Composable Security of Decoy-State BB84 with Semiconductor Laser Phase Noise: A Unified Source–Channel Model and Parameter-Dependent Key-Rate Bounds

Wadhah Al-Zofi

doi:10.15625/0868-3166/24012

Author affiliations

Authors

Wadhah Al-Zofi Changchun University of Science and Technology https://orcid.org/0009-0002-8105-0597

DOI:

https://doi.org/10.15625/0868-3166/24012

Abstract

Decoy-state BB84 security proofs often assume fully phase-randomized coherent pulses, yet gain-switched semiconductor lasers exhibit parameter-dependent phase diffusion that leaves inter-pulse correlations. We present a unified source–channel model that maps measurable linewidth, pulse spacing, and intensity modulation to effective photon-number statistics and state overlaps, and integrate it into a finite-size, composable security analysis against general attacks. The resulting key-rate bounds depend explicitly on phase-noise parameters, revealing how residual phase memory tightens decoy estimates and increases privacy amplification. Simulations with 0.2 dB/km fiber loss, InGaAs APDs (10% efficiency, 6×10^−7 dark counts/gate), misalignment 5×10^−3, and 10^8–10^12 pulses show reach reductions of tens of km, while MHz linewidths enable secure 100–130 km operation at total security ≈10^−15.

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