## 5 Discussion  
The analysis presented above met only the minimum requirements necessary to establish sufficient SKRs between a satellite and an OGS without incorporating any advanced equipment in either the space or ground segments. Recent technological advances have significantly enhanced the performance of quantum sources and the capabilities of OGSs, including AO and SNSPDs. The subsequent section will explore a forward-looking approach, assuming the satellite is equipped with a GHz-scale source and uses the DS-BB84 protocol. The OGS is assumed to be ideally located, such as at the Helmos Observatory (which is situated at 2340 m above sea level), and the design features of the Helmos telescope were considered. Additionally, the link budget analysis considered the distribution of information beyond the OGS to assess the potential convergence of space and ground fiber segments using state-of-the-art equipment. For this use case, although the quantum signal was captured by the telescope at the OGS site, it was not detected there but was instead transmitted through a low-loss SMF (ca. 0.2 dB/km loss), as described in Section 4.5. The modeling included additional losses attributed to the fiber distribution segment to determine if the remaining link budget can support effective fiber transmission. More specifically, as shown in Figure 13, the calculated SKR for (a) single pass of a LEO satellite, (b) a single pass of a MEO satellite, and (c) a 12 h communication  
Figure 12: SKR over time accounting for the additional fiber distribution losses of (a) \(D_{rx}=1\) m with SNSPD detection, (b) \(D_{rx}=2.3\) m telescope receiver with SNSPD detection, and (c) \(D_{rx}=1\) m telescope receiver with SPAD detection. between the OGS and a GEO satellite was depicted. To facilitate the comparison for the different fiber losses added, the \(y\)-axis was displayed on a semi-logarithmic scale. The updated assumptions for this case are listed in Table 2, while the parameters that are not included in Table 2 have remained unchanged and can be obtained from Table 1. Briefly, the 2.3 m Helmos telescope was considered as the receiver equipped with SNSPDs, whereas AO correction was also considered. Regarding the AO correction, we considered an almost perfect AO system (\(\gamma_{7}=0.05\) in Equation (11)) capable of achieving an overall coupling efficiency of 36% (at an elevation angle of 90\({}^{\circ}\)), in which an infinite control bandwidth was considered. For the case of AO, a more precise modeling would be needed, but that is beyond the scope of this work. As expected from the analysis conducted on Section 4, LEO orbits yield higher link budgets, thus enabling the transmission of qubits over greater fiber distances compared to MEO and GEO orbits. Figure 13a illustrates that, even with an additional 30 dB penalty due to fiber transmission, the achievable SKR can reach kbps for high elevation angles values. This 30 dB loss corresponds, ideally, to a fiber distance of 150 km. Such a distance typically exceeds the usual span of a single metropolitan area, often extending to a regional scale that could include multiple metropolitan areas or stretch from an urban center to rural or less densely populated regions. For the case of MEO and GEO that are presented in Figure 13b,c, respectively, the key rates dropped to zero when the additional fiber loss exceeded 15 dB for the case of MEO and 10 dB for the case of GEO. Nevertheless, the remaining link budgets of 15 dB and 10 dB ideally correspond to fiber distances of 75 km and 50 km, respectively, thus yielding tens of bps of SKRs. These distances typically fall within the range of a metropolitan area, enabling the information to be potentially distributed to urban centers and their surrounding suburbs. Since this section explores a more futuristic approach, it might also be of interest to consider that, with the advent of novel low loss optical fibers [80], there will be telecom wavelengths with losses below 0.14 dB/km.