This manuscript discusses the most relevant aspects of the practical implementation of a long-range Quantum Key Distribution (QKD) link with trusted nodes, achieving the highest possible secret key rate generation within the security and system-level constraints. To this purpose, it describes the implementation of an end-to-end QKD system, including implementation aspects from the physical transmission of photon states through a standard telecommunications grade optical fiber, to consideration of quantum metrology and information reconciliation protocols based on forward error correction codes. In addition, since there are circumstances when a fiber optical link may not be available, it examines the problems involved with the implementation of a Free Space Optics (FSO) QKD link. The manuscript also discusses the problem of information reconciliation in Continuous Variable (CV) QKD scenarios on FSO links, showing that in long distance links, since the sign of the received Gaussian samples contains the largest fraction of information, Unequal Error Protection (UEP) reverse reconciliation schemes can be designed. The presented results have been achieved within the NATO SPS project “Analysis, design and implementation of an end-to-end 400 km QKD link”.
This paper is focused on the problem of Information Reconciliation (IR) for continuous variable Quantum Key Distribution
(QKD). The main problem is quantization and assignment of labels to the samples of the Gaussian variables observed at
Alice and Bob. Trouble is that most of the samples, assuming that the Gaussian variable is zero mean which is de-facto
the case, tend to have small magnitudes and are easily disturbed by noise. Transmission over longer and longer distances
increases the losses corresponding to a lower effective Signal to Noise Ratio (SNR) exasperating the problem. Here we
propose to use Permutation Modulation (PM) as a means of quantization of Gaussian vectors at Alice and Bob over a
d-dimensional space with d ≫ 1. The goal is to achieve the necessary coding efficiency to extend the achievable range of
continuous variable QKD by quantizing over larger and larger dimensions. Fractional bit rate per sample is easily achieved
using PM at very reasonable computational cost. Ordered statistics is used extensively throughout the development from
generation of the seed vector in PM to analysis of error rates associated with the signs of the Gaussian samples at Alice
and Bob as a function of the magnitude of the observed samples at Bob.
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