In-field experiment of Intermodal Quantum Key Distribution over a 18km-long free-space link with Adaptive Optics for fiber-injection

In the modern era of information, secure communication is a fundamental societal need. Quantum Key Distribution (QKD), once combined with the One-Time- Pad protocol, offers information-theoretic security for data transmission. While significant progress has been made in QKD over fiber , extending secure links over longer distances remains challenging due to intrinsic fiber losses. Free-space QKD offers a promising alternative for establishing long-distance links with lower transmission losses, especially in scenarios where deployed fibers are not viable or satellite-based applications to reach global coverage. This thesis presents the realization of a 18-km long free-space QKD link, we focus on beam propagation modeling and channel efficiency calculations, validated through the field experiment where adaptive optics technology has been used for single-mode- fiber coupling at the receiving telescope. This experiment demonstrates successful key exchanges with polarization-based decoy-state BB84 QKD protocol. Furthermore, we investigate the application of QKD in short-range free-space communication, developing a compact, rapidly deployable system incorporating custom control software for active pointing and tracking. Our results contribute to advancing high-performance, field-deployable QKD systems for both long-distance backbone links and agile short-range communication setups.