## 5 Discussion
With these advancements, the feasible distances for transmitting qubits may be significantly extended in the upcoming years, enabling the transmission of qubits from beyond LEO orbits to even longer distances. To summarize our analysis, in Figure 14, we included all the orbits using, as a metric, the yearly distilled key bits achievable for each orbit, with the same assumptions. A figure of merit was provided, and bar plots of the annual distilled key bits available from each orbit were illustrated. Since, for LEO cases, the satellite is not always visible with the same elevation angle at each pass, a longer time window should be simulated to accurately compare the capability of each orbit to produce keys. Therefore, the satellite pass for the three different orbits over an entire year was modeled. Figure 14 below shows the yearly distilled key bits volume in the LEO, MEO, and GEO orbits. In the case of LEO, a volume of 285 Gbits was recorded under the given assumptions. For MEO and GEO, the volume of key bits produced was significantly lower, approximately one order of magnitude less, at 35.5 Gbits and 22 Gbits, respectively. This indicates that, even when using state-of-the-art equipment, LEO remains the preferred option over MEO and GEO. While MEO and GEO have distinct advantages and disadvantages, LEO currently appears to be the most promising orbit for space-to-Earth quantum communication. Improvements could potentially be achieved by using a larger aperture emitter on the satellite for MEO and GEO; however, this consideration is beyond the scope of this study. The above KPIs allow key distillation rates with specified end-to-end loss budgets of approximately ~60 dB, which offer QKD link implementations well beyond the current state-of-the-art commercially available systems offered by QKD vendors. More specifically, Toshiba and IDQ offer robust long-distance BB84 QKD system implementations operating at a 30 dB maximum link loss [81; 82]. The difference between these commercially available systems (which report only link loss) and the reported results (which report end-to-end loss) in our study stems from the use of SNSPDs, which allow for increased detection efficiencies with lower jitter and low DCR detector noise. It has been shown that, when taking advantage of superconducting single-photon detectors optimized for QKD links, link efficiencies lower than \(10^{-6}\) can be supported [83]. Recently, Toshiba and Single Quantum  
and intensity modulators before being multiplexed in a single optical beam emitted from the satellite telescope. By assuming the satellite is a trusted node, the WDM multiplexer can be considered as part of multiple QKD sender stations without introducing additional optical loss. Multiple WDM-enabled QKD links were collected from the same OGS before being demultiplexed in a low-loss, polarization-insensitive, and passive demultiplexer. The detection of single photons will be performed from superconducting nanowires sharing the same cryostat, thus allowing for a cost-effective solution for topologies where a large number of nanowires can be used in a centralized detection station. Time multiplexing techniques can also be considered in centralized detection stations in terms of further reducing the required number of SNSPDs in the proposed architecture. The use of fully synchronized electronic boards on the satellite payload feeding the WDM QKD sender stations facilitates the Alice/Bob synchronization steps that are required for the QBER estimation and post-processing in the centralized detection station based on SNSPD arrays. Since the WDM single-photon pulses are emitted from the same satellite, countermeasures against the polarization rotation due to satellite motion can be applied in the same way for different QKD links [85]. Beyond the system parameters considered in our study, additional system engineering parameters should also be considered for realizing deployable satellite-QKD downlinks.