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.
This work was supported in part under Grant PID2020-118178RB-C22 and Grant AEI/10.13039/501100011033; in part by the Community of Madrid, Spain, under the Cybersecurity, Network Analysis And Monitoring for the next generation Internet (CYNAMON) Project, under Grant P2018/TCS-4566; in part by the European Social Fund and European Union (EU) Fondo Europeo de Desarrollo Regional (FEDER) funds; in part by the Spanish National Research Council (CSIC), under Project 202050E232; in part by the CSIC's Interdisciplinary Thematic Platform (PTIC) on Quantum Technologies (PTI-QTEPC); in part by the CSIC's Program for the Spanish Recovery, Transformation and Resilience Plan through the Recovery and Resilience Facility of the EU, established by the Regulation (EU) 2020/2094; and in part by the Ministerio de Ciencia e Innovación (MCIN) from EU NextGenerationEU under Grant PRTR-C17.I1.
