We present a study of modern security proofs of quantum key distribution based on complementarity. This is a quantum communication protocol allowing for provable, unconditional security, although there is still a gap between theoretical analyses and real, operational systems. We focus on a recent argument by Koashi, which enables the treatment of individual, basis-dependent imperfections in the source. A detailed review is given based partly on an alternate derivation, with the goal of providing a more accessible approach to utilizing the strength of the complementarity-based proof. Some subtleties in the argument are then pointed out, particularly regarding the requirement of perfectly independent states in the random sampling. A numerical approach is implemented to simulate arbitrary protocols in order to investigate the necessity of this requirement. For independent states, the results provide strong evidence in support of the established security bounds. We then focus on imperfect sources with basis-independent correlations, which reveals concrete counterexamples to the security claims, even in the operational regime. This increases the understanding of why security relies critically on perfect independence of the states. We also predict the possibility of attacking real-world devices as the results show a necessary, although not sufficient, condition for exploiting small, but finite basis-independent correlations in an imperfect source. Furthermore, we argue that analyzing such systems and finding capable attack operations is difficult to achieve by analytic methods; instead we develop further numerical techniques for sampling the parameter space of source states and attack operations in a computationally efficient manner. These parameters are then iteratively improved using a hybrid uniform search and genetic optimization routine. The numerical methods prove to be effective in both uncovering sources that are prone to correlation attacks as well as constructing explicit attack operations for a given system.
We present a study of modern security proofs of quantum key distribution based on complementarity. This is a quantum communication protocol allowing for provable, unconditional security, although there is still a gap between theoretical analyses and real, operational systems. We focus on a recent argument by Koashi, which enables the treatment of individual, basis-dependent imperfections in the source. A detailed review is given based partly on an alternate derivation, with the goal of providing a more accessible approach to utilizing the strength of the complementarity-based proof. Some subtleties in the argument are then pointed out, particularly regarding the requirement of perfectly independent states in the random sampling. A numerical approach is implemented to simulate arbitrary protocols in order to investigate the necessity of this requirement. For independent states, the results provide strong evidence in support of the established security bounds. We then focus on imperfect sources with basis-independent correlations, which reveals concrete counterexamples to the security claims, even in the operational regime. This increases the understanding of why security relies critically on perfect independence of the states. We also predict the possibility of attacking real-world devices as the results show a necessary, although not sufficient, condition for exploiting small, but finite basis-independent correlations in an imperfect source. Furthermore, we argue that analyzing such systems and finding capable attack operations is difficult to achieve by analytic methods; instead we develop further numerical techniques for sampling the parameter space of source states and attack operations in a computationally efficient manner. These parameters are then iteratively improved using a hybrid uniform search and genetic optimization routine. The numerical methods prove to be effective in both uncovering sources that are prone to correlation attacks as well as constructing explicit attack operations for a given system.
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.
In contrast to classical public-key cryptosystems, where the security of encoded messages relies on on computational assumptions, Quantum Key Distribution (QKD) enables two distant parties to establish a shared secret key that, when combined with a one-time pad, provides information-theoretically secure encryption, provided that the QKD protocol is supported by a rigorous 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.
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) 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 key distribution (QKD) offers theoretical security guarantees for sharing secure key, but its practical systems face challenges due to the imperfections of devices. Widespread quantum state preparation imperfections, such as correlations between multiple rounds, significantly undermine the real-world security of QKD. In this paper, we propose a protocol that is immune to almost all kinds of state-preparation imperfections over multiple correlated rounds arising from both encoding and unknown non-encoding dimensions. The protocol relies only on three assumptions: the imperfect encoding produces unknown product states rather than entangled ones, a lower bound on the vacuum components is known, and the correlation has a finite range. The proposed protocol is also measurement-device-independent, ensuring high security at both the source and measurement sides. We provide the finite-key security analysis against coherent attacks and conduct numerical simulations to see the performance. The results show that for small correlation ranges, the protocol achieves excellent performance with a maximal transmission loss exceeding 60 dB (>300 km in standard fiber). Even for extreme cases, where one encoding affects up to 500 neighboring rounds, the protocol can still generate secret keys over a 10 dB-loss channel.
In recent years, the gap between theory and practice in quantum key distribution (QKD) has been significantly narrowed, particularly for QKD systems with arbitrarily flawed optical receivers. The status for QKD systems with imperfect light sources is however less satisfactory, in the sense that the resulting secure key rates are often overly dependent on the quality of state preparation. This is especially the case when the channel loss is high. Very recently, to overcome this limitation, Tamaki et al proposed a QKD protocol based on the so-called 'rejected data analysis', and showed that its security—in the limit of infinitely long keys—is almost independent of any encoding flaw in the qubit space, being this protocol compatible with the decoy state method. Here, as a step towards practical QKD, we show that a similar conclusion is reached in the finite-key regime, even when the intensity of the light source is unstable. More concretely, we derive security bounds for a wide class of realistic light sources and show that the bounds are also efficient in the presence of high channel loss. Our results strongly suggest the feasibility of long distance provably secure communication with imperfect light sources.