Security–efficiency optimization in multi-threshold intensity-modulated discrete-variable secret-key distribution

This paper presents a new discrete-variable secret-key distribution (DV-SKD) protocol based on multi-threshold (MT) intensity-modulation and direct-detection (IMDD). The protocol employs a multi-symbol decision structure with two erasure regions and a joint optimization of modulation depths and decision thresholds, enabling enhanced control over the trade-off between symbol distinguishability and secrecy. This multi-threshold architecture increases the information extracted per accepted detection event while preserving the simplicity of IMDD hardware. We develop a full optimization framework for the modulation parameters under realistic noise models and evaluate performance using both forward and reverse reconciliation. Numerical simulations show that MT-IMDD achieves robust key-generation rates (KGR) under practical operating conditions and maintains strong resilience against both passive and active eavesdropping attacks.