This work studies the robustness of quantum key distribution (QKD) networks by combining graph-theoretic structure with a physics-based model of BB84 link capacity. Unlike classical optical networks, where traffic degrades gradually under link loss, QKD secret-key rates collapse exponentially due to fiber attenuation, detector inefficiency, misalignment, and dark counts. As a result, purely topological centrality measures fail to capture which nodes are truly critical for end-to-end secure connectivity. We model each fiber link using an asymptotic BB84 capacity function, convert it to bit/s, and then propagate capacities through the network using max–min (widest-path) routing via a Floyd–Warshall-style algorithm. Global robustness is quantified by the sum of all-pairs widest-path capacities under progressive node removal. We compare four adaptive attack strategies: random failure, degree-targeted removal, hop-based betweenness, and a new capacity-weighted betweenness that explicitly incorporates quantum-physical constraints. Experiments across multiple topology classes (random geometric, scale-free, star, and mesh) and many random network instances show that classical metrics such as degree and unweighted betweenness correlate only weakly with the actual loss of global capacity. In contrast, capacity-weighted betweenness consistently identifies nodes whose removal causes catastrophic collapse of the QKD backbone, especially in sparse and hub-dominated networks. The results lead to a clear design principle: topology alone is insufficient; QKD robustness is governed primarily by the physics-constrained capacity structure. The repository includes the full PDF manuscript and Python code used to generate the BB84 link capacities, widest-path capacity matrices, centrality metrics, and robustness curves. These materials are intended to support reproducibility and to provide a baseline for future work on quantum-aware network design, survivability optimization, and traffic engineering in national-scale QKD infrastructures.
This work studies the robustness of quantum key distribution (QKD) networks by combining graph-theoretic structure with a physics-based model of BB84 link capacity. Unlike classical optical networks, where traffic degrades gradually under link loss, QKD secret-key rates collapse exponentially due to fiber attenuation, detector inefficiency, misalignment, and dark counts. As a result, purely topological centrality measures fail to capture which nodes are truly critical for end-to-end secure connectivity. We model each fiber link using an asymptotic BB84 capacity function, convert it to bit/s, and then propagate capacities through the network using max–min (widest-path) routing via a Floyd–Warshall-style algorithm. Global robustness is quantified by the sum of all-pairs widest-path capacities under progressive node removal. We compare four adaptive attack strategies: random failure, degree-targeted removal, hop-based betweenness, and a new capacity-weighted betweenness that explicitly incorporates quantum-physical constraints. Experiments across multiple topology classes (random geometric, scale-free, star, and mesh) and many random network instances show that classical metrics such as degree and unweighted betweenness correlate only weakly with the actual loss of global capacity. In contrast, capacity-weighted betweenness consistently identifies nodes whose removal causes catastrophic collapse of the QKD backbone, especially in sparse and hub-dominated networks. The results lead to a clear design principle: topology alone is insufficient; QKD robustness is governed primarily by the physics-constrained capacity structure. The repository includes the full PDF manuscript and Python code used to generate the BB84 link capacities, widest-path capacity matrices, centrality metrics, and robustness curves. These materials are intended to support reproducibility and to provide a baseline for future work on quantum-aware network design, survivability optimization, and traffic engineering in national-scale QKD infrastructures.
Quantum communication networks—the substrate of a future quantum Internet—demand analytical tools that account for entanglement, fidelity, and quantum-specific constraints absent from classical models. In this paper, we introduce the Quantum Network Science (QNS) framework that adapts core network metrics to the quantum setting through fidelity-, success-probability-, and capacity-aware weighting. We formalize centrality (including Quantum PageRank and continuous-time quantum-walk variants), community structure on entanglement graphs, and robustness/percolation with fidelity thresholds. The framework is validated via analytic motifs and controlled simulations on Erdős–Rényi, scale-free, and small-world topologies, as well as satellite-assisted versus fiber-only designs. Our results show that (i) fidelity weighting reorders structural importance and can reconnect networks that appear fragmented classically; (ii) heavy-tailed degree patterns improve tolerance to random failures but heighten vulnerability to targeted hub attacks; (iii) small-world shortcuts induced by long-range quantum links shrink path lengths; and (iv) overlapping “connected components” emerge from entanglement swapping, motivating revised connectivity baselines. We also discuss design implications—degree caps and hub hardening, link-type diversity, multipath routing, and buffering policies—and outline extensions to temporal and multilayer modeling that couple the quantum plane with its classical control layer. QNS thus offers a principled, measurement-oriented foundation for analyzing, comparing, and engineering resilient, high-capacity quantum networks.
The scalability of Quantum Key Distribution (QKD) networks remains a primary challenge for the realization of the quantum internet. Although full mesh topologies offer superior performance, their quadratic scaling of quantum links renders them impractical for large-scale deployment. This paper proposes a novel Hierarchical Quantum Backbone (HQB) topology that addresses this challenge. It is being done by employing a hierarchical architecture composed of a core ring of quantum repeaters interconnected with distribution hubs and trusted nodes serving end-users. A statistical simulation framework incorporating realistic models for channel loss, latency, and quantum bit error rate evaluates the design’s eficiency. The performance evaluation demonstrate that HQB reduces the number of quantum links compared to full-mesh networks. The topology supports near-instantaneous, high-rate intra-hub key distribution as well as metropolitan-scale inter-hub rates. This comprehensive analysis confirms HQB as a practical, efficient, and fault-tolerant architecture suitable for scalable quantum network deployment.
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) enables information-theoretically secure cryptographic key exchange by exploiting quantum mechanical properties -- photon polarisation, entanglement, and the no-cloning theorem -- to detect any eavesdropping attempt. Scaling QKD from point-to-point laboratory demonstrations to large-scale metropolitan and wide-area networks requires addressing three engineering challenges: photon loss over long fibre distances (limiting practical QKD to ~100-150 km without quantum repeaters), network topology design for multi-party key delivery, and integration of QKD with classical network infrastructure. This paper proposes the Quantum Key Distribution Network Architecture (QKDNA) framework, a systematic evaluation of five QKD protocol families (BB84, E91, MDI-QKD, Twin-Field QKD, and CV-QKD) across four network topology models (star, ring, mesh, and hierarchical trusted-node) on 16 real-world metropolitan network scenarios. QKDNA achieves maximum secure key rate of 18.4 Mbps (Twin-Field QKD, 80 km fibre, mesh topology) and demonstrates scalable QKD delivery to 48 simultaneous users. The framework contributes a Network QKD Deployment Index (NQDI) and quantitative guidance for national quantum network infrastructure planning.
Quantum networks will rely on entanglement distribution to enable multi-user applications such as distributed quantum computing and cryptography. While multipartite entanglement distribution routing protocols have been extensively studied on idealised grid topologies, less is understood about how real network structure shapes their performance and resource requirements. We present a systematic study of four routing protocols for multipartite entanglement distribution, each characterised by the number of paths (single-path and multi-path) and routing strategy (star-based and tree-based), over 81 real network topologies. We identified four distinct topology-dependent performance regimes, where: (i) all protocols perform poorly, (ii) tree-based protocols dominate, (iii) multi-path protocols dominate, or (iv) all protocols perform well. By correlating clusters with graph metrics, we also provide structural explanations for the varied performance of specific protocols. Additionally, motivated by the anticipated high cost of repeaters, we investigated the impact of repeater trimming on the performance of multi-path protocols. Topology strongly governs how far repeater nodes can be removed from the network while maintaining a given performance (distribution rate). For instance, in networks where only 80% of nodes operate as repeaters, well-performing topologies are able to retain over 90% of the distribution rate; whereas sparse, weakly connected graphs exhibit rapid performance degradation, retaining less than half of the distribution rate. Our results provide a topology-aware framework for protocol selection and infrastructure optimisation in future quantum networks, bridging routing design with cost-aware deployment strategies.
Quantum Key Distribution (QKD) exploits the principles of quantum mechanics to enable information-theoretic security, offering significant advantages over classical cryptographic approaches. Despite extensive experimental demonstrations worldwide, many implementations remain confined to laboratory environments that may be impractical for large-scale deployment. This thesis addresses the critical challenge of making QKD viable within real-world telecommunications networks. Current field trials often prioritise metrics such as distance and key rate at the expense of practicality, employing ultra-low-loss fibres, custom components, and dedicated dark channels. These constraints could hinder integration with existing infrastructure. To overcome these limitations, this work introduces UKQNtel—a testbed designed to incorporate commercial-grade equipment and multiplex quantum and classical traffic. UKQNtel represents the first field trial of a coherent one-way QKD system integrated with a commercial-grade encrypted Dense Wavelength Division Multiplexing (DWDM) network, delivering 500 Gbps of encrypted data over 121 km of real-world fibre. The network comprises four trusted-node links, each achieving performance metrics suitable for practical deployment. Complementing the experimental work, novel simulations were developed to predict QKD performance using only fundamental parameters such as distance, wavelength, and optical power; marking a clear improvement over models that require broad assumptions or empirical measurements of an existing QKD link. This simulation develops new tools for simulating non-linear effects such as Raman scattering and four-wave mixing, as well as many other aspects of QKD noise, loss, and detection. Validation against UKQNtel measurements showed strong agreement in attenuation modelling and link-level performance. While secret key rate estimates demonstrate the need to account for additional losses, the model provides a robust foundation for network planning and scalability analysis. This research demonstrates a clear pathway toward practical QKD deployment, combining experimental integration with predictive modelling applicable to larger, more complex networks. The results underscore QKD’s potential to become a cornerstone of future secure communications.