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 states, 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 sidechannel 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.

Wavefront tilt correction is necessary for reducing beam deviations caused by atmospheric turbulence in the receiver of an optical system. This reduction allows decreasing the receiver's field of view, which in the case of free-space quantum key distribution (QKD), reduces the solar background noise that reaches the detectors, and with this, the quantum bit error rate (QBER). A wavefront tilt correcting system that stabilizes the beam in two different points of the receiver's optical axis has been developed and characterised, and it is capable of operating under realistic conditions of strong turbulence with almost perfect ideal correction. The reduced area of beam deviations at the focal plane of the receiver after correction enables a reduction of the QBER of more than 80%, which paves the way to free-space quantum key distribution in daylight under strong turbulence.

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