Quantum Key Distribution (QKD) can distribute keys with guaranteed security but remains susceptible to key exchange interruption due to physical-layer threats, such as high-power jamming attacks. To address this challenge, we first introduce a novel metric, namely Maximum Number of Affected Requests (maxNAR), to quantify the worst-case impact of a single physical-layer attack, and then we investigate a new problem of Routing and Wavelength Assignment with Minimal Attack Radius (RWA-MAR). We formulate the problem using an Integer Linear Programming (ILP) model and propose a scalable heuristic to efficiently minimize maxNAR. Our approach incorporates key caching through Quantum Key Pools (QKPs) to enhance resilience and optimize resource utilization. Moreover, we model the impact of different QKD network architectures, employing Optical Bypass (OB) for optical switching of quantum channels and Trusted Relay (TR) for secure key forwarding. Moreover, a tunable parameter is designed in the heuristic to guide the preference for OB or TR, offering enhanced adaptability and dynamic control in diverse network scenarios. Simulation results show our method significantly outperforms the baseline in terms of security and scalability.

We provide design guidelines for the integration of quantum key distribution (QKD) into legacy passive optical networks (PONs). Our study addresses the challenge of quantum coexistence with various PON standards in different network topologies. We develop a novel theoretical model to assess the impact of the passive optical distribution network on both the quantum signal and the generation of scattered Raman photons from classical signals. This work also includes an extensive Raman efficiency experimental evaluation over a wide frequency range. The study evaluates the integration of upstream QKD systems in 32- or 64-user PON scenarios, determining quantum-available bandwidths and maximum supported PON lengths. In single-fibre architectures the coexistence with standards having upstream and downstream classical channels in different telecommunication fibre windows (e.g. XG-PON) is extremely challenging due to the pervasive spectrum placement of the spontaneous Raman scattering noise. However dual-feeder architectures (i.e., placing the QKD receiver at the OLT-side of the protection feeder fibre) can significantly enhance QKD performance and support the key-exchange over the entire standardized PON length for most PON standards and even in case of two-PON standard presence. Finally, the dual-fibre topology allows the quantum coexistence in access networks where three classical standards share the same infrastructure.

Quantum Key Distribution (QKD) is currently being explored as a solution to the threats posed to current cryptographic protocols by the evolution of quantum computers and algorithms. However, single-photon quantum signals used for QKD permit to achieve key rates strongly limited by link performance (e.g., loss and noise) and propagation distance, especially in multi-node QKD networks, making it necessary to design a scheme to efficiently and timely distribute keys to the various nodes. In this work, we introduce the new problem of joint Routing, Channel, Key-rate and Time-slot Assignment (RCKTA), which is addressed with four different network settings, i.e., allowing or not the use of optical bypass (OB) and trusted relay (TR). We first prove the NP-hardness of the RCKTA problem for all network settings and formulate it using a Mixed Integer Linear Programming (MILP) model that combines both quantum channels and quantum key pool (QKP) to provide an optimized solution in terms of number of accepted key rate requests and key storing rate. To deal with problem complexity, we also propose a heuristic algorithm based on an auxiliary graph, and show that it is able to obtain near-optimal solutions in polynomial time. Results show that allowing OB and TR achieves an acceptance ratio of 39% and 14% higher than that of OB and TR, respectively. Remarkably, these acceptance ratios are obtained with up to 46% less QKD modules (transceivers) compared to TR and only few (less than 1 per path) additional QKD modules than OB.

Quantum networks, with applications like Quantum Key Distribution (QKD), are gaining significant attention. However, their implementation faces challenges due to low entanglement generation success rates and quantum decoherence. Recent quantum technology advancements have extended entanglement memory lifetimes to one minute, termed cutoff, opening new opportunities for entanglement routing. We propose the Adaptive Entanglement Routing (AER) algorithm, which optimizes resource utilization to improve the success probability of serving entanglement and ultimately reduce the time needed for entanglement establishment. AER includes two phases: 1) determine redundant paths based on load and 2) utilize shared entanglements for entanglement swapping. Moreover, we design the highest-success-path (HSP) algorithm to maximize the success probability of entanglement routing with limited quantum memory. These innovative routing algorithms significantly reduce entanglement request failures, resulting in up to 70% reduction in average waiting times.

Progressive network recovery is the problem arising when a network is subject to massive failures and the network operator has to identify the optimal sequence of repair actions to maximize carried traffic. As initial deployments of quantum-key-distribution (QKD) over optical networks start appearing in several locations worldwide, in this paper, for the first time, we model and solve the Progressive QKD Network recovery (PQNR) problem in QKD networks to accelerate the QKD network recovery after massive failures. Specifically, we formulate an Integer Linear Programming (ILP) model to model the achievable key rate for different QKD network architectures (w/o trusted relay and optical bypass). Due to the scalability issue of ILP, we also propose a scalable heuristic to solve the PQNR problem for large topologies. Our numerical results show that joint utilization of optical bypass and trusted node technologies leads to significant improvement in performance and that our heuristic solution has an acceptable optimality gap compared with ILP while reducing time.

Tb/s transmission matching the requirements of future beyond 5-G mobile systems and high-throughput services in the future sustainable metro/access network is demonstrated. An innovative photonic transmitter module of a bandwidth/bitrate variable transceiver has been designed and developed integrating multiple directly modulated long-wavelength InP vertical cavity surface emitting lasers (VCSELs) in a silicon-photonics platform. Multicarrier direct modulation of each VCSEL combined with dense WDM multiplexing with a modular approach provides flexibility in the transported capacity to match dynamic bandwidth allocation, fine granularity, and scalability according to a pay-as-you-grow scheme. The use of VCSELs directly modulated with discrete multitone (DMT) signals is a sustainable technological solution in terms of cost, power consumption and footprint. Moreover, the adoption of multi core fibers (MCFs) allows to also use the space dimension for multiplexing, perfectly matching the proposed modular approach and coping with the mobile systems increasing throughput in an effective way. Here we experiment the propagation over 129 km, including 21 km of a 7-core MCF, with 57 Gb/s capacity per DMT -modulated VCSEL channel and 25 GHz WDM spacing, demonstrating more than 1.2 Tb/s total throughput thanks to the proposed modular solution.

Quantum Key Distribution (QKD) is a recent technology for secure distribution of symmetric keys, which is currently being deployed to increase communications security against quantum attacks. However, the key rate achievable over a weak quantum signal is limited by the link performance (e.g., loss and noise) and propagation distance, especially in multi-node QKD networks, making it necessary to design a scheme to efficiently and timely distribute keys to the various nodes. In this work, we formulate, using a Mixed Integer Linear Programming (MILP) model, a novel Routing, Channel, and Key-rate Assignment (RCKA) problem for QKD with Quantum Key Pool (QKP), which exploits the opportunity of using trusted relays and optical bypass. Our formulation accounts for the possibility to build a quantum key distribution path that combines both quantum channels and trusted relays to increase the acceptance ratio of key rate requests. Leveraging different versions of the proposed MILP model, we evaluate several strategies exploiting different combinations of trusted relays and optical bypass for the RCKA problem. Results show how different trade-offs between security and resource-efficiency (expressed in terms of acceptance ratio of key rate requests vs. key storing rate in QKP) can be achieved when adopting trusted-relay and/or optical-bypass technologies. Trusted relays can provide a higher acceptance ratio when the number of QKD modules (transmitters or receivers) is sufficiently large, while optical bypass, which does not require the implementation of expensive trusted relays, is preferable when the number of QKD modules is a limiting factor.

We consider long-wavelength vertical cavity surface emitting lasers (VCSELs) directly modulated with discrete multitone (DMT) signals as a very promising solution for implementing energy-efficient transmitters for 50G passive optical networks (PONs). Two scenarios based on single sideband and dual sideband modulation are taken into account. Preliminary experimentation with already available short-cavity VCSELs operating in the third window is presented to show the PON performance as a function of the accumulated chromatic dispersion and of the received power. Experimental results are also used to validate a simulation tool, which is further used to provide the performance evaluation of transmitters based on next-generation short cavity VCSELs with higher bandwidth (up to 20 GHz), operating in the O band. Thanks to its water-filling nature, DMT is demonstrated to enable PON flexibility: in fact, considering the statistics regarding a commercially deployed PON, a significant increase of the maximum aggregated capacity is provided, optimizing the PON resource usage with respect to the losses and dispersion impairments.

In this paper we consider a centralized radio access network (C-RAN) architecture with a fully analog fronthaul link between remote radio heads (RRHs) and baseband units (BBUs) based on the radio over fiber (RoF) paradigm. Mode division multiplexing (MDM) and frequency division multiplexing (FDM) are employed to provide an additional multiplexing signal dimension to meet the huge bandwidth requirements of next generation (5G) wireless mobile systems. The main contribution of the paper is to prove that a smart resource assignment between the radio antennas and the mode/frequency dimensions allows the communication over the RRH-BBU link at rates that are comparable to those achieved by an ideal fronthauling where BBU and RRH are assumed to be co-located, even without any complex and costly optical equalization technique. Validation is on the radio-link capabilities employing multiple antennas to meet the demand for massive MIMO technology.