Researchers in Italy demonstrated quantum key distribution across an 18-kilometer free-space optical link connected to standard telecom fiber, using adaptive optics to counter atmospheric turbulence. The system generated secret keys at about 1,000 bits per second using superconducting detectors and 200 bits per second with room-temperature detectors, despite roughly 30 decibels of channel loss. The work used commercial BB84-protocol QKD equipment from ThinkQuantum and could inform future satellite and terrestrial quantum networks, including ESA's Eagle-1 and SAGA programs.
Table of contents
Quantum Keys Across Air and FiberCorrecting the AtmosphereRoom-Temperature DetectorsLimits and Future WorkQuestions this post answers
What secret key rate did the 18-kilometer free-space to fiber QKD experiment in Italy achieve?
The field trial achieved an average secret key rate near 1,000 bits per second using high-efficiency superconducting nanowire detectors, and about 200 bits per second with compact room-temperature indium gallium arsenide avalanche diode detectors. This occurred despite roughly 30 decibels of total channel loss across the 18-kilometer free-space link plus fiber segment, using commercial ThinkQuantum BB84 equipment at 1,565.50 nanometers. Anyone tracking real-world QKD performance benchmarks can follow experimental results like this on daily.dev.
How does adaptive optics help connect free-space quantum links to fiber optic networks?
Adaptive optics corrects atmospheric turbulence that distorts light before it enters a single-mode fiber, which is essential for coupling free-space quantum signals into fiber-based infrastructure. The Padua experiment used a 41-centimeter telescope, a Shack-Hartmann wavefront sensor, and a deformable mirror with 64 actuators, achieving about 12% coupling efficiency into the fiber after correction. Engineers exploring hybrid optical network designs can keep up with adaptive optics research via daily.dev.
What limits the correction speed of adaptive optics systems used in free-space quantum key distribution?
The experimental controller's correction bandwidth reached only about 10 hertz, well below its estimated theoretical maximum of roughly 24 hertz, due to communication delays, sensor noise, and use of a general-purpose computer rather than dedicated real-time hardware. Researchers suggest dedicated real-time controllers could push bandwidth above 100 hertz, enabling better performance under strong daytime turbulence. Those evaluating adaptive optics hardware tradeoffs for optical links can find related research on daily.dev.