Jin-Woo Kim

Optical phased arrays (OPAs) are beam-steering devices that control the direction of light by adjusting the relative phase of multiple emitters and have been widely studied in free-space optical communications and LiDAR. Recently, their potential has also been explored in quantum applications such as free-space quantum key distribution (QKD). However, the far-field pattern of OPAs inherently includes not only a main lobe in the desired direction but also sidelobes, whose impact on QKD performance has not yet been investigated. In this work, we study the effect of sidelobes on QKD performance from a system-level perspective. We relate the sidelobe suppression ratio (SSR) of an OPA transmitter to QKD performance based on a decoy-state BB84 model. We focus on how power leakage into sidelobes reduces the signal intensity in the main beam and lowers the secure key rate. To capture this effect, we adopt a simplified model in which sidelobe power is treated as a loss from the main beam. Under a linear optical assumption, this loss can be treated in the same way as channel attenuation in standard QKD analysis. This work connects OPA beam characteristics, including sidelobe effects, to QKD system performance.

We propose and experimentally demonstrate a passive polarization stabilization method for robust entanglement distribution using a cross-aligned pair of polarization-maintaining fibers (PMFs). Unlike standard single-mode fiber links, the proposed configuration inherently mitigates polarization and phase fluctuations without active feedback control. Using polarization-entangled photon pairs generated from a Sagnac interferometer, we compare SMF-based transmission and the proposed PMF configuration under stable and mechanically perturbed conditions. While the SMF-based system shows severe degradation under vibration and fiber bending, with the average visibility dropping to 0.444, the proposed PMF configuration maintains a high average visibility of 0.867 with minimal fluctuation. These results demonstrate that the proposed approach enables reliable entanglement distribution under field conditions without complex active compensation, providing a practical route toward scalable quantum communication systems.

Quantum information technologies that utilize entangled photon pairs assume a single- photon source. While this assumption poses no significant issues when the channel loss is low, high loss can have a detrimental impact on the system's performance. To overcome high loss, the most intuitive solution is to increase the gain of entangled photon pairs by sending a large quantity of them. However, high-gain sources tend to degrade the quantum quality of entangled photon pair sources. We derived the density matrix of the quantum state in the distribution of polarization-entangled photon pairs under the non- symmetric channel losses with threshold detectors. We analyzed the variation of the CHSH inequality parameter S and the effective photon state transfer probability 𝑁𝑚 by changing the non-linear gain γ. The increase and subsequent decrease in Nm with increasing γ can be interpreted as follows: when γ is small, the state is not properly transmitted due to high loss, but as γ increases, the error probability, such as double-click events, increases due to the influence of multi-photon events, leading to a decrease in Nm. This result indicates the need to optimize the brightness of the light source for practical implementation in quantum information technologies. This study is expected to contribute to the analysis of discrete variable quantum key distribution(DVQKD) systems like BBM92, E91, and long- distance quantum imaging systems in the future.