Nicola Biagi

High-dimensional quantum key distribution (QKD) with path-encoded qudits can largely benefit from the slower phase drifts characteristic of multicore fibers: however, such channels still require phase stabilisation systems to effectively transmit quantum states with an acceptable error rate. We propose a scheme that multiplexes a co-propagating wavelength to use as reference signal in a phase locked loop system, and simultaneously achieves state of the art repetition rates for the high-dimensional QKD system. These factors allow us to design a system that can reach a much higher secret key generation rate over a propagation distance that is order of magnitudes longer than what shown in previous results, making our path-encoded QKD system appealing and comparable in terms of performance with current quantum systems.

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.

Quantum technologies play a central role in establishing new ways of quantum-secured communication. We investigate Free Space Quantum Communication and explore the advantage of implementing Quantum Key Distribution with a light source in the Mid-Infrared (>3 μm) region of the electromagnetic spectrum. We simulate and show that, for non-optimal weather conditions, Mid-Infrared can outperform the most commonly used telecom wavelength.

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.

The demand for higher secret key rates, in conjunction with the need for extending the reach of quantum key distribution has led to the devising of multiple novel protocols. Most of these protocols make use of qubits, owing to the simplicity with which they can be encoded in quantum communication systems that are available today. On the other hand, high-dimensional quantum states, yet more challenging to generate and transmit, enable higher secret-key rates and are more robust against errors in the process of quantum key distribution. A promising implementation of high-dimensional QKD is the one based on path encoding in optical-fiber quantum channels [1], where the most straightforward choice would be the use of multiple fibers. This choice, however, is challenged by the intrinsic non-homogeneity of different fibers. A more practical alternative is the one offered by multi-core fiber (MCF) technology, which has matured in recent years in the context of space-division multiplexed classical optical communications. In both cases, a key requirement is that the relative phase between spatial paths is preserved, which requires some phase-stabilization procedure in the presence of propagation-induced random phase drift. High-dimensional QKD in MCFs has been recently investigated in [1], where 4-dimensional QKD on a 2-km-long MCF was demonstrated. This was possible thanks to a phase stabilization scheme in which the phase fluctuations of a co-propagating classical continuous-wave laser signal were monitored in order to compensate for the phase drift. The same stabilization system was successfully tested more recently in the unique SDM test-bed in L'Aquila [2], in Italy, on various strands of deployed MCFs, up to a total length of 26 km [2]. In this work, we aim at developing a real-time high-dimensional QKD system based on joint path and time-bin encoding in MCFs. By using two fiber cores and two time bins, we generate 4-dimensional states.