Secure Data Link Spans 18km Via Free Space

Researchers are addressing the critical challenge of secure long-distance communication through the demonstration of intermodal quantum key distribution over free-space links. Edoardo Rossi, Ilektra Karakosta-Amarantidou, and Matteo Padovan, from the Dipartimento di Ingegneria dell’Informazione, Universit`a degli Studi di Padova, led a collaborative effort with colleagues at ThinkQuantum s.r.l. and the Institute of Photonics and Nanotechnology, National Council of Research of Italy, to achieve this breakthrough. The team, including Marco Taffarello and Antonio Vanzo, successfully implemented a real-time key distribution field trial across an 18km free-space channel, utilising adaptive optics to mitigate turbulence-induced wavefront aberrations. This work represents a significant step towards scalable and interoperable quantum networks, demonstrating secure key generation at 200 bit/s with compact, room-temperature detectors and providing a validated model for predicting fibre coupling efficiency in future intermodal systems.

Secure communications could soon span vast distances without relying on vulnerable infrastructure. This advance paves the way for unhackable networks linking cities and even countries. By successfully transmitting encryption keys through the air over 18 kilometres, utilising standard detectors and adaptive optics, a practical quantum network is now within reach.

Scientists are increasingly focused on building quantum networks capable of seamlessly integrating different transmission methods, such as optical fibres and free-space links. Achieving this interoperability is vital for creating scalable and flexible quantum communication systems, potentially linking terrestrial and satellite-based terminals. Extending quantum key distribution (QKD) to long-distance free-space links presents considerable challenges.

Atmospheric turbulence distorts the wavefront of light, severely hindering efficient coupling into the single-mode fibres essential for receiving quantum signals. Researchers have successfully demonstrated a real-time intermodal QKD field trial over an 18km free-space link, connecting a remote terminal with an urban optical ground station. This achievement bypasses limitations previously encountered with long-distance implementations, where wavefront aberrations impeded effective signal reception.

By employing an adaptive optics system, the team corrected for turbulence-induced distortions, enabling secure key generation at a rate of 200 bit/s. This was accomplished using compact, room-temperature detectors, a departure from systems requiring cryogenic cooling. Atmospheric conditions pose a significant hurdle for free-space quantum communication.

Turbulence introduces distortions that diminish the signal strength and complicate the process of aligning the incoming light with the receiver’s optical fibre. Once inside the receiver, the signal must be efficiently coupled into a single-mode fibre to suppress background noise and enable spectral filtering. By implementing a high-order adaptive optics system, the team dynamically compensated for these phase aberrations, improving signal quality.

For years, scientists have sought to model the impact of turbulence on fibre coupling efficiency. Experimental data was used to refine and validate a turbulence-based model, offering practical guidelines for designing future intermodal quantum networks. Beyond the 18km demonstration, these findings provide a foundation for extending quantum communication over even greater distances and integrating diverse network architectures. The ability to generate secure keys at 200 bit/s represents a step towards practical, long-range quantum communication.

Free-space quantum key distribution over 18km with adaptive optics and varying detector efficiencies

At an overall channel loss of approximately 30 dB, secure key generation was demonstrated during a real-time intermodal key distribution field trial over an 18km free-space link, connecting a remote terminal with an urban optical ground station utilising a 410mm-class telescope. Employing an adaptive optics system with direct wavefront and high-order aberration correction enabled efficient single-mode fibre coupling and secure key generation at a rate of 200 bit/s.

This key rate was achieved using a compact state analyzer equipped with room-temperature detectors. Detector technology impacted performance; experiments utilised superconducting nanowire single-photon detectors (SNSPDs) with 80% efficiency and InGaAs single-photon avalanche diodes (SPADs) with 15% detection efficiency. Signal detection rates peaked at approximately 50kHz during periods of stable atmospheric conditions, while noise remained consistently below 20kHz.

Atmospheric turbulence presented a challenge, necessitating careful monitoring of key parameters. Temperature ranged between 10 and 30 degrees Celsius, while wind speeds fluctuated between 0 and 10km/h. The Fried parameter, r0, a measure of atmospheric turbulence strength, varied between 0 and 20cm. Data collected from the wavefront sensor validated a turbulence-based model for predicting fibre coupling efficiency.

Quantum bit error rates (QBER) remained below 8% throughout the experiments, with values consistently below 6% when using the Z and X bases. Secret key rates (SKR) reached 1600 bit/s with SNSPDs, and approximately 800 bit/s with SPADs. Comparing data from different days confirms the repeatability of the measurements and the stability of the experimental platform. The overall channel attenuation, accounting for free-space propagation, optical losses, fibre coupling, and transmission, was determined to be around 30 dB.

Modelling atmospheric transmission and telescope performance for free-space optical key distribution

A 40cm-class telescope underpinned the optical ground station used in this work, facilitating a real-time intermodal key distribution field trial over an 18km free-space link. Researchers measured the link efficiency up to the primary focus of the receiving telescope, obtaining values ranging from -17 to -10 dB during alignment procedures. This measurement informed a model predicting beam propagation along a horizontal channel, separating atmospheric absorption and telescope collection efficiency.

Atmospheric absorption efficiency was calculated considering a wavelength-dependent absorption coefficient and the 18km link distance, resulting in an estimated range of -6 to -1 dB. The study detailed modelling of telescope collection efficiency, expressed as a function of telescope reflectivity, beam radius, and the Fried parameter. The Fried parameter, r0, quantifies the atmospheric turbulence strength and was central to predicting the received beam waist.

The received beam waist was used to estimate collection efficiency, ranging from -13 to -6 dB according to simulations. The predicted beam waist, between 0.4m and 1m, aligned well with experimental observations. Achieving stable coupling necessitated an adaptive optics system. Implementing direct wavefront and high-order aberration correction compensated for turbulence-induced wavefront distortions, a major impediment to efficient single-mode fibre coupling.

By correcting these aberrations, the setup enabled secure key generation at 200 bit/s, utilising a compact state analyzer with room-temperature detectors. The adaptive optics system directly addresses atmospheric turbulence and is applicable to broader quantum networking tasks beyond key distribution. Validating a turbulence-based model for predicting single-mode fibre coupling efficiency provides practical design guidelines for future intermodal networks.

This validation exploited direct measurements of the Fried parameter, obtained using wavefront sensor data, to estimate achievable coupling efficiency. Comparing these estimates with average experimental values confirmed the accuracy of the free-space system design methodology.

Real-time adaptive optics enable extended range quantum key distribution via free-space links

Scientists have achieved a notable advance in quantum communication, successfully demonstrating intermodal key distribution over an 18km free-space link. Bridging the gap between controlled laboratory settings and the unpredictable conditions of outdoor environments has presented a major hurdle for practical quantum networks. Turbulence in the atmosphere distorts light signals, making it difficult to couple photons efficiently into optical fibres, limiting the range and reliability of these systems.

This demonstration offers a pathway towards scalable and interoperable networks combining fibre optics and free-space links. Unlike previous trials, this experiment incorporated real-time adaptive optics, actively correcting for atmospheric distortions. By implementing direct wavefront correction and addressing high-order aberrations, researchers maintained efficient single-mode fibre coupling, enabling a secure key generation rate of 200 bit/s using detectors operating at room temperature.

The need for extremely cold detectors is lessened, potentially reducing system complexity and cost. Challenges remain in predicting and modelling atmospheric turbulence accurately. Although the team validated a turbulence-based model for fibre coupling efficiency, extending this to diverse geographical locations and weather patterns will be essential.

Improvements in state analysis technology could further increase key generation rates. The 200 bit/s rate, while a positive step, is modest compared to conventional cryptographic systems. The future likely holds hybrid approaches, integrating satellite links with terrestrial networks to create a global quantum web. Projects like Eagle-1 and SAGA are already underway, and this work provides valuable insights for designing and optimising these systems. This research stands out in its practical focus, offering design guidelines for future intermodal networks and paving the way for secure communication infrastructure that blends the strengths of both fibre and free-space technologies.

Free-space quantum key distribution over 18km with adaptive optics and varying detector efficiencies. This experiment incorporated real-time adaptive optics, actively correcting for atmospheric distortions. By implementing direct wavefront correction and addressing high-order aberrations, researchers maintained efficient single-mode fibre coupling, enabling a secure key generation rate of 200 bit/s using detectors operating at room temperature. The need for extremely cold detectors is lessened, potentially reducing system complexity and cost.

Challenges remain in predicting and modelling atmospheric turbulence accurately. Although the team validated a turbulence-based model for fibre coupling efficiency, extending this to diverse geographical locations and weather patterns will be essential. Improvements in state analysis technology could further increase key generation rates. The 200 bit/s rate, while a positive step, is modest compared to conventional cryptographic systems.

The future likely holds hybrid approaches, integrating satellite links with terrestrial networks to create a global quantum web. Projects like Eagle-1 and SAGA are already underway, and this work provides valuable insights for designing and optimising these systems. This research stands out in its practical focus, offering design guidelines for future intermodal networks and paving the way for secure communication infrastructure that blends the strengths of both fibre and free-space technologies.