Preparation of free-space quantum key distribution using adaptive optics for long-distance communication

Abstract

The literature presents the preparation of free-space quantum key distribution (QKD) links enhanced by adaptive optics to improve coupling efficiency and extend the operational distance. This study demonstrates the construction of a quantum channel based on the BB84 protocol using polarisation encoding, which was successfully implemented over a 70-metre indoor free-space optical link. To overcome the limitations posed by atmospheric turbulence and optical misalignment, two major challenges in long-distance QKD, adaptive wavefront correction was introduced using a spatial light modulator (SLM). An optimisation algorithm was developed to generate an optimal phase mask based on Zernike polynomials. These phase masks are used to correct static optical aberrations by maximising fibre coupling efficiency at the receiver. The adaptive optics system demonstrated a coupling improvement of up to 188% in a controlled 1-meter testbed and enabled effective compensation for wavefront distortions equivalent to those caused by 5.9 km of free-space turbulence. This research shows the viability of using adaptive optics to enhance transmission channel performance in practical, long-distance scenarios. The method lays a foundation for integrating real-time wavefront correction into quantum networks, and has potential applications in ground-to-satellite links.

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