  Designing a practical Continuous Variable (CV) Quantum Key Distribution (QKD)
system requires an estimation of the quantum channel characteristics and the
extraction of secure key bits based on a large number of distributed quantum
signals. Meeting this requirement in short timescales is difficult. On standard
processors, it can take several hours to reconcile the required number of
quantum signals. This problem is exacerbated in the context of Low Earth Orbit
(LEO) satellite CV-QKD, in which the satellite flyover time is constrained to
be less than a few minutes. A potential solution to this problem is massive
parallelisation of the classical reconciliation process in which a large-code
block is subdivided into many shorter blocks for individual decoding. However,
the penalty of this procedure on the important final secured key rate is
non-trivial to determine and hitherto has not been formally analysed. Ideally,
a determination of the optimal reduced block size, maximising the final key
rate, would be forthcoming in such an analysis. In this work, we fill this
important knowledge gap via detailed analyses and experimental verification of
a CV-QKD sliced reconciliation protocol that uses large block-length
low-density parity-check decoders. Our new solution results in a significant
increase in the final key rate relative to non-optimised reconciliation. In
addition, it allows for the acquisition of quantum secured messages between
terrestrial stations and LEO satellites within a flyover timescale even using
off-the-shelf processors. Our work points the way to optimised global quantum
networks secured via fundamental physics.
