Impact of Crosstalk and Core Allocation on BB84 Quantum Key Distribution over Multicore Fiber

Abstract

Quantum key distribution (QKD) enables the exchange of encryption keys with unconditional security by exploiting the principles of quantum mechanics. This paper investigates the BB84 protocol implemented over multi-core fibers (MCFs), extending its conventional single-core deployment. The adoption of MCFs for QKD introduces several new challenges, including inter-core interference, spatial mode coupling, and complex resource allocation. Although MCFs enhance spatial bandwidth, they also impose additional design and security constraints. The study evaluates the performance of BB84 in terms of Quantum Bit Error Rate (QBER) and Secret Key Rate (SKR) across varying inter-core crosstalk levels, fiber designs (7-core step-indexed and trench assisted), C-band wavelengths, and transmission distances up to 100 km. The impact of fiber type on quantum state coherence is also examined. Furthermore, a novel metric, termed Mean Effective Adjacent Classical Core Count (MEACCC), is introduced to optimize core allocation strategies. A comprehensive decoherence model incorporating attenuation, Raman scattering, nonlinearities, and crosstalk highlights the influence of fiber structure on QKD scalability. © 2025 IEEE.