# Towards Digital-Twin Assisted Software-Defined Quantum Satellite Networks
## 1 Introduction
However, the issue of mobility and its management can be effectively addressed with SDN and specific algorithms operating on Temporal Graphs (TGs) [18]. Moreover, as the elevation of the satellite relative to the Ground Station (GS) decreases, the atmospheric path to be traversed may increase significantly, thereby amplifying the impact of scattering on the entangled state. However, LEO satellites offer distinct advantages for quantum communications. Attenuation and latency are also lower for inter-satellite links due to their shorter length compared to links between MEO or even GEO satellites. Additionally, shorter distances enhance the probability of successful entanglement generation and increase entanglement rates [19]. Furthermore, additional studies are required to understand the impact of issues associated with the upper layers of the atmosphere. Nevertheless, the creation of a QN on a global scale necessitates the conceptualization of dedicated architectures and protocols, which must also consider the integration of different technologies while ensuring their interoperability [20]. In the case of a classical distributed computing system, several services are located on multiple computers; therefore, the system response time is significantly conditioned by the communication over the network. This condition is also evident in distributed quantum systems, where the diverse higher-level operations dictated by the quantum algorithm require proper allocation among the memories of the involved QCs. Moreover, QMs currently have significantly limited storage time; therefore, the design and implementation of completely distributed QN management protocols are particularly difficult [21; 22]. Software-Defined Networking (SDN)is considered a promising architecture to overcome the limitations of a fully distributed algorithm. Indeed, in a QN, the SDN Controller can manage the Bell pair distribution mechanisms on an E2E basis. The management of such a complex system needs to be accurate, and a Digital Twin (DT) of a Quantum Satellite Backbone (QSB) can be a promising solution. As a matter of fact, a DT is a digital thread that synchronizes with physical systems to optimize products or performance. As reported in [23], Digital Model (DM), Digital Shadow (DS), and DT are terms often used as synonyms. Nevertheless, their definitions differ considering that DM, DS, and DT have different levels of data integration between the digital and the physical entities. For DTs, a bidirectional data flow is provided between a physical system and its digital replication, as represented in Figure 1. Therefore, the digital entity is also a controlling instance of the physical object, and a modification in the state of the physical system is directly reflected on the digital replica and vice versa. As reported in [24], a DT can improve the real-time analysis, perform prediction, and improve the control capabilities of SDN to satisfy the requirements of the application scenario. In the case of satellite networks, this means that there exists the possibility to uplink data or commands provided by the DT to improve performance [25]. Specifically, for a QSB, it is necessary to create a Quantum Digital Twin (QDT) [26], which allows for the addressing of potential problems. For instance, upon the detection of an error, the physical network will transmit an error report to the DT, which will then refine the routing schemes and the latest satellite network status using specific algorithms. The improved routing schemes will then be verified in the virtual networks, in which the QSRs chain with the best performance will be calculated and subsequently established in the physical network [27]. Given the above considerations, this paper proposes a QSB composed of Low Earth Orbit (LEO) satellites that can be controlled via a specifically designed Control Plane (CP) studied according to the SDN paradigm that operates in a coordinated manner with a DT.