The practical implementation of Quantum Key Distribution (QKD) is heavily constrained by the PLOB bound [1], exponential optical channel loss, and device-dependent side-channel attacks [2]. We deductively elevate these limitations by structurally embedding the quantum optical channel into the 8-Dimensional Seonggil Matrix Space. By redefining decoherence and measurement disturbance as non-commutative quantum friction, the Topological Fractal Brake algebraically suppresses information leakage, bypassing classical linear bounds. This rigorously establishes a theoretical framework for a global, topologically lossless quantum internet without macroscopic repeater overhead.
The practical implementation of Quantum Key Distribution (QKD) is heavily constrained by the PLOB bound [1], exponential optical channel loss, and device-dependent side-channel attacks [2]. We deductively elevate these limitations by structurally embedding the quantum optical channel into the 8-Dimensional Seonggil Matrix Space. By redefining decoherence and measurement disturbance as non-commutative quantum friction, the Topological Fractal Brake algebraically suppresses information leakage, bypassing classical linear bounds. This rigorously establishes a theoretical framework for a global, topologically lossless quantum internet without macroscopic repeater overhead.
Quantum Key Distribution (QKD) provides unconditional security guaranteed by the laws of quantum mechanics. However, practical implementations remain vulnerable to physical-layer intrusions, such as the Photon-Number-Splitting (PNS) attack, which exploits non-ideal multi-photon emissions. Current countermeasures heavily rely on asymptotic statistical error bounds or complex optical hardware, introducing inherent detection latency. Building upon our foundational theoretical framework of fractal-entropic scaling, we introduce a protocol-agnostic, deterministic detection framework grounded in the dynamics of open quantum systems. We establish an operational isomorphism between the purity loss (ΔSL) induced by an eavesdropper and the topological symmetry breaking of nonlinear dynamic systems in the complex plane. By mapping discretized stochastic noise into a generalized complex iterative map, microscopic quantum perturbations are non-linearly amplified, forcing an orbital divergence in finite time. Hardware validation via fixed-point arithmetic on a Field-Programmable Gate Array (FPGA) demonstrates real-time, Layer-1 intrusion detection without relying on ideal quantum measurements. Using the BB84 protocol as a canonical case study, we prove that active quantum eavesdropping is topologically incompatible with the continuous natural noise of a secure channel.
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.
Cryptography plays a key role in today's digital society. The most widely used encryption method is the public-key algorithm, but its security relies on unproven computational assumptions. In contrast, quantum key distribution (QKD) is an unbreakable encryption algorithm. QKD has been proven to provide information-theoretic security rigorously based on the laws of physics. Despite the beauty of QKD theory, in practice, QKD still suffers from practical side-channel attacks, due to the imperfections of real-life implementations. Such discrepancy between the theory and the practice is fatal to the security of a QKD system. This thesis addresses a number of the discrepancies in a standard QKD implementation, and it offers efficient countermeasures to resolve these discrepancies. A novel protocol -- measurement-device-independent (MDI) QKD - was proposed to remove all detector side-channel attacks. To apply MDI-QKD in real life, I have proposed practical protocols, conducted the first rigorous finite-key security proof, designed methodologies to optimize parameters, and demonstrated MDI-QKD experimentally. These results allow MDI-QKD mature enough to be implemented in real-life. Based on the framework of MDI-QKD, the only security issue left is the source. To solve the loophole due to the imperfect encoder, I have experimentally implemented a recent protocol, which enables QKD loss-tolerant to source flaws. This experiment for the first time shows secure QKD with imperfect state preparations at long distances. Another vulnerable part in the source is the ability to generate true randomness. I have developed a compact and cost-effective quantum random number generator (QRNG) prototype, which has a real-time generation rate of 1 Gbit/s with excellent stability. The simplicity and speed of my prototype prove the feasibility of a robust, low-cost, and fast QRNG. In summary, MDI-QKD enables new scientific developments in the field of quantum cryptography. This thesis solves the major practical issues in the implementation of MDI-QKD. The combination of MDI-QKD, imperfect sources, and QRNG consists of the foundation of a future practical side-channel-free QKD protocol. The success of this protocol can substitute current classical cryptosystems by offering people an unconditionally secure infrastructure, which will be the ultimate dream for researchers in secure communication.
Abstract QKD (Quantum Key Distribution) promises information-theoretical security by using quantum states to generate shared cryptographic key material and to reveal eavesdropping attempts through unavoidable physical disturbances. The practical value of this promise depends not only on quantum technologies such as DV-QKD, CV-QKD, entanglement-based QKD, MDI-QKD, and Twin-Field QKD, but also on authentication, post-processing, implementation quality, and operational architecture. Fiber-optic, satellite, and free-space QKD each impose distinct limits on distance, key rate, trusted nodes, costs, and resistance to side-channel attacks.
The impending realization of fault-tolerant quantum computers threatens the foundational public-key cryptography underpinning global telecommunications and financial infrastructure. While algorithmic Post-Quantum Cryptography (PQC) provides mathematical computational hardness against quantum cryptanalysis, Quantum Key Distribution (QKD) establishes an alternative information-theoretically secure physical paradigm based on the immutable laws of quantum mechanics: the No-Cloning theorem and Heisenberg's uncertainty principle. However, deploying scalable, wide-area quantum networks introduces formidable physical and systems engineering challenges: exponential optical fiber attenuation that restricts direct point-to-point QKD to beneath 150 km, high dark-count rates in superconducting nanowire single-photon detectors (SNSPDs), non-deterministic Bell-state entanglement swapping across quantum repeaters, and intense Raman scattering cross-talk when multiplexing quantum channels alongside multi-terabit classical DWDM optical networks. This paper delivers an exhaustive, mathematically formal technical investigation and empirical architectural evaluation of quantum networking and QKD systems. We formalize the asymptotic secret key rate bounds under the Decoy-State BB84 and Measurement-Device-Independent (MDI-QKD) protocols, model continuous entanglement routing over quantum repeaters utilizing bipartite Bell-pair purification, and establish optical co-existence bounds under Dense Wavelength Division Multiplexing (DWDM). Furthermore, we construct an empirical optical testbed evaluating an 8-node metropolitan quantum network co-propagating QKD over 100 Gbps classical optical traffic, paired with hybrid PQC-QKD key derivation functions (ML-KEM + QKD). Our empirical findings demonstrate that decoy-state BB84 achieves a sustainable secret key rate of 1.42 kbps across 100 km of standard single-mode fiber (SMF-28), while hybrid PQC-QKD integration eliminates single-point cryptographic failures with less than 2.4 ms handshake latency overhead. Finally, we formulate open challenges in memory-assisted atomic quantum repeaters, satellite-to-ground freespace entanglement links, and quantum internet transport protocols.
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.