Francesco Bruno Leonardo Santagiustina

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.

The successful application of Quantum Key Distribution is dependent on the accurate generation and detection of quantum states, and a communication mechanism that can withstand disturbances caused in the channel. The selection of the optimal encoding strategy is complex and is influenced by external elements such as the characteristics of the quantum channel. Polarization encoding is acknowledged for its dependability and low error rate, rendering it ideal for free-space links, whereas time-bin encoding is robust to birefringence, thereby making it suitable for optical fiber networks. The strength of polarization-based protocols is based on the full characterization of the receiver, to reconstruct the information encoded in the shared qubits. This is typically achieved through tomographic analysis, which adds to the complexity of the final protocol. In this research, we introduce a unique cross-encoded method, where high precision quantum states are produced using a self-regulating, calibration-free polarization modulator and then conveyed through a polarization-to-time-bin converter. A hybrid receiver is used to carry out both time-of-arrival and polarization measurements for decoding the quantum states. Moreover, the suggested receiver is optimized to perform a self-characterization process, utilizing the same photons in which the information is encoded. The adaptability of our approach can lead to a significant advancement in the creation of hybrid networks that incorporate both optical fiber and free-space networks.

Entanglement is a unique and invaluable resource for quantum information processing because it highlights the non-locality property, which allows for device-independent (DI) quantum communication protocols, such as Quantum Key Distribution and Quantum Random Number Generation. However, the distribution of entanglement over long distances is challenging due to propagation losses and instability. Time-bin entanglement is a promising solution since it is robust in long-distance distribution over fiber optics and immune to the polarization distortion such a channel can introduce. Time-bin has also been demonstrated to be compatible with NV center-based quantum technologies, representing a crucial interface between the different devices in quantum networks. Nevertheless, its most common implementation suffers from a post-selection loophole (PSL), which invalidates Bell non-locality tests and renders it vulnerable to quantum hacking attacks, thus preventing its use for device-independent protocols. We present a scheme using optical switches to obtain only detection events displaying non-local interference, thereby closing the PSL. Our scheme works with 1550nm biphotons for entanglement distribution over existing fiber-based telecom networks. The switches show a high extinction ratio of up to 30dB and stability over extended periods. We also measure interferometric visibilities of over 94%, which corresponds to a CHSH S parameter of 2.65

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.