Intermodal quantum key distribution at telecom wavelengths provides a hybrid interface between fiber connections and free-space links, both essential for the realization of scalable and interoperable quantum networks. Although demonstrated over short-range free-space links, long-distance implementations of intermodal quantum key distribution remain challenging, due to turbulence-induced wavefront aberrations which limit efficient single-mode fiber coupling at the optical receiver. Here, we demonstrate a real-time intermodal quantum key distribution field trial over an 18 km free-space link, connecting a remote terminal to an urban optical ground station equipped with a 40 cm-class telescope. An adaptive optics system, implementing direct wavefront sensing and high-order aberration correction, enables efficient single-mode fiber coupling and allows secure key generation of 200 bit/s using a compact state analyzer equipped with room-temperature detectors. We further validate through experimental data a turbulence-based model for predicting fiber coupling efficiency, providing practical design guidelines for future intermodal quantum networks.
@article{arxiv.2602.16680,
title = {Intermodal quantum key distribution over an 18 km free-space channel with adaptive optics and room-temperature detectors},
author = {Edoardo Rossi and Ilektra Karakosta-Amarantidou and Matteo Padovan and Marco Nardi and Marco Avesani and Francesco Bruno Leonardo Santagiustina and Marco Taffarello and Antonio Vanzo and Stefano Bonora and Giuseppe Vallone and Paolo Villoresi and Francesco Vedovato},
journal= {arXiv preprint arXiv:2602.16680},
year = {2026}
}