Implementation Security of Practical Decoy-State QKD: A Dynamic Source-Deviation Model
DOI:
https://doi.org/10.31861/sisiot2026.1.01021Keywords:
QKD, decoy-state, implementation security, source memory, laser dynamicsAbstract
This paper develops a theoretical model for source-side implementation security in practical decoy-state quantum key distribution (QKD). The research object is a weak-coherent-pulse transmitter in which the emitted mean photon number is affected not only by the selected decoy setting, but also by semiconductor-laser dynamics, electrical drive response, intensity-modulator memory, residual optical effects, and slow drift. The transmitter is described at two modelling levels. The first layer is a nonlinear continuous-time carrier-photon-phase model used to explain physical mechanisms. The second layer is a pulse-indexed validation surrogate obtained by sampling the emitted pulse train under a declared operating regime and stroboscopic extraction rule. This distinction avoids treating a local discrete model as a globally valid replacement for nonlinear gain-switched laser dynamics. The model maps the selected intensity sequence to the actual pulse mean photon number, defines an operational memory length, and introduces vacuum-safe deviation metrics, residual-correlation diagnostics, conditional intensity laws, and photon-number probability intervals. These quantities are explicitly distinguished from composable security parameters. The security interpretation is an assumption audit: if the emitted intensity or phase statistics are history dependent, the standard decoy-state reduction to setting-independent photon-number yields and error rates is no longer automatic. The proposed framework therefore does not claim a new complete QKD security proof; it identifies measurable source quantities that must be exported to an appropriate finite-key or imperfect-source proof. The result is a mathematically traceable bridge between transmitter dynamics, source validation, and implementation-security requirements for practical decoy-state QKD systems. The objective of this paper is to develop a mathematically traceable dynamic source-deviation model for practical decoy-state QKD transmitters and to show how transmitter memory affects the assumptions used in decoy-state security analysis.
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