Modelling efficient BB84 with applications for medium-range, terrestrial free-space QKD

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Terrestrial free-space (FS) quantum key distribution (QKD) is ideally suited for deployment in dense urban environments. The transition from laboratory to commercial deployment, however, raises a number of important engineering and deployment issues. Here, we investigate these issues for efficient BB84 using a weak coherent pulse-decoy state protocol. We calculate expected key lengths for different environmental conditions and when the scope for optimisation of protocol parameters is restricted due to practical considerations. In particular, we find that for a fixed receiver basis choice probability, it can be advantageous to allow the transmitter to have a different basis choice probability depending on varying channel loss and background light levels. Finally, we examine the effects of pulse intensity uncertainty finding that they can dramatically reduce the key length. These results can be used to determine the loss budget for the FS optics of a QKD systems and assist in their design.

Original languageEnglish
Article number075002
Number of pages16
JournalNew Journal of Physics
Issue number7
Early online date7 Jul 2022
Publication statusPublished - 5 Aug 2022


  • terrestrial free-space (FS)
  • quantum key distribution (QKD)
  • efficient BB84
  • weak coherent pulse-decoy state protocol
  • optimisation
  • bias choice


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