New technologies for broadband quantum key distribution: Sources, detectors, and systems

Daniel J. Rogers

Published Nov 12, 2008• 0 citations

Abstract

In this thesis I describe three independent projects that advance the development of broadband quantum cryptography. While each project pertains to a different part of the QKD chain, together they provide key developments in implementing QKD at bit rates that are practical for use in the modern telecommunications infrastructure. The first project comprises the bulk of the thesis and involves developing a novel source of correlated photon pairs for use in free-space QKD. This source is based on a birefringent semiconductor optical waveguide as a Kerr medium. We demonstrate the feasibility of using birefringent phase-matched four-wave mixing to generate correlated photon pairs. We further propose that, by reversing the process and pumping with conjugate wavelengths, one can use the same effect to produce entangled photon pairs with the same device. These pairs can then be used for QKD to realize the most secure and efficient quantum cryptographic data links. The second project examines the implications of operating a BB84 QKD protocol at clock rates that are faster than the recovery time of the constituent detectors. We show that operating such systems under conventional protocols results in a security violation that allows an eavesdropper to learn significant information about the key and present a modification to the BB84 protocol that maintains key security at fast transmission rates. This modification to the protocol will become vital to QKD viability as links become faster and clock rates go into the tens of gigahertz. We also demonstrate, rather counterintuitively, that there exists an optimal transmission rate for a QKD system that exceeds the inverse of an individual detector's dead time. The final project describes a new design for a free-space QKD link that centers around faster silicon detectors. These detectors have a peak quantum efficiency in the visible range, requiring that the system operate at a wavelength that is more susceptible to solar interference. To mitigate this effect, the link is designed around a Fraunhofer line in the solar spectrum where the background solar light levels are reduced by up to 90%. By implementing this system, we expect at least a two-fold increase in the secret key rate, coming ever closer to the goal of a 10 Mb/s QKD system compatible with first-generation ethernet technology.

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Real world quantum cryptography

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Quantum cryptography uses the quantum properties of individual photons to provide two or more users with means to communicate securely and efficiently. Specifically, quantum key distribution (QKD) focuses on the secure communication between two distant parties and guarantees the security of a transmitted message between them. Although QKD is the most mature application of quantum cryptography, applications of quantum mechanics in communication systems are not limited to QKD and other cryptographic protocols can be implemented as well. This thesis focuses on the implementation of quantum cryptographic systems over deployed fiber. A first QKD system was built and used to demonstrate the BB84 protocol over deployed fiber between the University of Calgary and SAIT. The system implemented a novel tool called quantum frames that allow for the quantum signals to be routed, for clock synchronization, and for channel stabilization in a network scenario. The same system was used to examine the scalability of the secret key generation rate in different steps of the process in order to detect potential bottlenecks for key generation rate of high speed systems. A second QKD system was developed to demonstrate the measurement-device independent QKD protocol. The system worked over deployed fiber between the University of Calgary, SAIT, and Foothills Hospital in the City of Calgary. The advantage of the measurement-device independent QKD protocol over previous QKD protocols is that it eliminates the possibility of side-channel attacks that target single photon detectors used in QKD systems. This demonstration also involved the first demonstration of a Bell state measurement over deployed fiber. Bell state measurements were further studied by using novel single photon detectors, which allowed demonstrating a highly efficient Bell measurement. Finally, a cryptographic protocol known as private queries was demonstrated between the University of Calgary and SAIT. Quantum private queries were demonstrated for the first time over deployed fiber by developing and implementing an error correction scheme to make the protocol noise and loss tolerant.

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Focus on Quantum Cryptography: Theory and Practice

Norbert Lütkenhaus, A. J. Shields

Quantum cryptography, and especially quantum key distribution (QKD), is steadily progressing to become a viable tool for cryptographic services. In recent years we have witnessed a dramatic increase in the secure bit rate of QKD, as well as its extension to ever longer fibre- and air-based links and the emergence of metro-scale trusted networks. In the foreseeable future even global-scale communications may be possible using quantum repeaters or Earth–satellite links. A handful of start-ups and some bigger companies are already active in the field. The launch of an initiative to form industrial standards for QKD, under the auspices of the European Telecommunication Standards Institute, described in the paper by Laenger and Lenhart in this Focus Issue, can be taken as a sign of the growing commercial interest. Recent progress has seen an increase in the secure bit rate of QKD links, by orders of magnitude, to over 1 Mb s −1 . This has resulted mainly from an improvement in the detection technology. Here changes in the way conventional semiconductor detectors are gated, as well as the development of novel devices based on non-linear processes and superconducting materials, are leading the way. Additional challenges for QKD at GHz clock rates include the design of high speed electronics, remote synchronization and high rate random number generation. Substantial effort is being devoted to increasing the range of individual links, which is limited by attenuation and other losses in optical fibres and air links. An important advance in the past few years has been the introduction of protocols with the same scaling as an ideal single-photon set-up. The good news is that these schemes use standard optical devices, such as weak laser pulses. Thanks to these new protocols and improvements in the detection technology, the range of a single fibre link can exceed a few hundred km. 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Our improved theoretical understanding places more stringent demands on the QKD devices. We are aware by now that finite size effects in key generation arise not only from parameter estimation. It will not be possible to generate a key from just a few hundred received signals. It is a stimulating challenge for the theory of security proofs to develop lean proof strategies that work with finite signal block sizes. As QKD advances to a real-world cryptographic solution, side channel attacks must be carefully analysed. Theoretical security proofs for QKD schemes are so far based on physical models of these devices. It is in the nature of models that any real implementation will deviate from this model, creating a potential weakness for an eavesdropper to exploit. There are two solutions to this problem: the traditional path of refining the models to reduce the deviations, or the radically different approach of device-independent security proofs, in which none or only a few well controlled assumptions about the devices are made. Clearly, it is desirable to find security proofs that require only minimal or fairly general model descriptions and are based on observable tests during the run of QKD sessions. It is now 25 years since the first proposal for QKD was published and 20 since the first experimental realization. The intervening years have brought several technological and theoretical advances, which have driven new insights into the application of quantum theory to the wider field of information technology. We are looking forward to the new twists and turns this field will take in the next 25 years! Focus on Quantum Cryptography: Theory and Practice Contents Security of continuous-variable quantum key distribution: towards a de Finetti theorem for rotation symmetry in phase space A Leverrier, E Karpov, P Grangier and N J Cerf Optical networking for quantum key distribution and quantum communications T E Chapuran, P Toliver, N A Peters, J Jackel, M S Goodman, R J Runser, S R McNown, N Dallmann, R J Hughes, K P McCabe, J E Nordholt, C G Peterson, K T Tyagi, L Mercer and H Dardy Proof-of-concept of real-world quantum key distribution with quantum frames I Lucio-Martinez, P Chan, X Mo, S Hosier and W Tittel Composability in quantum cryptography Jörn Müller-Quade and Renato Renner Distributed authentication for randomly compromised networks Travis R Beals, Kevin P Hynes and Barry C Sanders Feasibility of 300 km quantum key distribution with entangled states Thomas Scheidl, Rupert Ursin, Alessandro Fedrizzi, Sven Ramelow, Xiao-Song Ma, Thomas Herbst, Robert Prevedel, Lothar Ratschbacher, Johannes Kofler, Thomas Jennewein and Anton Zeilinger Decoy-state quantum key distribution with both source errors and statistical fluctuations Xiang-Bin Wang, Lin Yang, Cheng-Zhi Peng and Jian-Wei Pan High rate, long-distance quantum key distribution over 250 km of ultra low loss fibres D Stucki, N Walenta, F Vannel, R T Thew, N Gisin, H Zbinden, S Gray, C R Towery and S Ten Topological optimization of quantum key distribution networks R Alléaume, F Roueff, E Diamanti and N Lütkenhaus The SECOQC quantum key distribution network in Vienna M Peev, C Pacher, R Alléaume, C Barreiro, J Bouda, W Boxleitner, T Debuisschert, E Diamanti, M Dianati, J F Dynes, S Fasel, S Fossier, M Fürst, J-D Gautier, O Gay, N Gisin, P Grangier, A Happe, Y Hasani, M Hentschel, H Hübel, G Humer, T Länger, M Legré, R Lieger, J Lodewyck, T Lorünser, N Lütkenhaus, A Marhold, T Matyus, O Maurhart, L Monat, S Nauerth, J-B Page, A Poppe, E Querasser, G Ribordy, S Robyr, L Salvail, A W Sharpe, A J Shields, D Stucki, M Suda, C Tamas, T Themel, R T Thew, Y Thoma, A Treiber, P Trinkler, R Tualle-Brouri, F Vannel, N Walenta, H Weier, H Weinfurter, I Wimberger, Z L Yuan, H Zbinden and A Zeilinger Stable quantum key distribution with active polarization control based on time-division multiplexing J Chen, G Wu, L Xu, X Gu, E Wu and H Zeng Controlling passively quenched single photon detectors by bright light Vadim Makarov Information leakage via side channels in freespace BB84 quantum cryptography Sebastian Nauerth, Martin Fürst, Tobias Schmitt-Manderbach, Henning Weier and Harald Weinfurter Standardization of quantum key distribution and the ETSI standardization initiative ISG-QKD Thomas Länger and Gaby Lenhart Entangled quantum key distribution with a biased basis choice Chris Erven, Xiongfeng Ma, Raymond Laflamme and Gregor Weihs Finite-key analysis for practical implementations of quantum key distribution Raymond Y Q Cai and Valerio Scarani Field test of a continuous-variable quantum key distribution prototype S Fossier, E Diamanti, T Debuisschert, A Villing, R Tualle-Brouri and P Grangier Physics and application of photon number resolving detectors based on superconducting parallel nanowires F Marsili, D Bitauld, A Gaggero, S Jahanmirinejad, R Leoni, F Mattioli and A Fiore Device-independent quantum key distribution secure against collective attacks Stefano Pironio, Antonio Acín, Nicolas Brunner, Nicolas Gisin, Serge Massar and Valerio Scarani 1310 nm differential-phase-shift QKD system using superconducting single-photon detectors Lijun Ma, S Nam, Hai Xu, B Baek, Tiejun Chang, O Slattery, A Mink and Xiao Tang Practical gigahertz quantum key distribution based on avalanche photodiodes Z L Yuan, A R Dixon, J F Dynes, A W Sharpe and A J Shields Simple security proof of quantum key distribution based on complementarity M Koashi Feasibility of satellite quantum key distribution C Bonato, A Tomaello, V Da Deppo, G Naletto and P Villoresi Programmable instrumentation and gigahertz signaling for single-photon quantum communication systems Alan Mink, Joshua C Bienfang, Robert Carpenter, Lijun Ma, Barry Hershman, Alessandro Restelli and Xiao Tang Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation G B Xavier, N Walenta, G Vilela de Faria, G P Temporão, N Gisin, H Zbinden and J P von der Weid Feasibility of free space quantum key distribution with coherent polarization states D Elser, T Bartley, B Heim, Ch Wittmann, D Sych and G Leuchs A fully automated entanglement-based quantum cryptography system for telecom fiber networks Alexander Treiber, Andreas Poppe, Michael Hentschel, Daniele Ferrini, Thomas Lorünser, Edwin Querasser, Thomas Matyus, Hannes Hübel and Anton Zeilinger Dense wavelength multiplexing of 1550 nm QKD with strong classical channels in reconfigurable networking environments N A Peters, P Toliver, T E Chapuran, R J Runser, S R McNown, C G Peterson, D Rosenberg, N Dallmann, R J Hughes, K P McCabe, J E Nordholt and K T Tyagi Clock synchronization by remote detection of correlated photon pairs Caleb Ho, Antía Lamas-Linares and Christian Kurtsiefer Megabits secure key rate quantum key distribution Q Zhang, H Takesue, T Honjo, K Wen, T Hirohata, M Suyama, Y Takiguchi, H Kamada, Y Tokura, O Tadanaga, Y Nishida, M Asobe and Y Yamamoto Practical long-distance quantum key distribution system using decoy levels D Rosenberg, C G Peterson, J W Harrington, P R Rice, N Dallmann, K T Tyagi, K P McCabe, S Nam, B Baek, R H Hadfield, R J Hughes and J E Nordholt Detector decoy quantum key distribution Tobias Moroder, Marcos Curty and Norbert Lütkenhaus Daylight operation of a free space, entanglement-based quantum key distribution system Matthew P Peloso, Ilja Gerhardt, Caleb Ho, Antía Lamas-Linares and Christian Kurtsiefer Observation of 1.5 μm band entanglement using single photon detectors based on sinusoidally gated InGaAs/InP avalanche photodiodes Benjamin Miquel and Hiroki Takesue

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Design and construction of a high speed quantum key distribution device

Stuart Holt

Quantum cryptography has been proven to be the only practical method for fast distribution of secret keys needed for encryption. It enables absolutely secure information transfer from one party to another and allows for the detection of an eavesdropper without sacrificing security. This project involved designing and building a quantum cryptography device. This device has applications in many fields, including internet security as well as military communications. The data are encoded on single photons, which are sent through free space over a distance of about 30 cm. The electronics, which control the device however, are equally suitable for operation with fibre optic cables. The device was designed to operate in dark conditions within a laboratory. Currently, no research group has demonstrated transmission speeds faster than 8 kb/s. The aim of this project was to achieve bit rates faster than 10 kb/s, while operating under strict economic constraints. Although quantum cryptography was not demonstrated in the laboratory due to problems with the detector, the emission end, which generates single photons of light with one of two different polarisations, was shown to work as it was designed. The current design has limited commercial future due to the dark conditions under which the communication must take place. The project however has demonstrated that quantum cryptography is getting closer to becoming a reality.

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On Experimental Quantum Communication and Cryptography

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One of the most fascinating recent developments in research has been how different disciplines have become more and more interconnected. So much so that fields as disparate as information theory and fundamental physics have combined to produce ideas for the next generation of computing and secure information technologies, both of which have far reaching consequences. For more than fifty years Moore's law, which describes the trend of the transistor's size shrinking by half every two years, has proven to be uncannily accurate. However, the computing industry is now approaching a fundamental barrier as the size of a transistor approaches that of an individual atom and the laws of physics and quantum mechanics take over. Rather then look at this as the end, quantum information science has emerged to ask the question of what additional power and functionality might be realized by harnessing some of these quantum effects. This thesis presents work on the sub-field of quantum cryptography which seeks to use quantum means in order to assure the security of ones communications. The beauty of quantum cryptographic methods are that they can be proven secure, now and indefinitely into the future, relying solely on the validity of the laws of physics for their proofs of security. This is something which is impossible for nearly all current classical cryptographic methods to claim. \n \nThe thesis begins by examining the first implementation of an entangled quantum key distribution system over two free-space optical links. This system represents the first test-bed of its kind in the world and while its practical importance in terrestrial applications is limited to a smaller university or corporate campus, the system mimics the setup for an entangled satellite system aiding in the study of distributing entangled photons from an orbiting satellite to two earthbound receivers. Having completed the construction of a second free-space link and the automation of the alignment system, I securely distribute keys to Alice and Bob in two distant locations separated by 1,575 m with no direct line-of-sight between them. I examine all of the assumptions necessary for my claims of security, something which is particularly important for moving these systems out of the lab and into commercial industry. I then go on to describe the free-space channel over which the photons are sent and the implementation of each of the major system components. I close with a discussion of the experiment which saw raw detected entangled photon rates of 565 s^{-1} and a quantum bit error rate (QBER) of 4.92% resulting in a final secure key rate of 85 bits/s. Over the six hour night time experiment I was able to generate 1,612,239 bits of secure key. \n \nWith a successful QKD experiment completed, this thesis then turns to the problem of improving the technology to make it more practical by increasing the key rate of the system and thus the speed at which it can securely encrypt information. It does so in three different ways, involving each of the major disciplines comprising the system: measurement hardware, source technology, and software post-processing. First, I experimentally investigate a theoretical proposal for biasing the measurement bases in the QKD system showing a 79% improvement in the secret key generated from the same raw key rates. Next, I construct a second generation entangled photon source with rates two orders of magnitude higher than the previous source using the idea of a Sagnac interferometer. 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Integrated photonic transmitters for secure space quantum communication

Marc Jofre

An important issue in today’s information society is the security of data transmission against potential intruders, which always put at risk the confidentiality. Current methods to increase security require that the two parties wishing to transmit information, exchange or share one or more security keys. Once the key has been identified, the information can be transferred in a provable secure way using a one-time pad, i. e. the key length is as long as the plaintext. Therefore, the security of the information transmission is based exclusively on the security of the key exchange. Quantum cryptography, or more precisely quantum key distribution (QKD), guarantees absolutely secure key distribution based on the principles of quantum physics, according to which it is not possible to measure or reproduce a state (e.g. polarization or phase of a photon) without being detected. The key is generated out from the measurement of the information encoded into specific quantum states of a photon, named qubits. For example, a qubit can be created using properties such as the polarization or the phase of a photon. Achieved goals of this thesis are the development of a new class of high speed integrated photonic sources for applications in quantum key distribution systems, capable of producing unprecedented qubit rates (100 Mbps - 1 Gbps) and transmitting those over larger distances than those achieved so far (> 200 km). More specifically the work has been focused on developing faint pulse sources which can be used in very demanding environmental conditions, such as those in Space. For the development of these sources, apart from the optical design, essential is the opto-mechanical engineering as well as the integration with the electronics. One of the objectives was to achieve very high level of integration and power efficiency, e.g. volumes and power consumption between 10 and 100 times smaller than those typical of a laboratory experiment. Moreover, work in related parts of a whole QKD transmission system has been carried out. In particular, a new scheme for a compact, fast and simple random number generator has been demonstrated successfully achieving a random number generation rate of 1:1 Gbps. Also, during the course of this thesis, the development and engineering of a free-space QKD optical link has been initiated. This thesis makes use of novel ideas to alternatively demonstrate proof-of- concept experiments, which could then further develop into commercial products. To this end, close collaborations with world-wide leading companies in the field have been established. The Optoelectronics Group at ICFO has been involved in current European Space Agency (ESA) projects to develop a small footprint and low power consumption quantum transceiver and a high-flux entangled photon source.

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