HKU Scholars Hubhttp://hub.hku.hkThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Thu, 27 Jan 2022 03:11:06 GMT2022-01-27T03:11:06Z501821- Efficient quantum key distributionhttp://hdl.handle.net/10722/174756Title: Efficient quantum key distribution
Authors: Ardehali, Mohammed; Brassard, Gilles; Chau, H.F.; Lo, Hoi-Kwong
Abstract: We devise a simple modification that essentially doubles the efficiency of a well-known quantum key distribution scheme proposed by Bennett and Brassard (BB84). Our scheme assigns significantly different probabilities for the different polarization bases during both transmission and reception to reduce the fraction of discarded data. The actual probabilities used in the scheme are announced in public. As the number of transmitted signals increases, the efficiency of our scheme can be made to approach 100%. An eavesdropper may try to break such a scheme by eavesdropping mainly along the predominant basis. To defeat such an attack, we perform a refined analysis of accepted data: instead of lumping all the accepted data together to estimate a single error rate, we separate the accepted data into various subsets according to the basis employed and estimate an error rate for each subset individually.
Thu, 01 Jan 1998 00:00:00 GMThttp://hdl.handle.net/10722/1747561998-01-01T00:00:00Z
- Efficient quantum key distribution scheme and a proof of its unconditional securityhttp://hdl.handle.net/10722/80361Title: Efficient quantum key distribution scheme and a proof of its unconditional security
Authors: Lo, HK; Chau, HF; Ardehali, M
Abstract: We devise a simple modification that essentially doubles the efficiency of the BB84 quantum key distribution scheme proposed by Bennett and Brassard.We also prove the security of our modified scheme against the most general eavesdropping attack that is allowed by the laws of physics. The first major ingredient of our scheme is the assignment of significantly different probabilities to the different polarization bases during both transmission and reception, thus reducing the fraction of discarded data. A second major ingredient of our scheme is a refined analysis of accepted data: We divide the accepted data into various subsets according to the basis employed and estimate an error rate for each subset separately. We then show that such a refined data analysis guarantees the security of our scheme against the most general eavesdropping strategy, thus generalizing Shor and Preskill's proof of security of BB84 to our new scheme. Until now, most proposed proofs of security of single-particle type quantum key distribution schemes have relied heavily upon the fact that the bases are chosen uniformly, randomly, and independently. Our proof removes this symmetry requirement. © 2004 International Association for Cryptologic Research.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10722/803612005-01-01T00:00:00Z
- Quantum Encryptionhttp://hdl.handle.net/10722/285705Title: Quantum Encryption
Authors: Qi, Bing; Qian, Li; Lo, Hoi Kwong
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/10722/2857052011-01-01T00:00:00Z
- Quantum cryptographyhttp://hdl.handle.net/10722/285884Title: Quantum cryptography
Authors: Lo, Hoi Kwong; Zhao, Yi
Abstract: © 2012 Springer Science+Business Media, LLC. All rights reserved. Article Outline: Glossary Definition of the Subject Introduction Quantum Key Distribution: Motivation and Introduction Security Proofs Experimental Fundamentals Experimental Implementation of BB84 Protocol Other Quantum Key Distribution Protocols Quantum Hacking Beyond Quantum Key Distribution Future Directions Acknowledgments Bibliography
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10722/2858842012-01-01T00:00:00Z
- A high-speed quantum random number generator prototypehttp://hdl.handle.net/10722/285881Title: A high-speed quantum random number generator prototype
Authors: Xu, Feihu; Qi, Bing; Xu, He; Xuan, Jiancheng; Ma, Xiongfeng; Lo, Hoi Kwong; Qian, Li
Abstract: We present a high-speed real-time quantum random number generator prototype by measuring the quantum phase noise of a laser. The simplicity and robustness of our design suggest that it is readily commercialized for practical applications. © 2012 Optical Society of America.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2858812013-01-01T00:00:00Z
- Broadband multipoint sensing with single-arm frequency-shifted interferometryhttp://hdl.handle.net/10722/285742Title: Broadband multipoint sensing with single-arm frequency-shifted interferometry
Authors: Zhang, Yiwei; Ye, Fei; Qi, Bing; Lo, Hoi Kwong; Qian, Li
Abstract: Using a broadband source, a slow detector, and a modulator, we propose and demonstrate a single-arm frequency-shifted interferometer capable of sensing multiple weak reflections simultaneously in parallel and in series at 0.1-m spatial resolution. © 2013 The Optical Society.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2857422013-01-01T00:00:00Z
- Interrogation of multiplexed fiber grating sensors using frequency-shifted interferometerhttp://hdl.handle.net/10722/285642Title: Interrogation of multiplexed fiber grating sensors using frequency-shifted interferometer
Authors: Liu, Yu; Qian, Li; Qi, Bing; Lo, Hoi Kwong; Gu, Xijia
Abstract: We report a novel frequency-shifted interferometer for interrogating multiplexed fiber grating sensors. This cw technique is capable of distinguishing gratings even when their reflection spectra overlap, suitable for very large sensor arrays. © 2006 Optical Society of America.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10722/2856422006-01-01T00:00:00Z
- Multi-party agile QKD network with a fiber-based entangled sourcehttp://hdl.handle.net/10722/285758Title: Multi-party agile QKD network with a fiber-based entangled source
Authors: Zhu, E. Y.; Corbari, C.; Gladyshev, A. V.; Kazansky, P. G.; Lo, H. K.; Qian, L.
Abstract: A multi-party quantum key distribution scheme is experimentally demonstrated by utilizing a poled fiber-based broadband polarization-entangled source and dense wavelength-division multiplexing. Entangled photon pairs are delivered over 40-km of fiber, with secure key rates of more than 20 bits/s observed. © 2015 Optical Society of America.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10722/2857582015-01-01T00:00:00Z
- Quantum cryptologyhttp://hdl.handle.net/10722/285560Title: Quantum cryptology
Authors: Lo, Hoi Kwong
Abstract: The contest between code-makers and code-breakers has been going on for thousands of years. Recently, quantum mechanics has made a remarkable entry in the field. On the one hand, it is generally accepted that quantum cryptography can provide absolute security for communications between two users. On the other hand, code-breakers in possession of a quantum computer can easily break popular encryption schemes such as RSA and Data Encryption Standard (DES) which are essentially intractable by any classical computer.
Wed, 01 Jan 1997 00:00:00 GMThttp://hdl.handle.net/10722/2855601997-01-01T00:00:00Z
- A simple proof of the unconditional security of quantum key distributionhttp://hdl.handle.net/10722/285692Title: A simple proof of the unconditional security of quantum key distribution
Authors: Lo, Hoi Kwong
Abstract: Quantum key distribution is the most well-known application of quantum cryptography. Previous proposed proofs of security of quantum key distribution contain various technical subtleties. Here, a conceptually simpler proof of security of quantum key distribution is presented. The new insight is the invariance of the error rate of a teleportation channel: We show that the error rate of a teleportation channel is independent of the signals being transmitted. This is because the non-trivial error patterns are permuted under teleportation. This new insight is combined with the recently proposed quantum to classical reduction theorem. Our result shows that assuming that Alice and Bob have fault-tolerant quantum computers, quantum key distribution can be made unconditionally secure over arbitrarily long distances even against the most general type of eavesdropping attacks and in the presence of all types of noises.
Fri, 01 Jan 1999 00:00:00 GMThttp://hdl.handle.net/10722/2856921999-01-01T00:00:00Z
- Concentrating entanglement by local actions - beyond mean valueshttp://hdl.handle.net/10722/285559Title: Concentrating entanglement by local actions - beyond mean values
Authors: Lo, Hoi Kwong; Popescu, Sandu
Abstract: Previous investigations of entanglement manipulations have focused on the average properties of the outcomes and little is known about the actual probability distribution. Here we go beyond the average properties. We show that, for a pure entangled state shared between two separated persons Alice and Bob, the mathematical interchange symmetry of the Schmidt decomposition can be promoted into a physical symmetry between the actions of Alice and Bob. Consequently, the most general (multi-step two-way-communications) strategy of entanglement manipulation of a pure state is, in fact, equivalent to a strategy involving only a single (generalized) measurement by Alice followed by one-way communications of its result to Bob. One important question is whether coherent manipulations in quantum mechanics can enhance the probability of large deviations from the average behaviour. We answer this question in the negative by showing that, given n pairs of identical partly entangled pure states |Ψ〉 with entropy of entanglement E(|Ψ〉), the probability of getting nK (K>E(|Ψ〉)) singlets out of entanglement concentration tends to zero as n tends to infinity.
Wed, 01 Jan 1997 00:00:00 GMThttp://hdl.handle.net/10722/2855591997-01-01T00:00:00Z
- Security of high speed quantum key distribution with finite detector dead timehttp://hdl.handle.net/10722/285720Title: Security of high speed quantum key distribution with finite detector dead time
Authors: Burenkov, Viacheslav; Qi, Bing; Fortescue, Ben; Lo, Hoi Kwong
Abstract: The security of a high speed quantum key distribution system with finite detector dead time τ is analyzed. When the transmission rate becomes higher than the maximum count rate of the individual detectors (1/τ), security issues affect the scheme for sifting bits. Analytical calculations and numerical simulations of the Bennett-Brassard BB84 protocol are performed. We study Rogers et al.'s scheme (further information is available in [D. J. Rogers, J. C. Bienfang, A. Nakassis, H. Xu, and C. W. Clark, New J. Phys. 9, 319 (2007)]) in the presence of an active eavesdropper Eve who has the power to perform an intercept-resend attack. It is shown that Rogers et al.'s scheme is no longer guaranteed to be secure. More specifically, Eve can induce a basis-dependent detection efficiency at the receiver's end. Modified key sifting schemes that are basis-independent and thus secure in the presence of dead time and an active eavesdropper are then introduced. We analyze and compare these secure sifting schemes for this active Eve scenario, and calculate and simulate their key generation rate. It is shown that the maximum key generation rate is 1/(2τ) for passive basis selection, and 1/τ for active basis selection. The security analysis for finite detector dead time is also extended to the decoy state BB84 protocol for one particular secure sifting scheme. © Rinton Press.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10722/2857202014-01-01T00:00:00Z
- Security proof of quantum key distribution with detection efficiency mismatchhttp://hdl.handle.net/10722/285650Title: Security proof of quantum key distribution with detection efficiency mismatch
Authors: Fung, Chi Hang Fred; Tamaki, Kiyoshi; Qi, Bing; Lo, Hoi Kwong; Xiongfeng, Ma
Abstract: In theory, quantum key distribution (QKD) offers unconditional security based on the laws of physics. However, as demonstrated in recent quantum hacking theory and experimental papers, detection efficiency loophole can be fatal to the security of practical QKD systems. Here, we describe the physical origin of detection efficiency mismatch in various domains including spatial, spectral, and time domains and in various experimental set-ups. More importantly, we prove the unconditional security of QKD even with detection efficiency mismatch. We explicitly show how the key generation rate is characterized by the maximal detection efficiency ratio between the two detectors. Furthermore, we prove that by randomly switching the bit assignments of the detectors, the effect of detection efficiency mismatch can be completely eliminated. © Rinton Press.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/10722/2856502009-01-01T00:00:00Z
- One-way continuous-variable quantum key distribution over 5km of standard telecom fiberhttp://hdl.handle.net/10722/285525Title: One-way continuous-variable quantum key distribution over 5km of standard telecom fiber
Authors: Huang, Lei Lei; Qi, Bing; Qian, Li; Lo, Hoi Kwong
Abstract: We report the first experimental demonstration of one-way Gaussian-modulated coherent state quantum key distribution system over kilometers of standard telecom fiber. Under realistic assumptions, the achievable secrete key rate is over 10kb/s. ©2007 Optical Society of America.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2855252007-01-01T00:00:00Z
- Quantum key distribution with an untrusted sourcehttp://hdl.handle.net/10722/285865Title: Quantum key distribution with an untrusted source
Authors: Zhao, Yi; Qi, Bing; Lo, Hoi Kwong
Abstract: The "Plug & play" quantum cryptography scheme has significant advantage in real-life applications over other schemes and is adopted in most commercial quantum cryptosystems. Here, we present a rigorous security proof of it. © 2009 Optical Society of America.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/10722/2858652009-01-01T00:00:00Z
- Battling with quantum hackershttp://hdl.handle.net/10722/285785Title: Battling with quantum hackers
Authors: Lo, H.
Abstract: © OSA 2017. Quantum hacking threatens the security of practical quantum key distribution (QKD) systems. Fortunately, measurement-device-independent(MDI) QKD automatically foils all attacks on detectors. I will survey MDI-QKD together with other recent progress on QKD security research.
Description: Session: Quantum Communications (FTu4F)
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2857852017-01-01T00:00:00Z
- Measurement-device-independent quantum key distributionhttp://hdl.handle.net/10722/285688Title: Measurement-device-independent quantum key distribution
Authors: Lo, Hoi Kwong; Curty, Marcos; Qi, Bing
Abstract: How to remove detector side channel attacks has been a notoriously hard problem in quantum cryptography. Here, we propose a simple solution to this problem-measurement-device-independent quantum key distribution (QKD). It not only removes all detector side channels, but also doubles the secure distance with conventional lasers. Our proposal can be implemented with standard optical components with low detection efficiency and highly lossy channels. In contrast to the previous solution of full device independent QKD, the realization of our idea does not require detectors of near unity detection efficiency in combination with a qubit amplifier (based on teleportation) or a quantum nondemolition measurement of the number of photons in a pulse. Furthermore, its key generation rate is many orders of magnitude higher than that based on full device independent QKD. The results show that long-distance quantum cryptography over say 200km will remain secure even with seriously flawed detectors. © 2012 American Physical Society.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10722/2856882012-01-01T00:00:00Z
- Decoy state quantum key distributionhttp://hdl.handle.net/10722/285771Title: Decoy state quantum key distribution
Authors: Lo, Hoi Kwong
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10722/2857712005-01-01T00:00:00Z
- Dual detectors scheme in practical quantum key distribution systemshttp://hdl.handle.net/10722/285633Title: Dual detectors scheme in practical quantum key distribution systems
Authors: Qi, B.; Zhao, Y.; Ma, X.; Lo, H. K.; Qian, L.
Abstract: In this study, a "dual detectors" method is proposed to overcome the limitation imposed by the speed-noise trade-off and improve the performance of a practical quantum key distribution (QKD) system based on realistic detectors. The "dual detectors" scheme can be employed to improve the performance of a Gaussian-modulated coherent states (GMCS) QKD system based on realistic homodyne detectors, as in the case of BB84 protocol. With the "dual detectors" method, we see a significant improvement of the key rate (more than one order of magnitude) at a relatively short distance (up to 5 km).
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2856332007-01-01T00:00:00Z
- Decoy state protocols for quantum cryptography with parametric down conversion sourceshttp://hdl.handle.net/10722/285631Title: Decoy state protocols for quantum cryptography with parametric down conversion sources
Authors: Ma, Xiongfeng; Fung, Chi Hang Fred; Lo, Hoi Kwong
Abstract: In this paper, we show that decoy states are very simple to implement for quantum key distribution (QKD) with parametric down conversion (PDC) sources. Indeed, with no modification in the hardware at all, by using the AYKI scheme, one can achieve a key generation rate that is close to the theoretical limit of infinite decoy states. Therefore, we expect decoy state QKD to become a standard technique not only in the coherent state QKD, but also in QKD with PDC sources.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2856312007-01-01T00:00:00Z
- Experimental implementation of a three-party quantum key distribution protocolhttp://hdl.handle.net/10722/285641Title: Experimental implementation of a three-party quantum key distribution protocol
Authors: Adamson, R. B.A.; Fortescue, B.; Lo, H. K.; Steinberg, A. M.
Abstract: We present a proof-of=principle experimental implementation of provably secure three-party quantum key distribution. The protocol's security relies on the quantum correlations of GHZ states but it is implemented using bipartite entanglement. © 2006 Optical Society of America.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10722/2856412006-01-01T00:00:00Z
- Experimental quantum key distribution with active phase randomizationhttp://hdl.handle.net/10722/285880Title: Experimental quantum key distribution with active phase randomization
Authors: Zhao, Yi; Qi, Bing; Lo, Hoi Kwong
Abstract: Phase-randomization is an important assumption in many security proofs of practical quantum key distribution (QKD) systems. Here, we present the first experimental QKD with reliable active phase-randomization. A polarization-insensitive phase-modulator is designed for our experiment. ©2007 Optical Society of America.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2858802007-01-01T00:00:00Z
- Some attacks on quantum-based cryptographic protocolshttp://hdl.handle.net/10722/285585Title: Some attacks on quantum-based cryptographic protocols
Authors: Lo, Hoi-Kwong; Ko, Tsz-Mei
Abstract: Quantum-based cryptographic protocols are often said to enjoy security guaranteed by the fundamental laws of physics. However, even carefully designed quantum-based cryptographic schemes may be susceptible to subtle attacks that are outside the original design. As an example, we give attacks against a recently proposed "secure communication using mesoscopic coherent states", which employs mesoscopic states, rather than single-photon states. Our attacks can be used either as a known-plaintext attack or in the case where the plaintext has not been randomized. One of our attacks requires beamsplitters and the replacement of a lossy channel by a lossless one. It is successful provided that the original loss in the channel is so big that Eve can obtain 2k copies of what Bob receives, where k is the length of the seed key pre-shared by Alice and Bob. In addition, substantial improvements over such an exhaustive key search attack can be made, whenever a key is reused. Furthermore, we remark that, under the same assumption of a known or non-random plaintext, Grover's exhaustive key search attack can be applied directly to "secure communication using mesoscopic coherent states", whenever the channel loss is more than 50 percent. Therefore, as far as information-theoretic security is concerned, optically amplified signals necessarily degrade the security of the proposed scheme, when the plaintext is known or non-random. Our attacks apply even if the mesoscopic scheme is used only for key generation with a subsequent use of the key for one-time-pad encryption. Studying those attacks can help us to better define the risk models and parameter spaces in which quantum-based cryptographic schemes can operate securely. Finally, we remark that our attacks do not affect standard protocols such as Bennett-Brassard BB84 protocol or Bennett B92 protocol, which rely on single-photon signals. © Rinton Press.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10722/2855852005-01-01T00:00:00Z
- Multi-partite quantum cryptographic protocols with noisy ghz stateshttp://hdl.handle.net/10722/285619Title: Multi-partite quantum cryptographic protocols with noisy ghz states
Authors: Chen, Kai; Lo, Hoi Kwong
Abstract: We propose a wide class of distillation schemes for multi-partite entangled states that are CSS-states. Our proposal provides not only superior effciency, but also new insights on the connection between CSS-states and bipartite graph states. We then apply our distillation schemes to the tri-partite case for three cryptographic tasks|namely, (a) conference key agreement, (b) quantum sharing of classical secrets and (c) third-man cryptography. Moreover, we construct \prepare-and-measure" protocols for the above three cryptographic tasks which can be implemented with the generation of only a single entangled pair at a time. This gives significant simplifcation over previous experimental implementations which require two entangled pairs generated simultaneously. We also study the yields of those protocols and the threshold values of the fdelity above which the protocols can function securely. Rather surprisingly, our protocols will function securely even when the initial state does not violate the standard Bell-inequalities for GHZ states. © Rinton Press.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2856192007-01-01T00:00:00Z
- Security of quantum key distribution using weak coherent states with nonrandom phaseshttp://hdl.handle.net/10722/285600Title: Security of quantum key distribution using weak coherent states with nonrandom phases
Authors: Lo, Hoi Kwong; Preskill, John
Abstract: We prove the security of the Bennett-Brassard (BB84) quantum key distribution protocol in the case where the key information is encoded in the relative phase of a coherent-state reference pulse and a weak coherent-state signal pulse, as in some practical implementations of the protocol. In contrast to previous work, our proof applies even if the eavesdropper knows the phase of the reference pulse, provided that this phase is not modulated by the source, and even if the reference pulse is bright. The proof also applies to the case where the key is encoded in the photon polarization of a weak coherent-state pulse with a known phase, but only if the phases of the four BB84 signal states are judiciously chosen. The achievable key generation rate scales quadratically with the transmission in the channel, just as for BB84 with phase-randomized weak coherent-state signals (when decoy states are not used). For the case where the phase of the reference pulse is strongly modulated by the source, we exhibit an explicit attack that allows the eavesdropper to learn every key bit in a parameter regime where a protocol using phase-randomized signals is provably secure. © Rinton Press.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2856002007-01-01T00:00:00Z
- Time-shift attack in practical quantum cryptosystemshttp://hdl.handle.net/10722/285912Title: Time-shift attack in practical quantum cryptosystems
Authors: Qi, Bing; Fung, Chi Hang Fred; Lo, Hoi Kwong; Ma, Xiongfeng
Abstract: Recently, a new type of attack, which exploits the efficiency mismatch of two single photon detectors (SPD) in a quantum key distribution (QKD) system, has been proposed. In this paper, we propose another "time-shift" attack that exploits the same imperfection. In our attack, Eve shifts the arrival time of either the signal pulse or the synchronization pulse or both between Alice and Bob. In particular, in a QKD system where Bob employs time-multiplexing technique to detect both bit "0" and bit "1" with the same SPD, Eve, in some circumstances, could acquire full information on the final key without introducing any error. In addition, we prove that if Alice and Bob are unaware of our attack, the final key they share is insecure. We emphasize that our attack is simple and feasible with current technology. Finally, we discuss some counter measures against our and earlier attacks. © Rinton Press.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2859122007-01-01T00:00:00Z
- Sequential attacks against differential-phase-shift quantum key distribution with weak coherent stateshttp://hdl.handle.net/10722/285605Title: Sequential attacks against differential-phase-shift quantum key distribution with weak coherent states
Authors: Curty, Marcos; Zhang, Lucy Liuxuan; Lo, Hoi Kwong; Lütkenhaus, Norbert
Abstract: We investigate limitations imposed by sequential attacks on the performance of differential-phase-shift quantum key distribution protocols that use pulsed coherent light. In particular, we analyze two sequential attacks based on unambiguous state discrimination and minimum error discrimination, respectively, of the signal states emitted by the source. Sequential attacks represent a special type of intercept-resend attacks and, therefore, they provide ultimate upper bounds on the maximal distance achievable by quantum key distribution schemes. © Rinton Press.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2856052007-01-01T00:00:00Z
- Proof of unconditional security of six-state quantum key distribution schemehttp://hdl.handle.net/10722/285545Title: Proof of unconditional security of six-state quantum key distribution scheme
Authors: Lo, Hoi Kwong
Abstract: We prove the unconditional security of the standard six-state scheme for quantum key distribution (QKD). We demonstrate its unconditional security up to a bit error rate of 12.7 percents, by allowing only one-way classical communications in the error correction/privacy amplification procedure between Alice and Bob. This shows a clear advantage of the six-state scheme over another standard scheme - BB84, which has been proven to be secure up to only about 11 percents, if only one-way classical communications are allowed. Our proof technique is a generalization of that of Shor-Preskill's proof of security of BB84. We show that a advantage of the six-state scheme lies in the Alice and Bob's ability to establish rigorously from their test sample the non-trivial mutual information between the bit-flip and phase error patterns. A modified version of the degenerate quantum codes studied by DiVincenzo, Shor and Smolin is employed in our proof.
Mon, 01 Jan 2001 00:00:00 GMThttp://hdl.handle.net/10722/2855452001-01-01T00:00:00Z
- Getting something out of nothinghttp://hdl.handle.net/10722/285594Title: Getting something out of nothing
Authors: Lo, Hoi Kwong
Abstract: We study quantum key distribution with standard weak coherent states and show, rather counter-intuitively, that the detection events originated from vacua can contribute to secure key generation rate, over and above the best previous result. Our proof is based on a communication complexity/quantum memory argument. The key observation is that Eve does not have to store anything, if Alice sends out a vacuum state. © Rinton Press.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10722/2855942005-01-01T00:00:00Z
- Quantum key distribution based on a Sagnac loop interferometer and polarization-insensitive phase modulatorshttp://hdl.handle.net/10722/285611Title: Quantum key distribution based on a Sagnac loop interferometer and polarization-insensitive phase modulators
Authors: Qi, Bing; Huang, Lei Lei; Lo, Hoi Kwong; Qian, Li
Abstract: We present a design for a quantum key distribution (QKD) system in a Sagnac loop configuration, employing a novel phase modulation scheme based on frequency shift, and demonstrate stable BB84 QKD operation with high interference visibility and low quantum bit error rate (QBER). The phase modulation is achieved by sending two light pulses with a fixed time delay (or a fixed optical path delay) through a frequency shift element and by modulating the amount of frequency shift. The relative phase between two light pulses upon leaving the frequency-shift element is determined by both the time delay (or the optical path delay) and the frequency shift, and can therefore be controlled by varying the amount of frequency shift. To demonstrate its operation, we used an acousto-optic modulator (AOM) as the frequency-shift element, and vary the driving frequency of the AOM to encode phase information. The interference visibility for a 40km and a 10km fiber loop is 96% and 99%, respectively, at single photon level. We ran BB84 protocol in a 40-km Sagnac loop setup continuously for one hour and the measured QBER remained within the 2%-5% range. A further advantage of our scheme is that both phase and amplitude modulation can be achieved simultaneously by frequency and amplitude modulation of the AOM's driving signal, allowing our QKD system the capability of implementing other protocols, such as the decoy-state QKD and the continuous-variable QKD. We also briefly discuss a new type of Eavesdropping strategy ("phase- remapping" attack) in bidirectional QKD system. © 2006 IEEE.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10722/2856112006-01-01T00:00:00Z
- Simulation and implementation of decoy state quantum key distribution over 60km telecom fiberhttp://hdl.handle.net/10722/285612Title: Simulation and implementation of decoy state quantum key distribution over 60km telecom fiber
Authors: Zhao, Yi; Qi, Bing; Ma, Xiongfeng; Lo, Hoi Kwong; Qian, Li
Abstract: Decoy state quantum key distribution (QKD) has been proposed as a novel approach to improve dramatically both the security and the performance of practical QKD setups. Recently, many theoretical efforts have been made on this topic and have theoretically predicted the high performance of decoy method. However, the gap between theory and experiment remains open. In this paper, we report the first experiments on decoy state QKD, thus bridging the gap. Two protocols of decoy state QKD are implemented: one-decoy protocol over 15km of standard telecom fiber, and weak+vacuum protocol over 60km of standard telecom fiber. We implemented the decoy state method on a modified commercial QKD system. The modification we made is simply adding commercial acousto-optic modulator (AOM) on the QKD system. The AOM is used to modulate the intensity of each signal individually, thus implementing the decoy state method. As an important part of implementation, numerical simulation of our set-up is also performed. The simulation shows that standard security proofs give a zero key generation rate at the distance we perform decoy state QKD (both 15km and 60km). Therefore decoy state QKD is necessary for long distance secure communication. Our implementation shows explicitly the power and feasibility of decoy method, and brings it to our real-life. © 2006 IEEE.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10722/2856122006-01-01T00:00:00Z
- Improve the efficiency of a practical quantum key distribution systemhttp://hdl.handle.net/10722/285613Title: Improve the efficiency of a practical quantum key distribution system
Authors: Qi, Bing; Zhao, Yi; Ma, Xiongfeng; Lo, Hoi Kwong; Qian, Li
Abstract: The performance of a practical quantum key distribution (QKD) system is often limited by the multi-photon state emission of its source and the dark counts of its detectors. Here, we present two methods to improve its performance. The first method is decoy state QKD: the sender randomly sends out weak coherent states with various average photon numbers (which are named as signal state and decoy states). In [14,15], we have performed the first experimental implementation of decoy state QKD over 15km and 60km respectively, thus dramatically increasing the distance and secure key generation rate of practical QKD systems. Our work has been followed up by many research groups worldwide [16-18]. The second scheme is QKD with "dual detectors" [19]: the legitimate receiver randomly uses either a fast (but noisy) detector or a quiet (but slow) detector to measure the incoming quantum signals. The measurement results from the quiet detector can be used to upper bound the eavesdropper's information, while the measurement results from the fast detector are used to generate a secure key. We applied this idea to various QKD protocols. Simulation results demonstrated significant improvements in both BB84 protocol with ideal single photon source and Gaussianmodulated coherent states protocol.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2856132007-01-01T00:00:00Z
- A survey on quantum cryptographic protocols and their securityhttp://hdl.handle.net/10722/285627Title: A survey on quantum cryptographic protocols and their security
Authors: Fung, Chi Hang Fred; Lo, Hoi Kwong
Abstract: Communications in secrecy are often required in many commercial and military applications. Unfortunately, many cryptographic schemes in use today such as public-key cryptography based on the RSA algorithm would be broken with either unanticipated advances in hardware and algorithm or the advent of quantum computers. Quantum cryptography, on the other hand, has been proven secure even against the most general attack allowed by the laws of physics and is a promising technology poised for widespread adoption in realistic cryptographic applications. Quantum cryptography allows two parties to expand on a secret key that they have previously shared. Various quantum cryptographic protocols have been proposed to perform this task. In this paper, we survey some popular quantum cryptographic protocols (including the famous Bennett-Brassard 1984 protocol) and discuss their security. Specifically, we consider their security in two cases: the ideal case where a perfect single-photon source is used and the practical case where a realistic laser source is used. We compare the protocols and find that the efficient six-state protocol outperforms the others both in the tolerable quantum bit error rate and in the key generation rate when a realistic laser source is used. ©2007 IEEE.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2856272007-01-01T00:00:00Z
- Quantum key distribution with vacua or dim pulses as decoy stateshttp://hdl.handle.net/10722/285629Title: Quantum key distribution with vacua or dim pulses as decoy states
Authors: Lo, Hoi Kwong
Abstract: Recently, Hwang has proposed a decoy state method in quantum key distribution (QKD). In Hwang's proposal, the average photon number of the decoy state is about two. Here, we propose a new decoy state scheme using vacua or very weak coherent states as decoy states and discuss its advantages.
Thu, 01 Jan 2004 00:00:00 GMThttp://hdl.handle.net/10722/2856292004-01-01T00:00:00Z
- Gaussian-modulated coherent state quantum key distribution experiment over 20km telecommunication fiberhttp://hdl.handle.net/10722/285639Title: Gaussian-modulated coherent state quantum key distribution experiment over 20km telecommunication fiber
Authors: Qi, Bing; Huang, Lei Lei; Chi, Yue Meng; Qian, Li; Lo, Hoi Kwong
Abstract: In most implementations of quantum-key-distribution (QKD) protocols the secure keys originate from single-photon signals. However, due to the unavoidable channel losses and the low efficiencies of single photon detectors, the key generation rate of a single-photon QKD system is low. Recently, there has been a growing interest in the Gaussian-modulated coherent state (GMCS) QKD protocol because it can be implemented with conventional laser sources and high efficiency homodyne-detectors. Here, we present our experimental results with a fully fiber-based one-way GMCS QKD system. Our system employed a double Mach-Zehnder interferometer (MZI) configuration in which the weak quantum signal and the strong local-oscillator (LO) go through the same fiber between Alice and Bob. We employed two novel techniques to suppress system excess noise. First, to suppress the LO's leakage, an important contribution to the excess noise, we implemented a scheme combining polarization and frequency multiplexing, achieving an extinction ratio of 70dB. Second, to further minimize the system excess noise due to phase drift of the double MZI, the sender simply remaps her data by performing a rotation operation. Under a "realistic model", the secure key rates determined with a 5km and a 20km fiber link are 0.3bit/pulse and 0.05bit/pulse, respectively. These key rates are significantly higher than that of a practical BB84 QKD system.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10722/2856392008-01-01T00:00:00Z
- Quantum hacking: Attacking practical quantum key distribution systemshttp://hdl.handle.net/10722/285620Title: Quantum hacking: Attacking practical quantum key distribution systems
Authors: Qi, Bing; Fung, Chi Hang Fred; Zhao, Yi; Ma, Xiongfeng; Tamaki, Kiyoshi; Chen, Christine; Lo, Hoi Kwong
Abstract: Quantum key distribution (QKD) can, in principle, provide unconditional security based on the fundamental laws of physics. Unfortunately, a practical QKD system may contain overlooked imperfections and violate some of the assumptions in a security proof. Here, we report two types of eavesdropping attacks against a practical QKD system. The first one is "time-shift" attack, which is applicable to QKD systems with gated single photon detectors (SPDs). In this attack, the eavesdropper, Eve, exploits the time mismatch between the open windows of the two SPDs. She can acquire a significant amount of information on the final key by simply shifting the quantum signals forwards or backwards in time domain. Our experimental results in [9] with a commercial QKD system demonstrate that, under this attack, the original QKD system is breakable. This is the first experimental demonstration of a feasible attack against a commercial QKD system. This is a surprising result. The second one is "phase-remapping" attack [10]. Here, Eve exploits the fact that a practical phase modulator has a finite response time. In principle, Eve could change the encoded phase value by timeshifting the signal pulse relative to the reference pulse.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10722/2856202007-01-01T00:00:00Z
- Security of quantum key distribution with imperfect deviceshttp://hdl.handle.net/10722/285630Title: Security of quantum key distribution with imperfect devices
Authors: Gottesman, Daniel; Lo, Hoi Kwong; Lütkenhaus, Norbert; Preskill, John
Abstract: We prove the security of the Bennett-Brassard (BB84) quantum key distribution protocol in the case where the source and detector are under the limited control of an adversary. Our proof applies when both the source and the detector have small basis-dependent flaws, as is typical in practical implementations of the protocol. We estimate the key generation rate in some special cases: sources that emit weak coherent states, detectors with basis-dependent efficiency, and misaligned sources and detectors.
Thu, 01 Jan 2004 00:00:00 GMThttp://hdl.handle.net/10722/2856302004-01-01T00:00:00Z
- Excess noise control in Gaussian-modulated coherent state quantum key distribution systemhttp://hdl.handle.net/10722/285634Title: Excess noise control in Gaussian-modulated coherent state quantum key distribution system
Authors: Qi, Bing; Huang, Lei Lei; Chi, Yue Meng; Qian, Li; Lo, Hoi Kwong
Abstract: A novel polarization-frequency-multiplexing scheme is implemented to suppress noise in a fiber-based Gaussian-modulated coherent-state quantum key distribution system. The achievable secrete key rate is 0.30bit/pulse with a 5km-fiber and 0.05bit/pulse with a 20km-fiber. © 2008 Optical Society of America.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10722/2856342008-01-01T00:00:00Z
- A quantum analog of Huffman codinghttp://hdl.handle.net/10722/285931Title: A quantum analog of Huffman coding
Authors: Braunstein, S. L.; Fuchs, C. A.; Gottesman, D.; Lo, Hoi Kwong
Abstract: We construct a Huffman-coding inspired scheme for quantum data compression. The number of computational steps in the encoding and decoding processes of N quantum signals can be made to be polynomial in log N by a massively parallel implementation of a quantum gate array. This is to be compared with the O(N 3) computational steps required in the sequential implementation by Cleve and Di-Vincenzo (see Phys. Rev. A54, p.2636, 1996) of the Schumacher (see Phys. Rev. A51, p.2738, 1995) quantum noiseless block coding scheme. © 1998 IEEE.
Thu, 01 Jan 1998 00:00:00 GMThttp://hdl.handle.net/10722/2859311998-01-01T00:00:00Z
- Quantum key distribution secure against partly malicious deviceshttp://hdl.handle.net/10722/285939Title: Quantum key distribution secure against partly malicious devices
Authors: Curty, Marcos; Lo, Hoi Kwong
Abstract: © OSA 2017. We show that using verifiable secret sharing and multiple optical devices and classical post-processing units, one could re-establish the security of quantum key distribution against covert channels and partly malicious classical post-processing units.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10722/2859392017-01-01T00:00:00Z
- Quantum cloning with a broadband optical amplifierhttp://hdl.handle.net/10722/285944Title: Quantum cloning with a broadband optical amplifier
Authors: Qi, Bing; Qian, Li; Lo, Hoi Kwong
Abstract: We study the quantum cloning fidelity of a broadband optical amplifier followed by optimal filters. Counter-intuitively, when the average input photon number is below one, such system is no better than a random cloning machine. © 2006 Optical Society of America.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10722/2859442006-01-01T00:00:00Z
- Practical measurement device independent quantum key distributionhttp://hdl.handle.net/10722/285725Title: Practical measurement device independent quantum key distribution
Authors: Xu, Feihu; Curty, Marcos; Qi, Bing; Cui, Wei; Lim, Charles Ci Wen; Tamaki, Kiyoshi; Lo, Hoi Kwong
Abstract: We analyze real-life implementations of measurement-device-independent quantum-key-distribution (MDI-QKD): a general system model, a finite-decoy protocol and a finite-key analysis. Our work is relevant to not only QKD but also general experiments on quantum interference. © 2013 Optical Society of America.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10722/2857252013-01-01T00:00:00Z
- Sagnac quantum key distribution using novel polarization-insensitive phase modulators based on frequency shifthttp://hdl.handle.net/10722/285917Title: Sagnac quantum key distribution using novel polarization-insensitive phase modulators based on frequency shift
Authors: Huang, Lei Lei; Qi, Bing; Mong, Roger; Qian, Li; Lo, Hoi Kwong
Abstract: We demonstrate a stable Sagnac quantum key distribution implementing BB84 phase-encoding protocol based on acousto-optic frequency shifters. The quantum bit error rate remains 4-6.5% for at least one hour without recalibration or feedback control. © 2006 IEEE.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10722/2859172006-01-01T00:00:00Z
- Conference key agreement and quantum sharing of classical secrets with noisy GHZ stateshttp://hdl.handle.net/10722/285907Title: Conference key agreement and quantum sharing of classical secrets with noisy GHZ states
Authors: Chen, Kai; Lo, Hoi Kwong
Abstract: We propose a wide class of distillation schemes for multi-partite entangled states that are CSS-states. Our proposal provides not only superior efficiency, but also new insights on the connection between CSS-states and bipartite graph states. We then consider the applications of our distillation schemes for two cryptographic tasks - namely, (a) conference key agreement and (b) quantum sharing of classical secrets. In particular, we construct "prepare-and- measure" protocols. Also we study the yield of those protocols and the threshold value of the fidelity above which the protocols can function securely. Surprisingly, our protocols will function securely even when the initial state does not violate the standard Bell-inequalities for GHZ states. Experimental realization involving only bi-partite entanglement is also suggested.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10722/2859072005-01-01T00:00:00Z
- Unconditionally secure key distillation from multi-photons in a single-photon polarization based quantum key distributionhttp://hdl.handle.net/10722/285908Title: Unconditionally secure key distillation from multi-photons in a single-photon polarization based quantum key distribution
Authors: Tamaki, Kiyoshi; Lo, Hoi Kwong
Abstract: In this presentation, we show some counter-examples to a naive belief that the security of QKD is based on no-cloning theorem. One example is shown by explicitly proving that one can indeed generate an unconditionally secure key from Alice's two-photon emission part in "SARG04 protocol" proposed by V. Scarani et al., in Phys. Rev. Lett. 92, 057901 (2004). This protocol differs from BB84 only in the classical communication. It is, thus, interesting to see how only the classical communication of QKD protocol might qualitatively change its security. We also show that one can generate an unconditionally secure key from the single to the four-photon part in a generalized SARG04 that uses six states. Finally, we also compare the bit error rate threshold of these protocols with the one in BB84 and the original six-state protocol assuming a depolarizing channel.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10722/2859082005-01-01T00:00:00Z
- Frequency-shifted interferometer and its applicationshttp://hdl.handle.net/10722/285910Title: Frequency-shifted interferometer and its applications
Authors: Qi, Bing; Qian, Li; Tausz, Andrew; Liu, Yu; Lo, Hoi Kwong
Abstract: We propose a frequency-shifted interferometer and demonstrate its various applications, including fiber length and dispersion measurement, multiple weak reflection sites locating along a fiber link. This could be a useful interrogation technique for sensor multiplexing. © 2006 Optical Society of America.
Sun, 01 Jan 2006 00:00:00 GMThttp://hdl.handle.net/10722/2859102006-01-01T00:00:00Z
- Cryptography's quantum barrierhttp://hdl.handle.net/10722/285665Title: Cryptography's quantum barrier
Authors: Lo, Hoi Kwong
Sat, 01 Jan 2000 00:00:00 GMThttp://hdl.handle.net/10722/2856652000-01-01T00:00:00Z
- A tight lower bound on the classical communication cost of entanglement dilutionhttp://hdl.handle.net/10722/285579Title: A tight lower bound on the classical communication cost of entanglement dilution
Authors: Harrow, Aram W.; Lo, Hoi Kwong
Wed, 01 Jan 2003 00:00:00 GMThttp://hdl.handle.net/10722/2855792003-01-01T00:00:00Z
- How to share a quantum secrethttp://hdl.handle.net/10722/285563Title: How to share a quantum secret
Authors: Cleve, Richard; Gottesman, Daniel; Lo, Hoi Kwong
Abstract: We investigate the concept of quantum secret sharing, where a secret quantum state is distributed between n parties in such a way that certain subsets of the parties can jointly recover the secret, while other subsets of the parties can acquire absolutely no information about it. In a ((k, n)) threshold scheme, any subset of k or more parties can reconstruct the secret, while any subset of k-1 or fewer parties can obtain no information. We show that the only constraint on the existence of threshold schemes comes from the quantum `no-cloning theorem', which requires that n<2k, and, in all such cases, we give an efficient construction of a ((k, n)) threshold scheme. We also explore similarities and differences between quantum secret sharing schemes and quantum error-correcting codes. One remarkable difference is that, while most existing quantum codes encode pure states as pure states, quantum secret sharing schemes must use mixed states in some cases. For example, if k≤n<2k-1 then any ((k, n)) threshold scheme must distribute information that is globally in a mixed state.
Thu, 01 Jan 1998 00:00:00 GMThttp://hdl.handle.net/10722/2855631998-01-01T00:00:00Z
- A Quantum Jump in Securityhttp://hdl.handle.net/10722/285593Title: A Quantum Jump in Security
Authors: Lo, Hoi Kwong
Abstract: Quantum mechanics can revolutionize the art of code-making by breaking standard encryption schemes and by making unbreakable codes. In this document, I survey the motivation, power and limitation of quantum code-making.
Wed, 01 Jan 2003 00:00:00 GMThttp://hdl.handle.net/10722/2855932003-01-01T00:00:00Z