Davide Bacco

We demonstrate an entanglement-based quantum key distribution (QKD) system employing frequency-bin encoding. The entangled state is generated using two independent high-finesse ring resonators fabricated on a silicon photonic chip. The system implements the BBM92 protocol with a passive basis selection scheme and enables simultaneous acquisition of sixteen projective measurements across two mutually unbiased bases. To counteract random phase fluctuations induced by thermal instabilities in the transmission fiber, we apply a real-time adaptive phase correction to the measurement basis. We achieve stable QKD over a 26 km fiber spool with a secure key rate exceeding 4.5 bit/s.

We experimentally tested an alternative fiber-based setup for 4D-QKD, with time and phase encoding and one-decoy technique. We evaluated the secret key rate achievable in a finite-key scenario and we compared it with the binary-encoded BB84 protocol, which was tested with the same experimental setup. Our 4D-QKD system makes it possible to improve the secret key rate by more than a factor 2 in the saturation-regime of single-photon detectors, without requiring additional expensive resources to the 2D-QKD setup. In comparison to previous works, our scheme allows to measure the 4D states with a simplified and compact receiver, thus making it a cost-effective solution for practical and fiber-based QKD.

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.

In this work we report the first chip-to-chip multidimensional entanglement distribution. The faithful generation and transmission of the multidimensional quantum states relies on two main ingredients: silicon integrated photonics, offering a compelling platform for quantum information processing, and multicore fibres, which allows the reliable transmission of path encoded qudits.

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.

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.

Ensuring information privacy in modern communication systems has become increasingly critical. Quantum key distribution (QKD), leveraging the principles of quantum mechanics, provides information-theoretically secure key sharing and has matured into the most advanced quantum communication application. Despite successful demonstrations and emerging commercial deployments, the widespread adoption of QKD is hindered by the high cost of building dedicated quantum networks. A promising and cost-effective alternative is the integration of QKD into classical fiber-optic infrastructure, particularly using standard single-mode fibers. However, this approach is limited by noise and nonlinear effects such as spontaneous Raman scattering. Recent advancements in space-division multiplexing (SDM) have led to the development of uncoupled-core multi-core fibers (MCFs), which offer spatial separation between quantum and classical signals, mitigating interference. While previous QKD-MCF coexistence studies have been restricted to lab environments and non-standard large-diameter fibers, we demonstrate, for the first time, the coexistence of QKD and classical communication channels, in a realistic field-deployed scenario. One of the cores was dedicated to QKD and the other cores to classical transmission. The system was tested with 110-Tb/s traffic over 25.2 km of field-deployed MCF with a 125-µm cladding. Our results mark a significant step forward in integrating QKD with classical communication based on uncoupled-core MCF technology.

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 coexistence of classical and quantum communication within the same fiber optics infrastructure is still an open challenge to be solved. In fact, most of the practical implementations of quantum key distribution (QKD) are accomplished by taking advantage of dark fiber channels, i.e. fiber-optics links totally dedicated to the transmission of quantum signals. This prevents the intense classical light to affect the qubit error rate, but strongly reduces the possibilities for a full deployment of QKD technologies in large-scale and realistic applications. Looking for a solution several approaches have been tested, generally based on multiplexing of different degrees of freedom of photons. In our work we combined a dense-wavelength-division-multiplexing scheme with two different home-made single photon detection stages able to convert C-band photons into photons detectable by a silicon photon counter, by exploiting sum-frequency-generation process in nonlinear crystals. We compared the results with an off-the-shelf InGaAs single-photon detector, equipping it with polarization and wavelength filters, that was tested under the same experimental conditions. Injecting an intense light laser into a different DWDM channel to simulate a real-worl QKD scenario, we demonstrated that our up-conversion based detector makes QKD feasible with a classical launch power of 4 dB higher than the one affordable by the InGaAs detector. This result paves the way to the employment of quantum communication in many realistic situations, by enabling the usage of already existing telecom infrastructures, where the noise levels are not manageable by current single photon avalanche detectors.

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.

In practical Quantum Key Distribution, there exist scenarios where the number of exchanged qubits is limited by physical constraints. A notable case is that of satellite QKD, where channel losses and visibility play a major role in limiting the efficiency and the availability of the quantum link. With this perspective in mind, the need for evaluating finite-key effects on the secret key rate is urgent. In this work, we experimentally evaluate, in a realistic setup and with different channel conditions, the robustness of a recent finite-key tight theoretical bound that ensures secrecy against the most general quantum attacks. We compare the experimental results obtained by this bound with the ones achieved with a new finite-key bound tailored for ensuring secrecy against individual attacks. We then show, the minimum number of raw bits to be exchanged in order to obtain a given secret key length, for different values of the QBER. This provides a valuable tool for practical QKD, as it allows to know in advance, depending on the channel noise, how may bits have to be exchanged for obtaining the desired secret key length. The results indicate that viable conditions for effective symmetric, and even one-time-pad, cryptography are achievable.