ThinkQuantum and University of Padua Demonstrate 18 km Free-Space QKD

Researchers at the University of Padua and ThinkQuantum s.r.l. have reported a real-time intermodal quantum key distribution field trial over an 18 km free-space channel in northeastern Italy. The work, published August 26, 2026, in npj Quantum Information, used adaptive optics at a 410 mm-class urban optical ground station to couple the telecom-wavelength quantum signal into single-mode fiber feeding a commercial QUKY QKD receiver and generated secret keys at about 200 bit/s with room-temperature detectors. The transmitter sat in the Colli Euganei and the receiver in Padova, with the free-space segment followed by about 0.5 km of deployed fiber.

Adaptive Optics and Fiber Coupling

The free-space segment connected a remote optical transmitter on Monte Grande in the Colli Euganei to an optical ground station at the Department of Information Engineering in Padova. A PRORC400 telescope from Officina Stellare collected the beam with a 410 mm aperture. The quantum signal ran at 1565.50 nm (ITU channel 15) and was multiplexed with auxiliary beacons at 1545.32 nm and 850 nm for tracking and wavefront sensing.

Atmospheric turbulence distorts the arriving wavefront and otherwise prevents efficient injection into single-mode fiber. The receiver used a Shack-Hartmann wavefront sensor and a deformable mirror correcting up to 35 Zernike terms, with a 10 Hz rejection bandwidth. Measured single-mode-fiber coupling was −9.2 dB, compared with −7.2 dB estimated from wavefront-sensor data.

Overall channel attenuation, including free-space propagation, optics, coupling, and the short fiber run, was about 29 dB. Coauthors from the Institute of Photonics and Nanotechnology of the National Research Council of Italy contributed to the adaptive-optics implementation.

The protocol was a 3-state, 1-decoy efficient BB84 implementation with Qubit4Sync synchronization. Field sessions on 2, 3, and 8 April 2025 produced quantum bit error rates below 1% with superconducting nanowire single-photon detectors at about 80% detection efficiency and about 2% with room-temperature InGaAs single-photon avalanche diodes at about 15% efficiency.

Corresponding secret-key rates were about 1 kbit/s and 200 bit/s. The team also validated a turbulence-based model of fiber-coupling efficiency against wavefront-sensor data, as detailed in the peer-reviewed paper and the earlier arXiv preprint.

Principal link parameters included:

18 km free-space path plus about 0.5 km of deployed fiber to the analyzer

50 nm quantum channel with 1545.32 nm and 850 nm beacons

410 mm PRORC400 receiver telescope and adaptive optics correcting up to 35 Zernike terms

Secret-key rates of about 200 bit/s (SPADs) and about 1 kbit/s (SNSPDs)

Commercial Platform and Network Design

The terminals were QUKY units from ThinkQuantum (QUKY-TX transmitter and QUKY-RX receiver), originally designed for fiber networks and operated here without functional modification.

ThinkQuantum is a University of Padua spin-off established with academic founders and Officina Stellare as its major industrial partner. Intermodal operation places the analyzer away from the telescope aperture, so the free-space segment does not dictate the location of the trusted QKD hardware. Authors are also affiliated with the Padua Quantum Technologies Research Center.

The architecture is intended for hybrid fiber and free-space quantum networks where deployed fiber is unavailable or uneconomic over the last span. The paper notes that the same coupling approach is protocol-agnostic and could support entanglement distribution.

Funding included the Italian Space Agency’s Q-SecGround Space project and the Horizon Europe Quantum Secure Networks Partnership (QSNP). The authors flag limited adaptive-optics bandwidth in strong turbulence and point to higher-bandwidth systems for satellite downlinks associated with missions such as ESA Eagle-1 and SAGA.

Bottom Line

Adaptive optics enabled a commercial QKD platform to generate secret keys over an 18 km Colli Euganei-to-Padova ground link.

Find out more here.

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