On August 26, 2026, researchers from the University of Padova, Italian photonics company ThinkQuantum, and the National Research Council of Italy published a field trial of an intermodal quantum key distribution (QKD) system in npj Quantum Information. The experiment sent quantum signals across an 18-km horizontal free-space link from Monte Grande to an optical ground station at the University of Padova, then coupled the light into a 0.5-km deployed fiber to reach a receiver lab.
The setup used ThinkQuantum's QUKY polarization-encoded QKD platform running an efficient three-state one-decoy BB84 protocol at a telecom wavelength of 1565.5 nm. The paper and Quantum Computing Report's summary supply the numbers below.
Adaptive optics made the link work
Atmospheric turbulence over an 18-km terrestrial path distorts the wavefront of a light beam and makes it hard to inject photons into single-mode fiber. The team built a high-order adaptive-optics bench at the receiving telescope to correct the distortion in real time. It used a 64-actuator piezoelectric deformable mirror, a Shack-Hartmann wavefront sensor, and closed-loop feedback to correct up to 35 Zernike modes. The system reached single-mode fiber coupling efficiencies up to 19%, high enough to pass the free-space signal directly into standard telecom fiber without an untrusted intermediate measurement.
Room-temperature detectors change the economics
The headline performance figure is the secure key rate with uncooled detectors. Using room-temperature indium gallium arsenide single-photon avalanche diodes with about 15% detection efficiency, the team generated a secure key rate of roughly 200 bit/s with a quantum bit error rate near 2%. For comparison, the same setup paired with cryogenic superconducting nanowire single-photon detectors at 80% efficiency produced about 1 kbit/s with error rates under 1%.
Cryogenic detectors perform better, but they need liquid helium cooling and are harder to deploy outside a lab. Showing that a room-temperature receiver works over this distance is the practical advance. It removes one cryogenic burden from field deployments and from future satellite-to-ground receivers.
Where this fits in quantum networking
The architecture is protocol-agnostic and intermodal, meaning it combines free-space and fiber in one continuous link. That matches the deployment model planned for the European Quantum Communication Infrastructure and for upcoming European Space Agency satellite missions such as Eagle-1 and SAGA. The experiment is a step toward terrestrial and satellite-compatible quantum networks, not a finished product.
The trial ran under specific atmospheric conditions and does not prove all-weather performance. The key rates, error rates, and coupling efficiency are reported results from the paper and from Quantum Computing Report's write-up. Independent replication under different weather and turbulence conditions would strengthen the claim.
For more context, read our quantum networking explainer and our post-quantum cryptography guide. QKD provides physical-layer key distribution, but the broader cryptographic migration problem is being handled by post-quantum algorithms rather than by quantum networks alone.