Impact of Physical Channel Impairments and Detector Noise on BB84 Quantum Key Distribution: A Python-Based Simulation Study

Abstract

The absence of real single-photon sources is a significant limitation to the experimental implementation of the BB84 quantum key distribution (QKD) protocol. When substantially attenuated laser pulses are substituted for ideal sources in practical systems, the outcome is weak coherent states where each pulse has a nonzero probability of having multiple photons rather than pure single-photon emission. To evaluate quantum bit error rate (QBER), key generation rate and overall system security it is essential to describe the exact impact of the optical and physical system on such pulses. This practical tradeoff presents additional issues. It is crucial to understand how QBER is impacted by attenuation, loss, dispersion, depolarisation and detector properties particularly in adversarial scenarios such as photon number splitting (PNS) and beam-splitting attacks. Although a significant amount of research has been conducted on algorithmic simulations of BB84, there are few complete frameworks that can physically explain these effects and their impact on QBER. This work models the BB84 protocol at the physical level by developing a Python-based simulation platform that takes into account all significant channel impairments and system inefficiencies. The framework supports both theoretical and practical optimisation of QKD systems by further isolating the effects of different optical losses and eavesdropping techniques on QBER and secure key generation. © 2026 IEEE.