Tommaso Occhipinti

Quantum Key Distribution (QKD) protocols require Information‑Theoretically Secure (ITS) authentication of the classical channel to preserve the unconditional security of the distilled key. Standard ITS schemes are based on one-time keys: once a key is used to authenticate a message, it must be discarded. Since QKD requires mutual authentication, two independent one-time keys are typically consumed per round, imposing a non-trivial overhead on the net security key rate. In this work, we present the \emph{authentication-with-response} scheme, a novel ITS authentication scheme based on $\varepsilon$-Almost Strongly Universal\textsubscript{2} ($\varepsilon$-ASU\textsubscript{2}) functions, whose IT security can be established in the Universal Composability (UC) framework. The scheme achieves mutual authentication consuming a single one-time key per QKD round, halving key consumption compared to the state-of-the-art.

The advent of cryptographically relevant quantum computers poses an existential threat to classical public-key infrastructure. Quantum Key Distribution (QKD) addresses this challenge by providing information-theoretic security for key establishment, independently of any computational hardness assumption. In this work, the deployment and experimental validation of a metropolitan-scale quantum-secure network between data centers in Milan is reported. The network operates over installed fiber infrastructure and implements a layered architecture integrating QKD hardware, standards-compliant Key Management (KM), and centralized Software-Defined Networking (SDN) orchestration. Dynamic path reconfiguration via active optical switching and trusted-node routing allow automated fail-over solutions. Application-layer validation across diverse protocols and workloads confirms the seamless interoperability of all system components. These results establish the technical and operational readiness of metropolitan QKD networks for production deployment, and offer a replicable blueprint for building quantum-secure communication infrastructure at metropolitan scale.

Free space quantum key distribution (QKD) has now achieved a groundbreaking advancement in secure communication, enabling long-distance private key exchange and ensuring unbreakable encryption. However, complete compatibility between fiber and free-space infrastructures remains a challenge for a fully integrated QKD system. Indeed, free space and fiber-based QKD commonly utilize different wavelengths and qubit encoding schemes that optimize photon transmission in their respective channels. Free-space QKD state generators usually employ visible light due to their lower beam divergence compared to longer wavelengths and polarization encoding for their resilience against turbulence. In contrast, fiber-based QKD primarily utilizes the C-band, which exhibits the lowest losses in silica fibers, and employs time-bin encoding to mitigate the effects of polarization instability in optical fibers. In our field trial, we demonstrate the viability of performinging QKD from a remote sender (Alice) to a fiber-based receiver (Bob) using the same signal without any wavelength or encoding conversion. We employ a time-bin encoded QKD protocol operating in the C-band through horizontally turbulent free-space channels and a pre-existing dark fiber infrastructure. We tested the setup over 50 m and 500 m free space long links, reaching an average secure key rate of 793 kbps and 40 kbps during several hours of measurement. The results put a step forward the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid systems.

The advances in quantum key distribution (QKD) during the last 30 years have been outstanding in terms of reachable distance and key generation rate. However, the integration of quantum systems in real telecommunication networks generates multiple challenges, from technology availability to the design of inter-operable QKD systems, interconnected to key management layers and cyphers, and that can be embedded in existing telecommunication network topologies. We present several use-cases of implementation and integration of our QKD systems, in different contexts and involving Italy and neighboring countries in Europe.

Quantum key distribution (QKD) is introduced to make encryption and transmission of data over any public channel unconditionally secure. A key requirement of such a promise is to have access to an encryption key with a similar length as the message and data itself. While QKD has become mature and the key rate significantly increased over the past 20 years, there is still a notable gap between data transmission and key generation rates. High-dimensional QKD is proposed as a method to respond to this demand. Here, we demonstrate a 4-dimensional path-\&-time encoding QKD system with more than 100\% improvement compared to a standard 2D system in the same test-bed, a 52-km deployed multicore fiber link.