Jin-Kyu Yang

We present an experimentally grounded security-analysis framework for decoy-state BB84 quantum key distribution (QKD) using an imperfect GaN-based single-photon source (SPS). A single GaN emitter is identified by confocal mapping and characterized using a Hanbury Brown–Twiss interferometer, yielding g(2)(0) = 0.29 ± 0.10. The multi-photon character of the source is further confirmed by measuring the n-fold coincidence counts (N0–N4). Rather than assuming a Poissonian or thermal photon-number model, the truncated photon-number distribution {P0, P1, P2} is analytically inferred from the measured g(2)(0) and mean photon number, and propagated through the encoding-path transmittance to obtain the effective signal and decoy photon-number distributions. These are incorporated directly into a two-decoy-state BB84 security analysis to estimate single-photon yields and error rates. In a laboratory free-space QKD experiment, the system achieves a signal-state sifted key rate of 6.90 ± 0.14 kbps and a quantum bit error rate (QBER) of 3.82 ± 0.24%, confirming positive asymptotic key-rate performance and providing a practical route for connecting solid-state SPS characterization with QKD security modeling.