https://doi.org/10.1140/epjqt/s40507-025-00340-7
Research
Characterising higher-order phase correlations in gain-switched laser sources with application to quantum key distribution
1
Vigo Quantum Communication Center, University of Vigo, E-36310, Vigo, Spain
2
Escuela de Ingeniería de Telecomunicación, Department of Signal Theory and Communications, University of Vigo, E-36310, Vigo, Spain
3
AtlanTTic Research Center, University of Vigo, E-36310, Vigo, Spain
4
Instituto de Física de Cantabria, CSIC-University of Cantabria, E-39005, Santander, Spain
5
Faculty of Engineering, University of Toyama, Gofuku 3190, 930-8555, Toyama, Japan
Received:
17
January
2025
Accepted:
10
March
2025
Published online:
19
March
2025
Multi-photon emissions in laser sources represent a serious threat for the security of quantum key distribution (QKD). While the decoy-state technique allows to solve this problem, it requires uniform phase randomisation of the emitted pulses. However, gain-switched lasers operating at high repetition rates do not fully satisfy this requirement, as residual photons in the laser cavity introduce correlations between the phases of consecutive pulses. Here, we introduce experimental schemes to characterise the phase probability distribution of the emitted pulses, and demonstrate that an optimisation task over interferometric measures suffices in determining the impact of arbitrary order correlations, which ultimately establishes the security level of the implementation according to recent security proofs. We expect that our findings may find usages beyond QKD as well.
Key words: Quantum communications / Quantum cryptography / Quantum key distribution / Gain-switched laser / Optical phase correlations
Supplementary Information The online version contains supplementary material available at https://doi.org/10.1140/epjqt/s40507-025-00340-7.
© The Author(s) 2025
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