## 3 Method and Results
The experimental results for controlling the intensity of the pulse are shown in Figure 4. The signal, decoy, and vacuum state can be generated by driving the OA input currents at 350, 420, and 0 mA, respectively. In particular, when OA was in the off state (=0 mA), the extinction ratio reached 46 dB, and thus, the vacuum state can be generated more reliably than a commercial IM of A2-055-10 (Eospace) (>20 dB). This result shows that pulses with different average photon numbers for the decoy-state protocol can be generated well. The experimental results for the visibility of the MZ interferometer are shown in Figure 5. Figure 5 shows the results of the measurement while changing the driving voltage of the PM from \(-4.8\) to \(4.8\). For both signal and decoy pulses, the \(V_{\pi}\) value was 3.05 V, and the interference visibility was measured as 0.9437 and 0.9540, respectively. This means that the quantum bit error rate (QBER) for the signal and decoy pulses can be, at least, 2.82% and 2.29%, respectively, when QKD operates. Additionally, if considering the detector noise of dark count and afterpulse noise, the QBER may increase to 3.12% and 3.67%, respectively. The detector noises are likely to influence decoy pulses more than signal pulses. Interference can occur and QKD can be performed even if the proposed double amplification method changes the intensity of the pulse. Figure 4: Intensity of the output pulse according to the magnitude of the current input to the OA. Figure 3: Double amplification method results: (**a**) intensity of the source and amplification pulses of \(|f\rangle\) and \(|s\rangle\), (**b**) comparison of \(|s\rangle/|f\rangle\) values of original pulse and amplitude pulse. Finally, the experimental results during 26 h of operation of the PnP QKD system are shown in Figure 6. Signal and decoy pulses obtained average key generation rates of 5.37 \(\times\) 10\({}^{-3}\) bit per pulse (b/p) and 1.037 \(\times\) 10\({}^{-3}\) b/p, respectively, and QBER obtained averages of 3.35% and 3.95%, respectively. This result shows the stable key generation rate and QBER even after a long time of experimentation. Thus, it was shown that the proposed decoy generation could operate as stable, as we expected. The decrease in the key generation rate and increase in QBER are mainly due to the slight mismatch between the arrival of the light pulse and the gate timing of the single-photon detector. This mismatch may be due to changes in the laboratory environment and variations in device and equipment performance that may occur over the time of the experiment. This can be improved by adjusting the operating timing of the single-photon detector [24]. Because no automatic timing control system had been configured in our experiments yet, we adjusted optimal timing manually at 24 h. As soon as an adjustment of 1 nsec was made, the key generation rate and QBER were returned, as shown in Figure 1.0  
Figure 1.1 Interference visibility results in (**a**) signal and (**b**) decoy pulse. The measured counts of APD_1 (red) and APD_2 (blue) versus input voltage of PM are shown. It can be seen that the measured counts are well modeled with the sine function. The visibility of signal and decoy pulse are 0.9437 and 0.9540, respectively. Figure 1.2 Key generation rate and QBER (signal and decoy) of the proposed PnP QKD. The experiment was conducted for a total of 26 h. After 24 h, the key generation rate and QBER were restored by manual control of the detector’s gate timing. After that, measurements were taken for 2 more hours.