## 3 Results
### Performance Evaluation of Shared Fiber Topology  
Moving to the shared fiber configuration, the Raman noise photons generated by the interaction of intense classical flows with the fiber medium strongly degrade the quantum layer performance, as is shown in Figure 8 for the P2P scenario. In Figure 8a, SKR calculations, while the downlink is located at 1310 nm and the uplink at 1490 nm, are presented. Kpps rates are achievable but only for few km-long fiber segments, since eventually the quantum passband becomes severely contaminated with the Raman noise photons, and subsequently the SKR approaches close to zero at \(\sim\)2.8 km. An improved performance can be obtained by also shifting the wavelength allocation of the uplink at 1310 nm. This enhanced version of coexistence scheme is feasible since the Raman scattering photons generated by an O-band pump has a relatively low impact to the C-band spectral windows (i.e., lower effective Raman cross-section, \(\rho_{ul}\)). For this channel allocation, the distillation of secret keys is feasible even for 10.5 km (Figure 8b), a significant improvement compared to the previous case. We can also conclude, in both the above cases, that the noise coming from the SpRS is much stronger than the noise component  
Figure 8: SKR as a function of fiber length, for the P2P shared fiber link, for both InGaAs and upconverted CMOS-based QKD setups, where: **(a)** the quantum channel at 1550 nm is multiplexed with the classical signals at 1310 nm (downlink) and at 1490 nm (uplink); **(b)** the quantum channel at 1550 nm is multiplexed with the classical signals at 1310 nm for both downlink and uplink. The horizontal lines with different styles correspond to the AES limits for key refresh times equal to 5.14 min (0.83 bps), 1 min (4.3 bps) and 1.4 s (183 bps), respectively. Figure 7: SKR as a function of fiber length, for the P2 MP dark fiber link serving \(N=4\), 16 or 64 users, for **(a)** InGaAs; and **(b)** upconverted CMOS-based QKD setups. The horizontal lines with different styles correspond to the AES limits for key refresh times equal to 5.14 min (0.83 bps), 1 min (4.3 bps) and 1.4 s (183 bps), respectively. current of the photon counter, since the blue (SKR for CMOS) and the orange (SKR for InGaAs) plots exhibit practically identical performance. In a potential case of a fully loaded optical topology, where many classical channels in WDM systems are co-transmitted along with the quantum channel, the Raman noise would be significantly increased. To mitigate the presence of these Raman contamination photons in this scenario, the realization of a quantum channel within L-band could be an option, benefited from the even lower Raman gains for these longer wavelength assignments [53]. It is worth noting that the DCR of the photon counters at Bob stations affects only the transmission distance, not the maximum key rate that can be obtained. Since we assumed, for the upconverted modules, a value of total efficiency 10% which is equal to the assumed value of quantum efficiency for the InGaAs counters, as mentioned in Section 2.1, there is no performance benefit for using the CMOS counters in this shared fiber P2P topology. To make clear how the detector's efficiency affects the SKR, we also assumed a hypothetical scenario with an upgraded CMOS counter efficiency equal to 20%. This slightly improved SKR performance is depicted in Figure 9.  
The evaluation of SKR for the P2 MP topology is illustrated in Figure 10. From this point forward, we will consider only the allocation where both classical signals are located at 1310 nm, since in this configuration the quantum passband is more resilient to the Raman scattered photons generated by the O-band pumps, as explained above. As expected, the key rate drops significantly as the number of terminal users increases, which has an immediate impact on the key rotation times.