## 4 Discussion  
Beyond the reported studies and outcomes, this section seeks to open the door to the future of secured 5G/B5G edge topologies, focusing on the technological options and deployment scenarios in support of the QKD integration within the 5G/B5G-oriented fronthaul infrastructure. In this direction, the roadmap of quantum communication blocks assisting in the end-to-end security of the converged optical/wireless can be built upon the following pillars:  
Practical on-chip QKD blocks: Moving towards the deployment of 5G network components within street furniture such as lampposts [62], the ultra-compact size of QKD Bob stations emerges as an essential feature for this kind of metropolitan 5G/B5G terminal nodes. To satisfy the demands of the low-cost and compact on-chip circuitries for this kind of terminal node [63], upconversion assisted modules can be exploited for quantum engines, based on the use of practical CMOS SPADs for single-photon detection operated at room temperature. The advantage gained from the detection of the C-band photons with integrated CMOS SPADs instead of bulky and expensive InGaAs SPADs can be combined with advanced chip-scale photonic solutions for frequency conversion of photons, thereby allowing for miniaturized detectors. In this direction, the UNIQORN project pursues an upconversion-assisted module based on sum-frequency generation inside a waveguide inscribed ppLN crystal on the polymer platform [33]. Quantum resources as part of future WDM-PON infrastructure: The integration of QKD links within WDM-enabled architecture for securely distributing symmetric keys to 5G/B5G terminal nodes along the optical segments is also fully aligned with the strong will of the telecom operator to reuse their installed PON infrastructure as the fronthaul/midhaul connectivity layer of 5G services [64]. In this  
Figure 14: SKR as a function of fiber length, for the P2 MP shared fiber link (blue line) and for the Fi-Wi topology (orange line) serving \(N=64\), where the quantum channel at 1550 nm is multiplexed with the classical signals at 1310 nm for both downlink and uplink, for (**a**) InGaAs; and (**b**) upconverted CMOS-based QKD setups. The horizontal lines with different styles correspond to the AES limits for different key refresh times: 0.83 bps corresponds to 5.14 min refresh time for the data encryption for P2 MP; 3.7 bps corresponds to 5.14 min refresh time for the data encryption and for the controlled functions for Fi-Wi; 183 bps corresponds to 1.4 s refresh time for the data encryption for P2 MP; and 198 bps corresponds to 1.4 s and 1 min refresh times for the data encryption and the controlled functions, respectively, for Fi-Wi. context, the QKD research community has experimentally verified that the coexistence of quantum channels with several classical channels is feasible under fully loaded modern access communication standards operating at different spectral bands [65]. QKD/PLS-assisted security for converged fiber/wireless fronthaul segments: The concept in which low-cost mmWave mesh nodes are operated as part of a network connecting access devices (e.g., WiFi hotspot, 5G RRH) has been proposed to enable flexible edge connectivity in 5G/B5G fronthaul [66]. The realization of P2 MP transport nodes at above 100 GHz relying on existing optical fiber infrastructure and by extending its reach has been recently demonstrated for use cases in ultra-dense environments using D-band radio equipment [67]. The end-to-end security framework of these hybrid optical/wireless mesh networks can be guaranteed through the combination of QKD-assisted AES engines in the optical fibers with the keyless secure transmission of highly directional Line-of-Sight (LOS) by mmWave/THz nodes [68]. Entering the era of Quantum Internet [69], the co-design and synergy between applications based on photonic qubits propagation over the deployed fiber infrastructure can be initiated.