Impact of Atmospheric Turbulence on Spatial Mode Mismatch Attacks in Free‐Space QKD Implementation

ABSTRACT In free‐space quantum key distribution (QKD), atmospheric turbulence affects quantum signals by causing intensity fluctuations, phase distortions, and beam wander, which may be exploited by an eavesdropper. Accurate characterization of the quantum channel and receiver is, therefore, essential. Detector efficiency mismatch has previously been identified as a possible side‐channel vulnerability. This study examines the impact of atmospheric turbulence on the security of a BB84 QKD receiver under a spatial‐mode mismatch attack. Turbulence is emulated in the laboratory using phase screens implemented on a spatial light modulator. Results show that turbulence exacerbates inherent detection probability mismatches, increasing the receiver's vulnerability. Information leakage to an eavesdropper is characterized and quantified based on both detection mismatches and turbulence strengths. A comparative analysis between Gaussian and Laguerre‐Gaussian (LG) modes shows that LG modes, particularly those of higher order, exhibit enhanced resilience against turbulence‐induced information leakage. These findings highlight the importance of receiver characterization and spatial mode selection for the secure implementation of QKD in long‐distance and satellite‐based systems under realistic atmospheric conditions.