Vulnerability in Free Space QKD Due to Detection Coupling Mismatch

Practical implementations of QKD protocols involve devices which are not perfect, and an eavesdropper may exploit this to gain information leading to attacks. Here we have considered the effects of coupling mismatch between the detectors. We find possible information leakage to Eve due to coupling mismatch at the receiver's detectors in terms of mutual information between the eavesdropper and receiver. The experiment has been performed for the Gaussian and Laguerre-Gaussian modes of the signal. This aspect becomes essential while implementing free space QKD using a satellite. The results suggest that accounting for detection coupling mismatch is crucial to avoid side-channel attacks.