## 1 Introduction  
Quantum key distribution (QKD) is a mature technology widely known to be the closest to commercialization among quantum application technologies [1]. QKD allows two authenticated distant participants, [PERSON] and [PERSON], to share quantum secret keys securely based on quantum effects [2]. It has evolved over the decades since the first BB84 protocol was proposed [3], and the first commercial product appeared in 2007. To achieve a more prevalent technology, many researchers are still conducting long-distance transmission (optical fiber, satellite) [4; 5], network [6], chip-based [7], and long-term stability tests in actual testbeds implemented in the local area [8; 9; 10].  
More specifically, in terms of commercialization, system stability is one of the key issues in QKD systems. Essentially, a quantum signal is fragile to changes in the external environment, and a delicate control is required for long-term stable operation. From this point of view, plug-and-play (PnP) QKD [11], proposed in 1997, has the advantage that a stable operation is possible even if environmental conditions change abruptly. Therefore, it has a round-trip structure. However, the PnP system still has some technical issues such as a relatively low secret key rate and the need for a bulky storage line. More than anything, the implementation of the decoy-state protocol [12; 13; 14; 15; 16; 17] is relatively tricky, unlike the conventional one-way QKD architecture. The decoy-state protocol was originally proposed to overcome photon splitting attacks. It allows QKD to operate with a higher average photon number, which increases the quantum key generation rate. In QKD using a decoy-state, the intensity of each pulse should be adjusted corresponding to a random number that determines its position and intensity. For adjusting pulse intensity, an intensity modulator (IM) is typically used. In a conventional one-way QKD, there is no issue with using a polarization-dependent IM because it can be implemented to use only a specific polarization within the transmitter (or Alice) system. However, in the case of PnP, it is impossible to implement with the simple connection of IM because the signal delivered from Bob to Alice has random polarization due to the quantum channel (QC) comprising single-mode fibers. If QC is implemented with polarization-maintaining fibers, this issue may disappear, but for cost reasons, almost all of the currently installed fibers are composed of single-mode-fibers. Therefore, for resolving this issue, several technologies have been proposed, including the use of a polarization beam splitter (PBS) [18,19] and the control of the \(V_{bias}\) value [20,21]. However, stability remains a problem, and it requires an additional separate feedback control. It is not easy to control by monitoring the output signal because it handles very tiny signals.  
We propose a structure that controls the intensity of light based on an optical amplifier (OA). This structure generates decoy pulses by controlling the amplification rather than the attenuation of pulses received from [PERSON]. The OA used for amplification is highly stable in terms of signal output, resulting in little variation in the output signal during long-term operation. Thus, it is possible to generate a stable and reliable signal and decoy pulse.