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For a compact quantum key distribution (QKD) sender for the polarization encoding BB84 protocol,an eavesdropper could take a side-channel attack by measuring the spatial information of photons to infer their polarizations.The possibility of this attack can be reduced by introducing an aperture in the QKD sender,however,the effect of the aperture on the QKD security lacks of quantitative analysis.In this paper,we analyze the mutual information between the actual keys encoded at this QKD sender and the inferred keys at the eavesdropper (Eve),demonstrating the effect of the aperture to eliminate the spatial side-channel information quantitatively.It shows that Eve’s potential on eavesdropping spatial side-channel information is totally dependent on the optical design of the QKD sender,including the source arrangement and the aperture.The height of compact QKD senders with integrated light-emitting diode (LED) arrays could be controlled under several millimeters,showing great potential on applications in portable equipment.