In this chapter, a short summary of research status of quantum cryptography and the workflow of BB84 protocol is introduced.Then, the security of QKD is discussed and some aspects of practical QKD system are focused on.In section 3, the implementation technolgies of a secure and stable point-to-point QKD system and QKD network are presented.Some field QKD network experiments and their results will be shown in this section.Another section will discuss some technical and non-technical aspects of the applications of QKD.Finally, a short conclusion aiming at the future trends of QKD is proposed. The research status of quantum cryptographyThe first QKD experiment was implemented by Bennett and Smolin in 1989 (Bennett, et al., 1992) which opened the gate to real-life QKD.According to the implementation scheme of QKD protocol, there are discrete variable, continuous variable and distributed phase reference coding.According to the physical carrier of quantum information, there are polarization encoding, phase encoding, frequency encoding , amplitude encoding and so on.Free-space QKD and fiber QKD can be used as the quantum channel for photon-based QKD.Implementation scheme should be selected according to the channel character, the performance requirements, the operating condition and so on.Fiber is the most widely used quantum channel, in which the polariztion and the phase encoding QKD schemes can be applied.Polarization encoding QKD schemes use the polarization states of photons to carry key information.Due to the intrinsic birefringence effect in fiber, the polarization states of photons are vulnerable to be interfered, which affects the polarization encoding rather than the phase encoding QKD system in common situation.Although there are some valuable polarization encoding QKD schemes (Ma, et al., 2008;Liu, et al., 2010), phase encoding QKD is still the major scheme used in fiber quantum channel.The encoder and decoder of phase encoding QKD system generally implemented with interferometer, which need to be precisely modulated and adjusted to meet the interference conditions.Some mature phase encoding systems based on different interferomter structures (Ribordy, et al.,
In this chapter, a short summary of research status of quantum cryptography and the workflow of BB84 protocol is introduced.Then, the security of QKD is discussed and some aspects of practical QKD system are focused on.In section 3, the implementation technolgies of a secure and stable point-to-point QKD system and QKD network are presented.Some field QKD network experiments and their results will be shown in this section.Another section will discuss some technical and non-technical aspects of the applications of QKD.Finally, a short conclusion aiming at the future trends of QKD is proposed. The research status of quantum cryptographyThe first QKD experiment was implemented by Bennett and Smolin in 1989 (Bennett, et al., 1992) which opened the gate to real-life QKD.According to the implementation scheme of QKD protocol, there are discrete variable, continuous variable and distributed phase reference coding.According to the physical carrier of quantum information, there are polarization encoding, phase encoding, frequency encoding , amplitude encoding and so on.Free-space QKD and fiber QKD can be used as the quantum channel for photon-based QKD.Implementation scheme should be selected according to the channel character, the performance requirements, the operating condition and so on.Fiber is the most widely used quantum channel, in which the polariztion and the phase encoding QKD schemes can be applied.Polarization encoding QKD schemes use the polarization states of photons to carry key information.Due to the intrinsic birefringence effect in fiber, the polarization states of photons are vulnerable to be interfered, which affects the polarization encoding rather than the phase encoding QKD system in common situation.Although there are some valuable polarization encoding QKD schemes (Ma, et al., 2008;Liu, et al., 2010), phase encoding QKD is still the major scheme used in fiber quantum channel.The encoder and decoder of phase encoding QKD system generally implemented with interferometer, which need to be precisely modulated and adjusted to meet the interference conditions.Some mature phase encoding systems based on different interferomter structures (Ribordy, et al.,
Quantum key distribution (QKD) is a straightforward cryptographic protocol for secure communication using quantum mechanics principles. This book chapter provides an introduction to QKD protocols based on theoretical principles, applicability, and recent advancements. The chapter provides an overview of the most popular BB84 protocol and its extensions, including measurement-device-independent QKD (MDI-QKD) and twin-field QKD (TF-QKD). The chapter includes continuous-variable QKD (CV-QKD) and its applicability to real networks. Security attacks, including side-channel attacks and photon-number splitting (PNS) attacks, are addressed with a focus on countermeasures, including decoy-state techniques and device-independent protocols. Application of QKD for network security, cloud computing, and critical infrastructure is addressed. Integration of QKD with classical cryptography techniques, post-quantum cryptography, and innovative quantum network topologies is addressed to investigate applicability for large-scale implementation. The chapter discusses recent advancements in superconducting nanowire single-photon detectors, quantum memory-assisted QKD, and free-space QKD in atmospheric channels. It also discusses innovative reconciliation techniques, authentication schemes, future developments, and deployment challenges and provides valuable information for researchers and practitioners.
In so-called private-key cryptographic systems, secure transmission of confi dential information through unprotected channels relies on the exchange of secret keys, known only to the sender, Alice, and the receiver, Bob.Quantum cryptography is based on the properties of quantum mechanics to obtain a provably secure key distribution.Most existing implement ations rely on either the polarization or the phase of very weak pulses of light sent through optical fibers.Recent results by research groups in British Telecom, Los Alamos National Laboratory and at the University of Geneva, have shown that quantum key distribution is possible over distances of tens of kilometers, using only standard telecom cables.However, due to birefringence and the effects of the environment, the polarization at the output of a fiber fluctuates randomly.Therefore, all existing systems need active polarization control.Systems based on phase also require interferometric detection, which necessitates active stabilization of the interferometers.To eliminate these problems, our team, led by Prof. N. Gisin in the Group of Applied Physics at the University of Geneva, is currently developing a new interferometric system implementing phase-encoded quantum key distribution (US patent pending).A schematic of the setup is given in Fig. 1.Its main features are:1) The system is based on time multiplexing, the interfering pulses following exactly the same spatial path, albeit with a small time delay.The interferometer is thus automatically aligned, and needs neither adjustments nor stabilization of the path lengths.2) Use of Faraday mirrors enables all birefringence effects in the fibers to be cancelled out.Therefore, the system does not require any polarization control.With this new system, Alice and Bob could exchange their cryptographic keys through standard telecom systems.They would be provided with a sending kit and
The rapid advancement of quantum computing poses a significant threat to classical cryptographic systems that rely on the computational hardness of mathematical problems such as integer factorization and discrete logarithm problems. In this context, quantum cryptography has emerged as a promising paradigm for secure communication based on the fundamental principles of quantum mechanics rather than on computational assumptions. This paper presents a comprehensive review of quantum cryptography, with a particular focus on Quantum Key Distribution (QKD), the most mature application. The study discusses the theoretical foundations of quantum security, including superposition, entanglement, and the No-cloning theorem, which collectively enable eavesdropping detection and guarantee information-theoretic security. Furthermore, the review examines major QKD protocols, such as BB84 and E91, as well as their advanced variants designed to address practical vulnerabilities and enhance performance. Recent progress in real-world implementations, including fiber-optic networks, free-space communication, and satellite-based systems such as the Micius satellite, is also analyzed. In addition, the paper highlights critical challenges related to scalability, hardware limitations, and security loopholes arising from imperfect devices. Finally, emerging research directions, including hybrid cryptographic frameworks that integrate quantum and post-quantum approaches, are discussed to provide insights into the future of secure communication. This review aims to provide a structured, up-to-date understanding of quantum cryptography, bridging the gap between theoretical developments and practical implementations, and emphasizing its crucial role in shaping next-generation cybersecurity systems.
Cyberspace has become the most popular carrier of information exchange in every corner of our life, which is beneficial for our life in almost all aspects. With the continuous development of science and technology, especially the quantum computer, cyberspace security has become the most critical problem for the Internet in near future. In this paper, we focus on analyzing characteristics of the quantum cryptography and exploring its advantages of it in the future Internet. It is worth noting that we analyze the quantum key distribution (QKD) protocol in the noise-free channel. To include real practical stimulators, there are developed various QKD Schemes for the noisy channels. Here we develop the unconditional security of quantum cryptography theoretically, which is suitable for the Internet as ever-increasing challenges are inevitable in the future. Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. In practice, the achievable distance for QKD has been limited to a few hundred kilometers, due to the channel loss of fibers or terrestrial free space that exponentially reduced the photon rate. Also experimentally it is proved that the Satellite-based QKD promises to establish a global-scale quantum network by exploiting the negligible photon loss and decoherence in the empty space. In this paper, we also give some introduction to various public-key cryptography and also touch on the various functions, advantages, and disadvantages of quantum cryptography and the need for quantum cryptography in the future as in data encoding, and digital signatures, and many more. It also covers the various securities issue of our current cryptography system. Different protocols BB84 i.e., the first QKD protocol also some other protocols, and finally the future directions of quantum cryptography followed by the conclusion of the paper.
This doctoral thesis summarizes research in quantum cryptography done at the Department of Electronics and Telecommunications at the Norwegian University of Science and Technology (NTNU) from 1998 through 2007.\n\nThe opening parts contain a brief introduction into quantum cryptography as well as an overview of all existing single photon detection techniques for visible and near infrared light. Then, our implementation of a fiber optic quantum key distribution (QKD) system is described. We employ a one-way phase coding scheme with a 1310 nm attenuated laser source and a polarization-maintaining Mach-Zehnder interferometer. A feature of our scheme is that it tracks phase drift in the interferometer at the single photon level instead of employing hardware phase control measures. An optimal phase tracking algorithm has been developed, implemented and tested. Phase tracking accuracy of +-10 degrees is achieved when approximately 200 photon counts are collected in each cycle of adjustment. Another feature of our QKD system is that it uses a single photon detector based on a germanium avalanche photodiode gated at 20 MHz. To make possible this relatively high gating rate, we have developed, implemented and tested an afterpulse blocking technique, when a number of gating pulses is blocked after each registered avalanche. This technique allows to increase the key generation rate nearly proportionally to the increase of the gating rate. QKD has been demonstrated in the laboratory setting with only a very limited success: by the time of the thesis completion we had malfunctioning components in the setup, and the quantum bit error rate remained unstable with its lowest registered value of about 4%.\n\nMore than half of the thesis is devoted to various security aspects of QKD. We have studied several attacks that exploit component imperfections and loopholes in optical schemes. In a large pulse attack, settings of modulators inside Alice's and Bob's setups are read out by external interrogating light pulses, without interacting with quantum states and without raising security alarms. An external measurement of phase shift at Alice's phase modulator in our setup has been demonstrated experimentally. In a faked states attack, Eve intercepts Alice's qubits and then utilizes various optical imperfections in Bob's scheme to construct and resend light pulses in such a way that Bob does not distinguish his detection results from normal, whereas they give Bob the basis and bit value chosen at Eve's discretion. Construction of such faked states using several different imperfections is discussed. Also, we sketch a practical workflow of breaking into a running quantum cryptolink for the two abovementioned classes of attacks. A special attention is paid to a common imperfection when sensitivity of Bob's two detectors relative to one another can be controlled by Eve via an external parameter, for example via the timing of the incoming pulse. This imperfection is illustrated by measurements on two different single photon detectors. Quantitative results for a faked states attack on the Bennett-Brassard 1984 (BB84) and the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocols using this imperfection are obtained. It is shown how faked states can in principle be constructed for quantum cryptosystems that use a phase-time encoding, the differential phase shift keying (DPSK) and the Ekert protocols. Furthermore we have attempted to integrate this imperfection of detectors into the general security proof for the BB84 protocol. For all attacks, their applicability to and implications for various known QKD schemes are considered, and countermeasures against the attacks are proposed.\n\nThe thesis incorporates published papers [J. Mod. Opt. 48, 2023 (2001)], [Appl. Opt. 43, 4385 (2004)], [J. Mod. Opt. 52, 691 (2005)], [Phys. Rev. A 74, 022313 (2006)], and [quant-ph/0702262].
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.