## 2 Materials and Methods
### Dark Fiber Topology
Therefore, the number of (N) Bob stations is equal to the number of (N) 5G terminal nodes assumed in our topology. In this way, each Bob (and so each 5G terminal node) is supplied with a unique, randomly selected subset of bits (1/N) from the bit string that Alice transmits at the feeder fiber [34]. These independent keys are then used for the security of each of the fiber segments connecting the BBU and the terminal nodes, through AES-based encryption/decryption engines. The independency of the multiple distilled keys lies in the fact that any single photon incident in the passive 50:50 splitter stage cannot split, but it follows only one output path in a completely random way. Even though an attenuated laser source may emit multiphoton states which could split when entering a splitter and the splitting ratio of each splitter may not be ideal (e.g., 51:49), it has been shown [34] that the level of correlation between the individual bit strings is extremely low. In our proposed topology, a single feeder is assumed to link the BBU and the splitter stage, and N terminal nodes were optically connected through drop fiber segments of equal lengths (small compared to feeder fiber length). Besides the 3 dB splitting ratio associated with each stage, an additional 0.2 dB insertion loss is assumed for each splitter to study on a more realistic scenario for the installed passive fiber segment. To combat the increased optical loss linked with the 1:N splitter stage and to increase the obtained SKR, photon counters with lower noise count rates were also considered. The performance evaluation of the proposed secured fronthaul segments was carried out through numerical simulations based on the physical connectivity parameters as well the AES-256 security layer. For the quantum layer of P2P implementation, we considered a plug and play phase coding BB84-QKD system, based on the well-known Cerberis2 [35] and id3100 Clavis2 [36] from idQuantique [37], operating at 1550 nm over a Standard Single-Mode Fiber (SSMF) with an attenuation of \(\alpha_{q}\) = 0.2 dB/km and visibility set at V = 98%, where the optical pulses are generated from a highly attenuated laser source at [PERSON]'s site, with a repetition rate of \(f_{\text{rep}}\)\(=\) 5 MHz. At this low pulse repetition rate, the chromatic dispersion of fiber medium cannot significantly broaden the pulsewidth [38]. The photons were assumed to be detected at [PERSON]'s station by a pair of Single-Photon Avalanche Diodes (SPADs) operated in gated mode with a detection time-window of 1 ns. Both SPAD units were assumed to exhibit a dead time of 0.1 us and an afterpulse probability of 0.8%. The obvious selection of photon counters at telecom wavelengths are the avalanche photodiodes working in Geiger mode, for which a dark count rate (DCR) of \(5\times 10^{-6}\) ns\({}^{-1}\) and a quantum efficiency of 10% were assumed. These values correspond to the typical performance of the widely used InGaAs modules photon counters for single-photon detection at telecom wavelengths [39]. The high DCR that these modules exhibit can significantly  
Figure 2: Quantum-secured multi-user topology with a centralized Alice station and multiple Bob stations located at the 5G terminal nodes. A passive optical distribution network based on 1:N splitter stage implements the P2 MP topology with dedicated fiber segments for classical and quantum layer respectively. limit the transmission distance where no secure key can be distilled anymore. In order to overcome this limitation, we also considered the use of Silicon (Si) SPAD modules in Bob stations, offering significant advantages such as operation at room temperatures without the need of complex cooling mechanisms, higher values of quantum efficiency with very low timing jitter and much lower noise and dark current. To take advantage of the above benefits at telecom wavelengths where Si-based photon counters are virtually blind, an upconversion module is required to translate the wavelength of the incoming telecom single-photons into the visible range.