Chigo Okonkwo

We analyse the secret-key rate (SKR) of continuous-variable quantum key distribution (CV-QKD) when the quantum channel shares a fibre with classical DWDM traffic. Using a physical model of an 88-channel, 50 GHz-spaced C-band link that captures four-wave mixing, spontaneous Raman scattering, Rayleigh backscatter, and out-of-band effects with full frequency dependence, we compute the asymptotic SKR under Gaussian collective attacks across guardband size, quantum-channel position, classical launch power, link distance, and propagation direction. We report three results. First, placing the quantum channel at the band edge with a 150 GHz guardband yields a 108% SKR gain at 3.4% classical capacity loss, half the cost of symmetric centre-band protection. Second, counter-propagation without any guardband produces the same SKR as guardbanded co-propagation at the same operating point. Third, a power threshold near -2 dBm/ch separates two operating regimes: below threshold the link is Raman-limited and the SKR is essentially insensitive to the guardband, while above threshold four-wave mixing dominates and the SKR depends sharply on guardband size and band-edge proximity. These rules apply across the operating regimes typical of installed DWDM metropolitan fibre.

Quantum Key Distribution (QKD) is currently being discussed as a technology to safeguard communication in a future where quantum computers compromise traditional public-key cryptosystems. We conduct a comprehensive security evaluation of QKD-based solutions, focusing on real-world use cases sourced from academic literature and industry reports. We analyze these use cases, assess their security, and identify the possible advantages of deploying QKD-based solutions. We further compare QKD-based solutions with Post-Quantum Cryptography (PQC), the alternative approach to achieving security when quantum computers compromise traditional public-key cryptosystems, evaluating their respective suitability for each scenario. Based on this comparative analysis, we critically discuss and comment on which use cases QKD is suited for, considering factors such as implementation complexity, scalability, and long-term security. Our findings contribute to a better understanding of the role QKD could play in future cryptographic infrastructures and offer guidance to decision-makers considering the deployment of QKD.

A multidimensional optimisation analysis for CV-QKD systems with practical constraints is presented. We demonstrate secret-key-rates >1Mb/s for 30km transmission with arbitrary discrete modulation, utilising 10dB receiver clearance and 100kHz summedlinewidth as a cost-effective implementation.