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 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, the implementation of an end-to-end QKD system will be discussed, including implementation aspects from physical transmission of photon states through a standard telecommunications grade optical fiber, to consideration of device imperfections, information reconciliation protocols. In addition, since there are circumstances when a fiber optical link may not be available, we will also discuss a test bench implementation of a Free Space Optics (FSO) QKD link.
Furthermore, in spite of the fact that Discrete Variable QKD (DV-QKD) systems have reached a maturity level that allows their potential full realization and implementation for creation of a secure network backbone for key distribution in nations, in realistic links DV-QKD is really limited by technology and physical constraints associated with construction of reliable high rate single photon (or at least low photon count) sources, and of fast and reliable single photon detectors with very low dark count rates. In these cases, the use of Continuous Variable QKD (CV-QKD) schemes may be advantageous. For this reason the paper also discusses the problem of information reconciliation in CVQKD scenarios, showing that in long distance links the sign of the received Gaussian samples contains the largest fraction of information, leading to the design of an Unequal Error Protection (UEP) reverse reconciliation scheme.
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