Beyond the 11% Threshold: Statistical Noise Decomposition for Enhanced Eavesdrop Detection in BB84 Quantum Key Distribution

Standard BB84 QKD detects eavesdropping via a fixed 11% error rate threshold, which does not distinguish legitimate channel noise from eavesdropper-induced errors. We present an enhanced method that decomposes the observed error rate into expected noise contributions (channel transmission, detector dark counts, optical misalignment, timing jitter) and applies statistical significance testing to the anomalous residual. The method provides a four-level detection verdict with calibrated confidence. We demonstrate catching an eavesdropper intercepting 30% of qubits at 9.8 sigma significance when standard BB84 reported "secure" (6.88% < 11%). The method reduces false positives by a factor of 80,000 in noisy channels while eliminating false negatives from low-intercept-rate attacks.