Cristian Antonelli

We analyse the secret-key rate (SKR) of continuous-variable quantum key distribution (CV-QKD) when the quantum channel shares a fibre with classical DWDM traffic. Using a physical model of an 88-channel, 50 GHz-spaced C-band link that captures four-wave mixing, spontaneous Raman scattering, Rayleigh backscatter, and out-of-band effects with full frequency dependence, we compute the asymptotic SKR under Gaussian collective attacks across guardband size, quantum-channel position, classical launch power, link distance, and propagation direction. We report three results. First, placing the quantum channel at the band edge with a 150 GHz guardband yields a 108% SKR gain at 3.4% classical capacity loss, half the cost of symmetric centre-band protection. Second, counter-propagation without any guardband produces the same SKR as guardbanded co-propagation at the same operating point. Third, a power threshold near -2 dBm/ch separates two operating regimes: below threshold the link is Raman-limited and the SKR is essentially insensitive to the guardband, while above threshold four-wave mixing dominates and the SKR depends sharply on guardband size and band-edge proximity. These rules apply across the operating regimes typical of installed DWDM metropolitan fibre.

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.

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.