Francesco Vedovato

Quantum communication is advancing toward large-scale quantum networks, with quantum key distribution (QKD) serving as a key driving technology. However, seamless interoperability between fiber-based and free-space links remains a major challenge for heterogeneous quantum networks. Here we report, to the best of our knowledge, the first continuous-variable QKD (CV-QKD) system distributing secret keys using both coherent and squeezed states over a hybrid channel composed of a 620m free-space link followed by 2km of optical fiber, corresponding to a total loss of 20 dB. Daylight operation is enabled by intrinsic mode filtering provided by a locally generated local oscillator, eliminating the need for complex spectral or spatial filtering. In addition, we introduce an optimized binning strategy that mitigates free-space transmittance fluctuations, resulting in an average of 45% increase in the secure key rate. These results demonstrate the feasibility of CV-QKD across hybrid optical channels and highlight its potential as a plug-and-play solution for heterogeneous quantum networks integrating fiber and free-space infrastructure.

Here we present a simple and robust polarization encoded QKD experiment where synchronization, polarization compensation and QKD are all performed with the same optical setup, without requiring any changes or any additional hardware, by exploiting only the transmission of quantum states. Furthermore, the developed polarization encoder exhibits high stability and the lowest intrinsic Quantum Bit Error Rate ever reported.

Space-based quantum key distribution would allow, in the near future, secure communications be- tween parties over continental distances, complementing short-range fiber-based quantum networks. However, further demonstrations of daylight operations over free-space channels and the full compatibility with the telecom-based fiber infrastructure are still necessary. Here we present the prototype for daylight QKD at 1550 nm we developed as a demonstrator for application of QKD both on ground and in Space. Our QKD source, exploiting integrated silicon photonics technology, allows to reach a QBER of 1% during the field-test performed over a 145 m link, and represents a promising resource to design quantum optical payloads to be implemented in future satellite missions.

Intermodal quantum key distribution (QKD) provides a fundamental interface for scalable quantum networks. Nevertheless, long-distance implementations are typically compromised by turbulence-induced aberrations, which decrease coupling efficiency into single-mode fiber. Here, we report a real-time intermodal QKD field trial over an 18 km free-space link, connecting a remote terminal to an urban optical ground station. By utilizing an adaptive optics system for high-order aberration correction, we achieved efficient single-mode fiber coupling and a secure key rate of 200 bit/s using room-temperature detectors. Furthermore, we validate a turbulence-based coupling model that provides practical design guidelines for future interoperable quantum architectures.

We report, to the best of our knowledge, the first experimental demonstration of wavelength-division multiplexing between independent continuous-variable (CV) and discrete-variable (DV) quantum key distribution (QKD) systems operating simultaneously on the same optical link. We validate coexistence over a fiber and a daylight free-space channel, benchmarking the Secret Key Rate (SKR) versus channel attenuation while both systems operate simultaneously. We observe the expected CV-DV complementarity, with CV-QKD providing higher SKR at low loss and DV-QKD becoming advantageous in the high-loss regime. In free-space daylight, both systems sustain Mbit/s key rates under atmospheric fluctuations. In all scenarios analyzed, simultaneous operation introduces negligible multiplexing-induced penalty. These results provide a validation of hybrid CV-DV architectures for heterogeneous quantum communication infrastructures, where high-throughput metropolitan users and long-reach links can be simultaneously served on the same physical channel.

Intermodal quantum key distribution enables the integration of fiber networks and free-space channels, essential components for developing a global quantum network. We conducted a field trial of an intermodal quantum key distribution system, featuring two polarization-based transmitters and a single receiver. In this trial, the active channel was alternately switched between a 620-meter free-space link and a 17-kilometer deployed fiber in the metropolitan area of Padova. The free-space channel's performance was assessed in relation to atmospheric turbulence strength. The field trial, conducted over several hours in daylight, demonstrated the intermodal functionality between fiber and free-space channels. Our switching system offers a cost-effective solution for a trusted quantum key distribution network, minimizing the number of necessary devices across different network topologies.

We describe a QKD field trial running on urban fibers deployed in Padua, Italy. This is the first validation outside of the laboratory environment of a new low-error and calibration-free polarization encoder, called iPOGNAC, which we also present here. Our system is resource- and cost-effective, and can be installed quickly in an existing fiber network.