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Breaking the Unbreakable: Exploiting Loopholes in Bell’s Theorem to Hack Quantum Cryptography
Linköping University, Department of Electrical Engineering, Information Coding. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-8032-1466
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. , p. 239
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1875
National Category
Atom and Molecular Physics and Optics Communication Systems
Identifiers
URN: urn:nbn:se:liu:diva-140912DOI: 10.3384/diss.diva-140912ISBN: 9789176854600 (print)OAI: oai:DiVA.org:liu-140912DiVA, id: diva2:1150887
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
List of papers
1. Energy-time entanglement, elements of reality, and local realism
Open this publication in new window or tab >>Energy-time entanglement, elements of reality, and local realism
2014 (English)In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 47, no 42, p. 424032-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
Keywords
bell inequalities; energy-time entanglement; elements of reality
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-112643 (URN)10.1088/1751-8113/47/42/424032 (DOI)000344222200033 ()
Available from: 2014-12-05 Created: 2014-12-05 Last updated: 2017-12-05
2. Hacking the Bell test using classical light in energy-time entanglement–based quantum key distribution
Open this publication in new window or tab >>Hacking the Bell test using classical light in energy-time entanglement–based quantum key distribution
Show others...
2015 (English)In: Science Advances, ISSN 2375-2548, Vol. 1, no 11, p. 1-7, article id e1500793Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Association for the Advancement of Science, 2015
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:liu:diva-114210 (URN)10.1126/sciadv.1500793 (DOI)000216604200020 ()
Note

At the time for thesis presentation publication was in status: Manuscript

At the time for thesis presentation name of publication was: A Classical-Light Attack on Energy-Time Entangled Quantum Key Distribution, and Countermeasures

Available from: 2015-02-13 Created: 2015-02-13 Last updated: 2018-03-09Bibliographically approved
3. Tight bounds for the Pearle-Braunstein-Caves chained inequality without the fair-coincidence assumption
Open this publication in new window or tab >>Tight bounds for the Pearle-Braunstein-Caves chained inequality without the fair-coincidence assumption
2017 (English)In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 96, no 2, article id 022102Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2017
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:liu:diva-139910 (URN)10.1103/PhysRevA.96.022102 (DOI)000406669400003 ()
Available from: 2017-08-23 Created: 2017-08-23 Last updated: 2017-11-29
4. High-visibility time-bin entanglement for testing chained Bell inequalities
Open this publication in new window or tab >>High-visibility time-bin entanglement for testing chained Bell inequalities
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2017 (English)In: Physical Review A, ISSN 2469-9926, Vol. 95, no 3, article id 032107Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2017
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-136599 (URN)10.1103/PhysRevA.95.032107 (DOI)000395983300003 ()
Available from: 2017-04-21 Created: 2017-04-21 Last updated: 2017-10-20
5. Quantum Bitcoin: An Anonymous and Distributed Currency Secured by the No-Cloning Theorem of Quantum Mechanics
Open this publication in new window or tab >>Quantum Bitcoin: An Anonymous and Distributed Currency Secured by the No-Cloning Theorem of Quantum Mechanics
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The digital currency Bitcoin has had remarkable growth since it was first proposed in 2008. Its distributed nature allows currency transactions without a central authority by using cryptographic methods and a data structure called the blockchain. In this paper we use the no-cloning theorem of quantum mechanics to introduce Quantum Bitcoin, a Bitcoin-like currency that runs on a quantum computer. We show that our construction of quantum shards and two blockchains allows untrusted peers to mint quantum money without risking the integrity of the currency. The Quantum Bitcoin protocol has several advantages over classical Bitcoin, including immediate local verification of transactions. This is a major improvement since we no longer need the computationally intensive and time-consuming method Bitcoin uses to record all transactions in the blockchain. Instead, Quantum Bitcoin only records newly minted currency which drastically reduces the footprint and increases efficiency. We present formal security proofs for counterfeiting resistance and show that a quantum bitcoin can be re-used a large number of times before wearing out - just like ordinary coins and banknotes. Quantum Bitcoin is the first distributed quantum money system and we show that the lack of a paper trail implies full anonymity for the users. In addition, there are no transaction fees and the system can scale to any transaction volume.

Keywords
Quantum Bitcoin, Bitcoin, Quantum Computing
National Category
Computer Sciences
Identifiers
urn:nbn:se:liu:diva-129217 (URN)
Available from: 2016-06-13 Created: 2016-06-13 Last updated: 2018-01-10Bibliographically approved
6. Comment on "Franson Interference Generated by a Two-Level System"
Open this publication in new window or tab >>Comment on "Franson Interference Generated by a Two-Level System"
2017 (English)Other (Other academic)
Abstract [en]

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

Place, publisher, year, pages
Ithaca, New York, USA: Cornell University Press, 2017. p. 1
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:liu:diva-142073 (URN)
Available from: 2017-10-20 Created: 2017-10-20 Last updated: 2017-10-20

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