Performance Evaluation of Quantum Key Distribution in Secure Communication Networks: A Link-to-User Simulation Study

Quantum Key Distribution (QKD) provides physical-layer security based on quantum state disturbance rather than computational complexity. However, operational viability depends heavily on physical channel parameters, optical loss, finite-key statistical bounds, and network key-sharing constraints. This paper presents a multi-metric, quantitative evaluation of a three-intensity decoy-state BB84 QKD link and its application within a time-shared star network. Using a Monte Carlo sampling framework (n=60 replications per configuration), we evaluate link distance (0–200 km), attenuation (0.18-0.30 dB/km), detector efficiency (10-40%), dark count probability (10-8-10-5 per gate), eavesdropping intensity (0-50%), privacy-amplification block size (106-109 pulses), and simultaneous user pair load (1-20 pairs). Under baseline settings (0.20 dB/km, 25% detector efficiency, 500 MHz pulse rate, signal intensity μ=0.50, 1.5% misalignment, block size N=5×107), the secure key rate (SKR) drops from 12.83 Mbit/s at 0 km to 295.63 kbit/s at 80 km, reaching non-positive values at 200 km due to finite statistical penalties. Intercept-resend attacks degrade secrecy, suppressing output to zero at ∼20% interception intensity (QBER=6.51%). In the shared network, a target demand of 20 kbit/s per user pair experiences depletion when active pairs exceed 12, despite a positive aggregate link generation rate. The findings demonstrate that QKD viability requires joint evaluation of SKR, QBER, finite-key confidence, latency, and multi-user key supply dynamics.