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arXiv:2602.16680 · npj Quantum Information

Intermodal quantum key distribution over an 18 km free-space channel with adaptive optics and room-temperature detectors

Edoardo Rossi, Ilektra Karakosta-Amarantidou, Matteo Padovan, Marco Nardi, Marco Avesani, Francesco Bruno Leonardo Santagiustina, Marco Taffarello, Antonio Vanzo, Stefano Bonora, Giuseppe Vallone, Paolo Villoresi, Francesco Vedovato

quant-phSubmitted Feb 18, 2026• Announced Feb 19, 2026• 1 citation

Abstract

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.

DOI: 10.1038/s41534-026-01358-0

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Quantum Key Distribution (QKD) promises a provably secure method to transmit information from one party to another. Free-space QKD allows for this information to be sent over great distances and in places where fibre-based communications cannot be implemented, such as ground-satellite. The primary limiting factor for free-space links is the effect of atmospheric turbulence, which can result in significant error rates and increased losses in QKD channels. Here, we employ the use of a high-speed Adaptive Optics (AO) system to make real-time corrections to the wavefront distortions on spatial modes that are used for high-dimensional QKD in our turbulent channel. First, we demonstrate the effectiveness of the AO system in improving the coupling efficiency of a Gaussian mode that has propagated through turbulence. Through process tomography, we show that our system is capable of significantly reducing the crosstalk of spatial modes in the channel. Finally, we show that employing AO reduces the quantum dit error rate for a high-dimensional orbital angular momentum-based QKD protocol, allowing for secure communication in a channel where it would otherwise be impossible. These results are promising for establishing long-distance free-space QKD systems. High-dimensional quantum key distribution will allow for higher information density and greater error tolerance in future quantum networks. This work experimentally demonstrates how implementing an adaptive optics system in a spatial-mode free-space optical link can allow for quantum communications where it would otherwise be impossible.

Intermodal quantum key distribution (QKD) enables seamless integration of fiber networks and free-space channels, which are essential to create a global quantum network. Here we report on a recent field trial of an intermodal QKD system within a 3-node heterogeneous quantum network, that we presented in details in the work by F. Picciariello et al., arXiv:2310.17441. The network consists of two polarization-based transmitters and a single receiver, where the active channel is alternately switched between a 620-meter free-space link and a 17-kilometer deployed fiber link in the metropolitan area of Padova. The trial evaluates the performance of the free-space channel in relation to the atmospheric turbulence strength. Conducted over several hours during daylight, the trial confirms the interoperability between fiber and free-space channels, achieving a secret key rate on the order of kbps for both types of channels. Notably, the QKD hardware and software function consistently across both channels without requiring different strategies, despite the distinct characteristics of the links. This switching system offers a cost-effective solution for a trusted QKD network by minimizing the number of devices needed across different networks. The results underscore the feasibility of intermodal QKD systems for future quantum networks.

Abstract Future quantum communication infrastructures will rely on both terrestrial and space-based links integrating high-performance optical systems engineered for this purpose. In space-based downlinks in particular, the loss budget and the variations in the signal propagation due to atmospheric turbulence effects impose a careful optimization of the coupling of light in single-mode fibers required for interfacing with the receiving stations and the ground networks. In this work, we perform a comprehensive study of the role of adaptive optics (AO) in this optimization, focusing on realistic baseline configurations of prepare-and-measure quantum key distribution, with both discrete and continuous-variable encoding, and including finite-size effects. Our analysis uses existing experimental turbulence datasets at both day and night time to model the coupled signal statistics following a wavefront distortion correction with AO, and allows us to estimate the secret key rate for a range of critical parameters, such as turbulence strength, satellite altitude and ground telescope diameter. The results we derive illustrate the interest of adopting advanced AO techniques in several practical configurations.