarcón Cuevas, Alvaro, 1991-
(författare)
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A Few-Mode-Fiber Platform for Quantum Communication Applications
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Society as we know it today would not have been possible without the explosive and astonishing development of telecommunications systems, and optical fibers have been one of the pillars of these technologies.Despite the enormous amount of data being transmitted over optical networks today, the trend is that the demand for higher bandwidths will also increase. Given this context, a central element in the design of telecommunications networks will be data security, since information can often be confidential or private.Quantum information emerges as a solution to encrypt data by quantum key distribution (QKD) between two users. This technique uses the properties of nature as the fundamentals of operation rather than relying on mathematical constructs to provide data protection. A popular alternative to performing QKD is to use the relative phase between two individual photon paths for information encoding. However, this method was not practical over long distances. The time-bin- based scheme was a solution to the previous problem given its practical nature, however, it introduces intrinsic losses due to its design, which increases with the dimension of the encoded quantum system.In this thesis we have designed and tested a fiber-optic platform using spatial-division- multiplexing techniques. The use of few-mode fibers and photonic lanterns are the cornerstone of our proposal, which also allow us to support orbital angular momentum (OAM) modes. The platform builds on the core ideas of the phase-coded quantum communication system and also takes advantage of the benefits proposed by the time-bin scheme. We have experimentally tested our proposal by successfully transmitting phase-coded single-photon states over 500 m few-mode fiber, demonstrating the feasibility of our scheme. We demonstrated the successful creation of OAM states, their propagation and their successful detection in an all in-fiber scheme. Our platform eliminates the post-selection losses of time-bin quantum communication systems and ensures compatibility with next-generation optical networks and opens up new possibilities for quantum communication.
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| 8. |
- Bengtsson, Ingemar, et al.
(författare)
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Mutually unbiased bases and Hadamard matrices of order six
- 2007
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Ingår i: Journal of Mathematical Physics. - : American Institute of Physics (AIP). - 0022-2488 .- 1089-7658. ; 48:5, s. 052106-1-052106-21
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Cabello, Adan, et al.
(författare)
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Minimum Detection Efficiency for a Loophole-Free Atom-Photon Bell Experiment
- 2007
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 98, s. 220402-1-220402-4
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Cederlöf, Jörgen, et al.
(författare)
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Security aspects of the Authentication used in Quantum Cryptography
- 2008
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Ingår i: IEEE Transactions on Information Theory. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9448 .- 1557-9654. ; 54:4, s. 1735-1741
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 11. |
- Gill, Richard D., et al.
(författare)
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Accardi contra Bell (cum mundi) : The Impossible Coupling
- 2003
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Ingår i: Mathematical Statistics and Applications. - Hayward, CA : Institute of Mathematical Statistic. - 0940600579 - 9780940600577 ; , s. 133-154
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- 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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| 12. |
- Jogenfors, Jonathan, 1988-
(författare)
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Breaking the Unbreakable : Exploiting Loopholes in Bell’s Theorem to Hack Quantum Cryptography
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we study device-independent quantum key distribution based on energy-time entanglement. This is a method for cryptography that promises not only perfect secrecy, but also to be a practical method for quantum key distribution thanks to the reduced complexity when compared to other quantum key distribution protocols. However, there still exist a number of loopholes that must be understood and eliminated in order to rule out eavesdroppers. We study several relevant loopholes and show how they can be used to break the security of energy-time entangled systems. Attack strategies are reviewed as well as their countermeasures, and we show how full security can be re-established.Quantum key distribution is in part based on the profound no-cloning theorem, which prevents physical states to be copied at a microscopic level. This important property of quantum mechanics can be seen as Nature's own copy-protection, and can also be used to create a currency based on quantummechanics, i.e., quantum money. Here, the traditional copy-protection mechanisms of traditional coins and banknotes can be abandoned in favor of the laws of quantum physics. Previously, quantum money assumes a traditional hierarchy where a central, trusted bank controls the economy. We show how quantum money together with a blockchain allows for Quantum Bitcoin, a novel hybrid currency that promises fast transactions, extensive scalability, and full anonymity.
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| 13. |
- Jogenfors, Jonathan, 1988-, et al.
(författare)
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Comment on "Franson Interference Generated by a Two-Level System"
- 2017
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 14. |
- Johansson, Niklas, 1987-, et al.
(författare)
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Conjugate Logic
- 2023
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Ingår i: The Quantum-Like Revolution. - Cham : Springer. - 9783031129858 - 9783031129865 ; , s. 157-180
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Bokkapitel (refereegranskat)abstract
- We propose a conjugate logic that can capture the behavior of quantum and quantum-like systems. The proposal is similar to the more generic concept of epistemic logic: it encodes knowledge or perhaps more correctly, predictions about outcomes of future observations on some systems. For a quantum system, these predictions are statements about future outcomes of measurements performed on specific degrees of freedom of the system. The proposed logic will include propositions and their relations, including connectives, but importantly also transformations between propositions on conjugate degrees of freedom of the systems. A key point is the addition of a transformation that allows to convert propositions about single systems into propositions about correlations between systems. We will see that subtle choices of the properties of the transformations lead to drastically different underlying mathematical models; one choice gives stabilizer quantum mechanics, while another choice gives Spekkens’ toy theory. This points to a crucial basic property of quantum and quantum-like systems that can be handled within the present conjugate logic by adjusting the mentioned choice. It also enables a discussion on what behaviors are properly quantum or only quantum-like, relating to that choice and how it manifests in the system under scrutiny.
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| 15. |
- Johansson, Niklas, 1987-, et al.
(författare)
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Quantum Simulation Logic, Oracles, and the Quantum Advantage
- 2019
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Ingår i: Entropy. - : MDPI. - 1099-4300. ; 21:8
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 16. |
- Larsson, Jan-Åke, 1969-
(författare)
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A contextual extension of Spekkens' toy model
- 2012
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Ingår i: AIP Conference Proceedings. - : American Institute of Physics (AIP). - 0094-243X .- 1551-7616. ; 1424, s. 211-220
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 17. |
- Larsson, Jan-Åke, 1969-
(författare)
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A Kochen-Specker inequality
- 2002
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Ingår i: Europhysics letters. - : Institute of Physics (IOP). - 0295-5075 .- 1286-4854. ; 58:6, s. 799-805
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Tidskriftsartikel (refereegranskat)abstract
- By probabilistic means, the concept of contextuality is extended so that it can be used in non-ideal situations. An inequality is presented, which at least in principle enables a test to discard non-contextual hidden-variable models at low error rates, in the spirit of the Kochen-Specker theorem. Assuming that the errors are independent, an explicit error bound of 1.42% is derived, below which a Kochen-Specker contradiction occurs.
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| 18. |
- Larsson, Jan-Åke, 1969-
(författare)
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A possible unification of the Copenhagen and the Bohm interpretations using local realism
- 2000
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Ingår i: Foundations of physics letters. - : Springer Netherlands. - 0894-9875 .- 1572-9524. ; 13:5, s. 477-486
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 19. |
- Larsson, Jan-Åke, 1969-
(författare)
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A practical Trojan Horse for Bell-inequality-based quantum cryptography
- 2002
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Ingår i: Quantum information & computation. - : Rinton Press. - 1533-7146. ; 2:6, s. 434-442
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Tidskriftsartikel (refereegranskat)abstract
- Quantum Cryptography, or more accurately, Quantum Key Distribution (QKD) is based on using an unconditionally secure "quantum channel" to share a secret key among two users. A manufacturer of QKD devices could, intentionally or not, use a (semi-) classical channel instead of the quantum channel, which would remove the supposedly unconditional security. One example is the BB84 protocol, where the quantum channel can be implemented in polarization of single photons. Here, use of several photons instead of one to encode each bit of the key provides a similar but insecure system. For protocols based on violation of a Bell inequality (e.g., the Ekert protocol) the situation is somewhat different. While the possibility is mentioned by some authors, it is generally thought that an implementation of a (semi-) classical channel will differ significantly from that of a quantum channel. Here, a counterexample will be given using an identical physical setup as is used in photon-polarization Ekert QKD. Since the physical implementation is identical, a manufacturer may include this modification as a Trojan Horse in manufactured systems, to be activated at will by an eavesdropper. Thus, the old truth of cryptography still holds: you have to trust the manufacturer of your cryptographic device. Even when you do violate the Bell inequality.
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| 20. |
- Larsson, Jan-Åke, 1969-
(författare)
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A probabilistic inequality for the Kochen-Specker paradox
- 2001
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Ingår i: Foundations of Probability and Physics,2000. - Singapore : World Scientific. - 9789812810809 ; , s. 236-245
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Konferensbidrag (refereegranskat)abstract
- A probabilistic version of the Kochen-Specker paradox is presented. The paradox is restated in the form of an inequality relating probabilities from a non-contextual hidden-variable model, by formulating the concept of "probabilistic contextuality." This enables an experimental test for contextuality at low experimental error rates. Using the assumption of independent errors, an explicit error bound of 0.71% is derived, below which a Kochen-Specker contradiction occurs.
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| 21. |
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| 22. |
- Larsson, Jan-Åke, 1969-
(författare)
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Bell inequalities for position measurements
- 2004
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Ingår i: Physical Review A. - : American Physical Society. - 1050-2947. ; 70:2, s. 022102-1-022102-5
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 23. |
- Larsson, Jan-Åke, 1969-
(författare)
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Bell’s inequality and detector inefficiency
- 1998
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 57:5, s. 3304-3308
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 24. |
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| 25. |
- Larsson, Jan-Åke, 1969-, et al.
(författare)
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Bell's inequality and the coincidence-time loophole
- 2004
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Ingår i: Europhysics letters. - : Institute of Physics (IOP). - 0295-5075 .- 1286-4854. ; 67:5, s. 707-713
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Tidskriftsartikel (refereegranskat)abstract
- 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".
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