Giulio Foletto

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.

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.