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Energy-time entanglement, elements of reality, and local realism
Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.
Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-1082-8325
2014 (English)In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 47, no 42, 424032- p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
IOP Publishing: Hybrid Open Access , 2014. Vol. 47, no 42, 424032- p.
Keyword [en]
bell inequalities; energy-time entanglement; elements of reality
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:liu:diva-112643DOI: 10.1088/1751-8113/47/42/424032ISI: 000344222200033OAI: oai:DiVA.org:liu-112643DiVA: diva2:769083
Available from: 2014-12-05 Created: 2014-12-05 Last updated: 2017-12-05
In thesis
1. A Classical-Light Attack on Energy-Time Entangled Quantum Key Distribution, and Countermeasures
Open this publication in new window or tab >>A Classical-Light Attack on Energy-Time Entangled Quantum Key Distribution, and Countermeasures
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum key distribution (QKD) is an application of quantum mechanics that allowstwo parties to communicate with perfect secrecy. Traditional QKD uses polarization of individual photons, but the development of energy-time entanglement could lead to QKD protocols robust against environmental effects. The security proofs of energy-time entangled QKD rely on a violation of the Bell inequality to certify the system as secure. This thesis shows that the Bell violation can be faked in energy-time entangled QKD protocols that involve a postselection step, such as Franson-based setups. Using pulsed and phase-modulated classical light, it is possible to circumvent the Bell test which allows for a local hidden-variable model to give the same predictions as the quantum-mechanical description. We show that this attack works experimentally and also how energy-time-entangled systems can be strengthened to avoid our attack.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 60 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1709
Keyword
Quantum Key Distribution, Energy-Time Entanglement, Quantum Information, Kvantkryptering, Energi-Tid-Snärjning, Kvantinformation
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-114073 (URN)10.3384/lic.diva-114073 (DOI)978-91-7519-118-8 (ISBN)
Presentation
2015-02-27, Visionen, B-huset, Campus Valla, Linköpings Universitet, Linköping, 13:15 (English)
Opponent
Supervisors
Available from: 2015-02-18 Created: 2015-02-06 Last updated: 2015-02-18Bibliographically approved
2. Breaking the Unbreakable: Exploiting Loopholes in Bell’s Theorem to Hack Quantum Cryptography
Open this publication in new window or tab >>Breaking the Unbreakable: Exploiting Loopholes in Bell’s Theorem to Hack Quantum Cryptography
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

En viktig konsekvens av kvantmekaniken är att okända kvanttillstånd inte kan klonas. Denna insikt har gett upphov till kvantkryptering, en metod för två parter att med perfekt säkerhet kommunicera hemligheter. Ett komplett bevis för denna säkerhet har dock låtit vänta på sig eftersom en attackerare i hemlighet kan manipulera utrustningen så att den läcker information. Som ett svar på detta utvecklades apparatsoberoende kvantkryptering som i teorin är immun mot sådana attacker.

Apparatsoberoende kvantkryptering har en mycket högre grad av säkerhet än vanlig kvantkryptering, men det finns fortfarande ett par luckor som en attackerare kan utnyttja. Dessa kryphål har tidigare inte tagits på allvar, men denna avhandling visar hur även små svagheter i säkerhetsmodellen läcker information till en attackerare. Vi demonstrerar en praktisk attack där attackeraren aldrig upptäcks trots att denne helt kontrollerar systemet. Vi visar också hur kryphålen kan förhindras med starkare säkerhetsbevis.

En annan tillämpning av kvantmekanikens förbud mot kloning är pengar som använder detta naturens egna kopieringsskydd. Dessa kvantpengar har helt andra egenskaper än vanliga mynt, sedlar eller digitala banköverföringar. Vi visar hur man kan kombinera kvantpengar med en blockkedja, och man får då man en slags "kvant-Bitcoin". Detta nya betalningsmedel har fördelar över alla andra betalsystem, men nackdelen är att det krävs en kvantdator.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. 239 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1875
National Category
Atom and Molecular Physics and Optics Communication Systems
Identifiers
urn:nbn:se:liu:diva-140912 (URN)10.3384/diss.diva-140912 (DOI)9789176854600 (ISBN)
Public defence
2017-11-17, Ada Lovelace, B House, Campus Valla, Linköping, 13:00 (English)
Opponent
Supervisors
Available from: 2017-10-23 Created: 2017-10-20 Last updated: 2017-10-23Bibliographically approved

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Jogenfors, JonathanLarsson, Jan-Åke

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