Giulio Foletto

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 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.

Quantum correlations between measurements of two or more separated observers play a fundamental role in many applications, such as randomness generation or key distribution. Although security can be certified from correlations with minimal assumptions in the device-independent scenario, the performance of such protocols is currently limited. This limitation motivates the exploration of sequential measurements, that is, defined with precise temporal ordering, as a means of improving performance through the reuse of the quantum states. To date, the study of sequential quantum protocols has been modest, lacking a comprehensive mathematical framework to explore the properties of the obtainable correlations. In this study, we adopt a geometric perspective to investigate sequential quantum correlations, providing a general mathematical framework. Here, we analytically prove a Tsirelson-like boundary for sequential quantum correlations, expressed as a trade-off between the amount of nonlocality shared by each sequential user. This boundary is particularly beneficial for the generation of secure quantum randomness. Indeed, observing a correlation on it can certify the maximum attainable bits per state in the case of one remote party and two sequential parties. In contrast to all previous schemes, this can happen even if one of the sequential users does not share any nonlocality. We demonstrate that this quantum boundary can be reached with a simple qubit protocol and investigate numerically the robustness of randomness generation under realistic noise conditions, finding that it greatly improved compared to previous proposals. Our proof-of-concept photonic implementation of the protocol confirms experimentally that our approach certifies more bits per state compared to the standard Clauser-Horne-Shimony-Holt scenario for the same noise, affirming both feasibility and robustness. This study marks a significant advance in understanding sequential quantum correlations, offering valuable insights and new mathematical tools for further fundamental studies and practical applications of efficient device-independent protocols.

The main goal of the NQCIS project is to build a quantum key distribution (QKD) network centered in Stockholm and that is adapted to the particular geographical properties of Sweden. The center of the network will be the AlbaNova hub, which will also be open to selected users to test the QKD technology. From there, fiber links will reach two nodes in the metropolitan area (<20 km in length) and two longer distance points (80-150 km-long links), which will require also a trusted node. The network will use both continuous and discrete-variable devices, the latter being augmented by superconducting nanowire detectors. Furthermore, we are refurbishing an astronomical telescope to serve as an optical ground station for QKD, giving satellite-tracking capabilities. The project has also goals that go beyond deployment. One is advancing research in quantum communication with a study of stabilization techniques for twin field QKD and of noise contribution hindering different QKD protocols, and the other is forming the Swedish quantum work force through coordinated courses, seminars, and outreach events. This poster gives an overview of the entire project.

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.

The 18-month project called PROtocols for Space sEcure Quantum cOmmunication (PROSEQO), funded by the European Space Agency, was coordinated by the University of Padova with Sitael and Qascom as industrial partners. The scope of the project was to assess the protocols feasible for Satellite QKD and then realize an analytical model to describe all the elements that contribute to the Secret Key Rate (SKR). The analytical model was integrated in a dedicated software able to get several input parameters and orbit descriptions and calculate the final SKR. The software was tested in 10 different case studies. Therefore, this can be a useful tool for future Satellite QKD missions as a preliminary step to evaluate mission feasibility. It could also be the starting point for a numerical overview on the practicability of a satellite QKD infrastructure.