Hoon 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.