Practical Side-Channel Attack on Free-Space QKD Systems With Misaligned Sources and Countermeasures

Practical implementations of quantum key distribution (QKD) protocols can introduce additional degrees of freedom in the quantum states that may render them distinguishable to an eavesdropper. This is the case of QKD systems using a different laser source to generate each quantum state, which can lead to temporal, spectral and/or spatial differences among them that can be exploited by a malicious party to extract information of the key. In this work we characterize, and experimentally verify, a side-channel attack on spatially distinguishable states against free-space QKD systems with misaligned laser sources. Specifically, for those emitting Gaussian beams, which is the most common case in free-space QKD. The attack makes theoretically unsafe any QKD system with any angular misalignment between the laser sources. Finally, we propose two countermeasures to eliminate the spatial distinguishability and secure the key exchange.