Spatial Mode Side Channels in Free-Space QKD Implementations

The quantum key distribution protocol uses one degree of freedom of a single quantum system to encode information. If this information has correlations with the system's other degrees of freedom, or if the measurement efficiencies on the receiver side depend on them, a security loophole called side channel is created. An eavesdropper can exploit it to gain information without disturbing the system, and thus, without revealing the attack. Here, we analyze side channels in a free-space QKD sender and receiver implementation and focus especially on the dependencies and side channels for the spatial degree of freedom.