Toward Converged Satellite/Fiber 1550 nm DS-BB84 QKD Networks: Feasibility Analysis and System Requirements  
Aristeidis Stathis  
School of Electrical and Computer Engineering, National Technical University of Athens, 9 Iron Polytechnique Str., 15780 Athens, Greece; [EMAIL_ADDRESS] (A.N. ); [EMAIL_ADDRESS] (N.K.L. ); [EMAIL_ADDRESS] (G.G. ); [EMAIL_ADDRESS] (A.D.P. ); [EMAIL_ADDRESS] (H.A.) Argiris Ntanos  
School of Electrical and Computer Engineering, National Technical University of Athens, 9 Iron Polytechnique Str., 15780 Athens, Greece; [EMAIL_ADDRESS] (A.N. ); [EMAIL_ADDRESS] (N.K.L. ); [EMAIL_ADDRESS] (G.G. ); [EMAIL_ADDRESS] (A.D.P. ); [EMAIL_ADDRESS] (H.A.) Aristeidis Stathis  
School of Electrical and Computer Engineering, National Technical University of Athens, 9 Iron Polytechnique Str., 15780 Athens, Greece; [EMAIL_ADDRESS] (A.N. ); [EMAIL_ADDRESS] (N.K.L. ); jgianmou@mail.ntua. repeaters faces a great deal of open questions [3] beyond practical deployments, there is an ambitious goal to establish a worldwide sophisticated network that should include satellites. This network will connect quantum computers, repeaters, and storage devices through both classical and quantum channels, where satellite nodes will communicate over quantum channels with terrestrial nodes, and distributed quantum computers will also connect around the world for unconditionally safe communication [4]. Such a network promises to revolutionize fields by enabling distributed quantum computing [5], secure quantum communication [6], enhanced imaging techniques [7], and improved precision in timing [8], among other potential benefits. Progress toward this vision includes the seminal experiments of the Micius satellite, which performed quantum communication between a satellite and the Earth, demonstrating Quantum Key Distribution (QKD) [9; 10; 11; 12], entanglement distribution [13], and quantum teleportation [14]. In recent years, the landscape of quantum communication and QKD has seen a growing interest worldwide, with several countries launching funding and implementation initiatives for satellite-based QKD research projects. This growing interest is justified by the plethora of scientific papers and infrastructure mission projects that highlight the stages of research, development, and the demonstration of technology. Notably, in Europe, significant strides are being made through collaborative efforts between the European Commission (EC), the European Space Agency (ESA), and various European space companies, which are exemplified by missions such as Eagle-1 [15]. This mission not only includes a QKD system, but also marks a transition from experimental demonstrations to operational service provision, and its launch is scheduled for 2025. Similarly, the SAGA 1G mission [16] represents the first generation of the European Quantum Communication Infrastructure's (EuroQCI) space segment, where the aim is for in-orbit validations of the QKD performance and system verification by 2027. Further enriching the European QKD landscape are the IRIS2 [17] and QUBE-II [18] missions. IRIS2 is envisioned as a future generation of the EU Secure Satellite Constellation, integrating EuroQCI and thereby enhancing resilience, interconnectivity, and security through satellite technology. QUBE-II, on the other hand, is a CubeSat mission equipped with two distinct BB84 Prepare and Measure (P&M)-QKD systems, and it is designed for LEO downlink experimental demonstration, with its launch also being anticipated in 2025. These initiatives, including the EC's EuroQCI and the ESA's SAGA mission underscore a concerted effort toward establishing a comprehensive network of satellite quantum communication systems. This effort is further supported by the inclusion of a terrestrial backbone component, which comprises fiber networks in major cities [19; 20; 21; 22; 23], thereby creating a synergistic space-ground quantum communication infrastructure.