Design and Modeling of a PAT System for Freespace Optical Links in Quantum Key Distribution.

This study introduces a novel free-space optical (FSO) communication system, designed for quantum key distribution (QKD) effective up to 2 kilometers and extendable to 10 kilometers. Suitable for terrestrial and unmanned aerial vehicle (UAV) applications, the system centers around an advanced Pointing, Acquisition, and Tracking (PAT) model, specifically tailored for the demands of free-space quantum key distribution (FSQKD) communication. The PAT system, with its dual coarse and fine-tracking capabilities, is crucial in establishing and sustaining robust, quantum-secured optical channels. The fine tracking module, in particular, achieves an impressive 0.11 milliradian accuracy and a swift 3.5 millisecond response time. Key to the system's performance is the optimization of beam wavelengths to minimize beam divergence and diffraction losses, essential for the stable operation of the QKD system. A detailed link budget analysis identifies specific losses: diffraction loss at 5dB for 2 kilometers, escalating to 18.4 dB at 10 kilometers, and receiver insertion losses, including optical, coupling, and wavefront losses, at 10.5 dB. The system's modular design enhances its adaptability for various QKD scenarios. This study gives a detailed approach towards practical, scalable quantum communication networks.