In contrast to classical cryptography, where the security of encoded messages typically relies on the inability of standard algorithms to overcome computational complexity assumptions, Quantum Key Distribution (QKD) can enable two spatially separated parties to establish an information-theoretically secure encryption, provided that the QKD protocol is underpinned by a security proof. In the last decades, security proofs robust against a wide range of eavesdropping strategies have established the theoretical soundness of several QKD protocols. However, most proofs are based on idealized models of the physical systems involved in such protocols and often include assumptions that are not satisfied in practical implementations. This mismatch creates a gap between theoretical security guarantees and actual experimental realizations, making QKD protocols vulnerable to attacks. To ensure the security of real-world QKD systems, it is therefore essential to account for imperfections in security analyses. In this article, we present an overview of recent analytical and numerical developments in QKD security proofs, which provide a versatile approach for incorporating imperfections and re-establishing the security of quantum communication protocols under realistic conditions.
In contrast to classical cryptography, where the security of encoded messages typically relies on the inability of standard algorithms to overcome computational complexity assumptions, Quantum Key Distribution (QKD) can enable two spatially separated parties to establish an information-theoretically secure encryption, provided that the QKD protocol is underpinned by a security proof. In the last decades, security proofs robust against a wide range of eavesdropping strategies have established the theoretical soundness of several QKD protocols. However, most proofs are based on idealized models of the physical systems involved in such protocols and often include assumptions that are not satisfied in practical implementations. This mismatch creates a gap between theoretical security guarantees and actual experimental realizations, making QKD protocols vulnerable to attacks. To ensure the security of real-world QKD systems, it is therefore essential to account for imperfections in security analyses. In this article, we present an overview of recent analytical and numerical developments in QKD security proofs, which provide a versatile approach for incorporating imperfections and re-establishing the security of quantum communication protocols under realistic conditions.
Quantum Key Distribution (QKD) is revolutionizing cryptography by promising information-theoretic security through the immutable laws of quantum mechanics. Yet, the challenge of transforming these idealized security models into practical, resilient systems remains a pressing issue, especially as quantum computing evolves. In this review, we critically dissect and synthesize the latest advancements in QKD protocols and their security vulnerabilities, with a strong emphasis on rigorous security proofs. We actively categorize contemporary QKD schemes into three key classes: uncertainty principle-based protocols (e.g., BB84), hybrid architectures that enable secure direct communication (eg, three-stage protocol), and continuous-variable frameworks. We further include two modern classes of QKD protocols, namely Twin-field QKD and Device-Independent QKD, both of which were developed to have practical implementations over the last decade. Moreover, we highlight important experimental breakthroughs and innovative mitigation strategies, including the deployment of advanced Quantum Error Correction Codes (QECCs), that significantly enhance channel fidelity and system robustness. By mapping the current landscape, from sophisticated quantum attacks to state-of-the-art error correction methods, this review fills an important gap in the literature. To bring everything together, the relevance of this review concerning quantum augmented networks (QuANets) is also presented. This allows the readers to gain a comprehensive understanding of the security promises of quantum key distribution from theoretical proofs to experimental validations.
Quantum Key Distribution (QKD) offers information-theoretic security grounded in the laws of quantum mechanics rather than computational assumptions. While its theoretical foundations are robust, practical implementations necessarily deviate from the idealized models assumed in security analyses, opening the door to side-channel attacks and implementation loopholes. In particular, most research to date has focused on mitigating vulnerabilities in the measurement devices, leading to the development of Measurement-Device-Independent QKD (MDI-QKD) and Twin-Field QKD (TF-QKD), which effectively close all receiver-side loopholes. By contrast, the security of the source has remained less explored, despite being critical for the overall integrity of QKD systems. The goal of this thesis is to develop security proofs that explicitly account for realistic imperfections in QKD sources. Special emphasis is placed on three main aspects: correlations between emitted pulses that violate independence assumptions, imperfect phase randomization arising from practical modulation schemes, and leakage of internal settings through passive or active side channels. By formulating rigorous methods to analyze these imperfections, the work aims to provide a framework that enables practitioners to quantify and mitigate implementation risks systematically. Additionally, this thesis seeks to clarify under which conditions the commonly used decoy-state method remains secure, and to what extent source assumptions can be relaxed without compromising performance. This thesis aims to advance the understanding of how real-world deviations impact QKD security and to contribute to the development of protocols and devices. Ultimately, this work is motivated by the ambition to help bring QKD from a theoretically promising idea to a mature technology deployable with confidence in practical scenarios.
Decoy-state quantum key distribution (QKD) is the most widely adopted approach for overcoming the limitations of imperfect single-photon sources. However, existing security proofs typically either neglect important device imperfections or rely on assumptions that are difficult to justify in realistic high-speed implementations, such as the independent and identically distributed nature of the emitted signals. In this work, we combine and extend several recent theoretical advances to provide a comprehensive analytical finite-key security proof for decoy-state QKD that simultaneously incorporates multiple practically relevant transmitter and receiver imperfections, including state-preparation flaws, bit and basis side-channel leakage and correlations, setting-independent intensity fluctuations, and detection-efficiency mismatches.
Quantum key distribution (QKD) is a method that enables two parties separated by distance to share a secret data string. QKD is based on the principles of quantum mechanics and offers information-theoretical security, making it a strong contender for the next generation of cryptography. However, the use of imperfect devices can lead to vulnerabilities and potential quantum hacking schemes. It is crucial to address these imperfections and restore the security of QKD, especially for individuals who use realistic devices for secure communication. QKD can be categorized into two types based on key encoding methods: discrete-variable (DV) QKD and continuous-variable (CV) QKD. It can also be classified based on the transmission medium: fiber-based QKD, free-space-based QKD, and chip-based QKD. This thesis focuses on chip-based DVQKD and fiber-based CVQKD. In the first part, we study the chip-based DVQKD system in the presence of phase-and polarization-dependent loss. We develop innovative methods to restore the security and improve the key rate. The solution is practical, requiring no hardware modifications, and has been adopted by other groups. The second part focuses on fiber-based CVQKD systems that use intensity fluctuating sources. We establish the security by incorporating the concept from DVQKD. Our proposed techniques are practical and do not require any hardware adjustments for existing CVQKD systems. Lastly, we present a general framework for passive CVQKD. Our proposed passive protocol offers a one-time solution to eliminate all potential side channels or imperfections in the source. Interestingly, we discover that the passive source is an excellent choice for the discrete-modulated CVQKD protocol, since the passive protocol offers the same key rate as its active counterpart, while removing all modulator side channels that have plagued the active ones. To sum up, this thesis introduces novel techniques and practical solutions to address imperfections in source devices in different QKD systems. We also propose a passive protocol that can handle all loopholes in source devices in a one-time process. We anticipate that this thesis will facilitate significant advancements in chip-based and fiber-based QKD implementations in the future.
Quantum key distribution (QKD) promises information-theoretic security based on quantum mechanics and idealized device models. Practical implementations, however, deviate from these models due to unavoidable device imperfections, and existing security proofs fall short of capturing the complexity of real-world systems. Here we introduce a versatile numerical finite-key security framework valid against general coherent attacks and applicable to a broad class of practical QKD setups. It accommodates most relevant imperfections at both transmitter and receiver, including non-independent-and-identically-distributed (non-IID) signals arising in high-speed QKD systems due to the limited bandwidth of optical modulators, while requiring only partial characterization of the apparatuses. We demonstrate the power of our framework by proving the security of a realistic decoy-state QKD implementation with laser sources, providing a practical route towards rigorous security certification of real-world QKD setups.
Quantum technology, and especially quantum computing, is advancing rapidly. Our every-day secure communication infrastructure relies heavily on public key cryptography; unfortunately, many public key schemes are in fact insecure against quantum algorithms. Furthermore, adversaries who, today, capture information encrypted using classical key distribution systems, can wait and, as soon as quantum computers of sufficient power become available, they may decipher all previously captured information. While some new “post-quantum” public key systems are assumed to be secure against quantum computers, this is only an assumption and, even if the assumption remains valid, still results in systems that are less efficient for end-users than the more commonly used public key systems employed today. While the advent of quantum technology may seem detrimental to security, in fact, one may harness the power of quantum through quantum key distribution (QKD) for stronger security guarantees. Such QKD protocols allow two (or more) parties to establish a shared secret key, secure against an all-powerful adversary. Perhaps surprising is that QKD technology is already here both experimentally and commercially. This tutorial serves as an introduction to basic QKD along with QKD technology from a practical perspective. Quantum communication is a highly interdisciplinary field of research and one of the goals of this article is to introduce a larger set of researchers and practitioners to its study in the hopes of furthering its progress and eventual wide-scale adoption.