Composable security analysis of an experimental continuous-variable quantum key distribution system operating with polar codes
Irina O. Vorontsova, R. Goncharov, I. M. Filipov, Vladimir Chistiakov, Boris Aleksandrovich Nasedkin, Peter Trifonov, Mikhail Gellert, Fedor Goncharov, D. V. Tupyakov, E.O. Samsonov, Vladimir Egorov
The integration of a continuous-variable (CV) quantum key distribution (QKD) system with a transmitted local oscillator into existing telecommunication infrastructure is presented and analyzed in terms of both its experimental implementation and theoretical security aspects. We studied the operation of the proposed system for its practical parameters and obtained the secure key. The choice of an algorithm for post-processing is also discussed and justified. Composable security analysis is fulfilled in the trusted noise paradigm for the real parameters of the proposed continuous-variable quantum key distribution system, with unambiguous state discrimination and photon-number-splitting attacks addressed in detail. The implemented CV-QKD system can be used to create metropolitan area quantum networks for both commercial and public organizations’ purposes requiring frequent secure key updates, as well as for scaling backbone quantum networks.
The integration of a continuous-variable (CV) quantum key distribution (QKD) system with a transmitted local oscillator into existing telecommunication infrastructure is presented and analyzed in terms of both its experimental implementation and theoretical security aspects. We studied the operation of the proposed system for its practical parameters and obtained the secure key. The choice of an algorithm for post-processing is also discussed and justified. Composable security analysis is fulfilled in the trusted noise paradigm for the real parameters of the proposed continuous-variable quantum key distribution system, with unambiguous state discrimination and photon-number-splitting attacks addressed in detail. The implemented CV-QKD system can be used to create metropolitan area quantum networks for both commercial and public organizations’ purposes requiring frequent secure key updates, as well as for scaling backbone quantum networks.
Abstract A quantum key distribution (QKD) system must fulfill the requirement of universal composability to ensure that any cryptographic application (using the QKD system) is also secure. Furthermore, the theoretical proof responsible for security analysis and key generation should cater to the number N of the distributed quantum states being finite in practice. Continuous-variable (CV) QKD based on coherent states, despite being a suitable candidate for integration in the telecom infrastructure, has so far been unable to demonstrate composability as existing proofs require a rather large N for successful key generation. Here we report a Gaussian-modulated coherent state CVQKD system that is able to overcome these challenges and can generate composable keys secure against collective attacks with N ≈ 2 × 10 8 coherent states. With this advance, possible due to improvements to the security proof and a fast, yet low-noise and highly stable system operation, CVQKD implementations take a significant step towards their discrete-variable counterparts in practicality, performance, and security.
Continuous-variable (CV) quantum key distribution (QKD) allows for quantum secure communication with the benefit of being close to classical coherent communication. In recent years, CV QKD protocols using a discrete number of displaced coherent states have been studied intensively as the modulation can be directly implemented with real devices with finite resolution. Until now, experiments only calculated key rates in the asymptotic regime. Here, we present a CV QKD system using discrete modulation that is especially designed for atmospheric channels. We use polarization encoding to exploit the nonbirefringent nature of the turbulent atmosphere. This allows to expand CV QKD networks beyond the existing fiber backbone. In a laboratory demonstration with a static 3-decibel loss channel, we implemented a recently developed security proof allowing to calculate composable finite-size key rates against independently and identically distributed collective attacks. We applied the full QKD protocol including a quantum random number generator, error correction, and privacy amplification to extract secret keys.
Quantum key distribution (QKD) has been proven to promise unconditionally secure data communication between remote legitimate users，based on the fundamental laws of quantum mechanics. QKD has great potential to be commercialized in the market and implemented in securing critical institutions, such as financial infrastructures, smart Grid, and national defence. Most theoretical security analyses of QKD protocols are based on several idealistic assumptions. However, the practical limitations, such as device imperfections, may bring security loopholes and leave QKD systems vulnerable to malicious eavesdropping attacks. Since QKD systems will be used as the practical applications in the real-world, it is of great importance to study the practical security and practical implementations of QKD protocols. Compared with dedicated single-photon based discrete variable (DV) QKD protocols, continuous variable (CV) QKD systems have attracted more attentions in recent years, as they benefit from their compatibility with commercial off-the-shelf (COTS) telecommunication components and high key rate over metropolitan networks. However, the requirement of a co-transmitted local oscillator (LO) brings a number of issues that compromises secure key generation and becomes a major obstacle to the practical implementation of CVQKD. To overcome these problems, a new CVQKD protocol with a locally generated LO (LLO) at Bob has been proposed. Based on previous studies, this thesis presents a novel quantum attack based on a discovered security loophole of the LLO CVQKD protocol, and demonstrates two high performance and low complexity practical LLO CVQKD systems. 1. After exploring the phase recovery scheme and the phase noise model, the first quantum attack on LLO CVQKD system is proposed, that exploits the phase estimation error associated with the amplitude of the phase reference pulses, in a ‘reference pulse attack’. Under this attack, a new system noise model and a refined linear quantum states transmission model have been developed when the phase noise is partially trusted. The attack performance is evaluated in terms of excess noise manipulation and mutual information extraction over both trusted – referred to as realistic, and untrusted – referred to as paranoid, security models of phase noise. Effective countermeasures are also given to improve the practical security and performance of the LLO CVQKD system under the trusted phase noise model. 2. Aside from the practical security analysis, two practical LLO CVQKD implementations developed in the laboratory are demonstrated here. The first practical LLO CVQKD system is featured as high speed (500 MHz) and low complexity, realizing the world’s fastest predicted key rate of 26.9 Mbps over a 15 km optical fibre link. To increase the signal repetition rate, and hence the potential secure key rate, our system is equipped with high-performance, wideband devices and components designed to support high repetition rate operations. To reduce the system complexity and correct for any phase shift during transmission, reference pulses are interleaved with quantum signals at Alice. Customized monitoring software has been developed allowing all parameters to be controlled in real-time without any modification to the physical setup. The system-level noise model analysis is introduced at high bandwidth and a new ‘combined optimisation’ technique is proposed to simultaneously optimize system parameters to high precision. It also has the potential for an even faster implementation. 3. The second demonstrated system presents the feasibility of the first practical LLO CVQKD system. The system comprises integrated real-time data-processing units (DPUs) and commercial off-the-shelf (COTS) telecommunication components. Instead of using conventional expensive, high-specification, standalone laboratory equipment (e.g. oscilloscope and arbitrary waveform generator (AWG) used in most CVQKD systems), each DPU consists of a high-speed, multi-channel data acquisition module, and is embedded in a computer at both Alice and Bob to achieve real-time quantum data generation, acquisition and analysis. In addition, a hardware-based filter unit has been implemented to amplify and stabilise modulation signals, together with a system-level noise model analysis and a combined parameter optimisation method to enhance the system key rate. In laboratory conditions, the system has been shown to run continuously, and the asymptotic secure key rates are estimated as 1.37 Mbps and 509 kbps over 15 km and 25 km optical fibre links, respectively, through stable excess noise measurements for over 10 hours at a 20 MHz repetition rate. The measurement results open up the possibility of building up a commercialized LLO CVQKD system.
Abstract Establishing secure data communication necessitates secure key exchange over a public channel. Quantum key distribution (QKD), which leverages the principles of quantum physics, can achieve this with information-theoretic security. The discrete modulated (DM) continuous variable (CV) QKD protocol, in particular, is a suitable candidate for large-scale deployment of quantum-safe communication due to its simplicity and compatibility with standard high-speed telecommunication technology. Here, we present the first experimental demonstration of a four-state DM CVQKD system, successfully generating composable finite-size keys, secure against collective attacks over a 20 km fiber channel with 2.3 × 109 coherent quantum states, achieving a positive composable key rate of 11.04 × 10−3 bits/symbol. This accomplishment is enabled by using an advanced security proof, meticulously selecting its parameters, and the fast, stable operation of the system. Our results mark a significant step toward the large-scale deployment of practical, high-performance, cost-effective, and highly secure quantum key distribution networks using standard telecommunication components.
Abstract Distributing cryptographic keys over public channels in a way that can provide information-theoretic security is the holy grail for secure communication. This can be achieved by exploiting quantum mechanical principles in so-called quantum key distribution (QKD). Continuous-variable (CV) QKD based on coherent states, in particular, is an attractive scheme for secure communication since it requires only standard telecommunication technology that can operate at room temperature. However, a recently discovered side-channel created in the process of state preparation leads to a leakage of information about the transmitted quantum state, opening a security loophole for eavesdroppers to compromise the security of the CVQKD system. Here, we present a CVQKD system without this modulation leakage vulnerability. Our implementation is based on a baseband modulation approach and uses an in-phase and quadrature (IQ) modulator for state preparation and radio frequency heterodyne detection together with carefully designed digital signal processing for state measurement. We consider practical aspects in the implementation and demonstrate the generation of a composable secret key secure against collective attacks. This work is a step towards protecting CVQKD systems against practical imperfections of physical devices and operational limitations without performance degradation.
Abstract Quantum key distribution (QKD) represents a cornerstone of secure communication in the quantum era. While discrete-variable QKD (DV-QKD) protocols were historically the first to demonstrate secure key exchange, continuous-variable QKD (CV-QKD) has emerged as a more practical alternative due to its seamless compatibility with current telecommunications infrastructure. CV-QKD relies on coherent and squeezed states of light, offering significant advantages for integration into modern optical networks. This review comprehensively explores the theoretical underpinnings, technological advancements, and practical challenges of CV-QKD. Special attention is given to the role of photonic integrated circuits (PICs) in enabling scalable and efficient implementation of CV-QKD systems. Furthermore, recent advances in machine learning have been leveraged to optimize CV-QKD performance, with data-driven techniques enhancing noise estimation, parameter optimization, and system security. Additionally, tensor networks provide efficient computational tools for analyzing complex quantum correlations, improving the efficiency and robustness of quantum key distribution protocols. These developments, combined with ongoing improvements in quantum photonic integration, pave the way for the practical deployment of large-scale, high-speed quantum-secure networks.