# Towards Digital-Twin Assisted Software-Defined Quantum Satellite Networks
## 2 Related Works  
Recently, quantum communications have had remarkable development, and the following step consists of the creation of a network of QCs on a global scale that might allow computations that can be beyond the capacity of a single extremely high-performing QC. The developments in satellite and QMs make this step more feasible [32], and many experiments have already been conducted. A comparison between the properties of the current satellite-based QNs with those of the OFs has been made in [33]. Moreover, ref. [34] provided a detailed analysis of multiple disturbance effects that the quantum communication channels are subjected to. A specific model that includes the losses typical of the quantum signals during the propagation along the atmosphere is provided in [35]. The LEO Micius satellite has been used to distribute quantum keys between several locations deployed on an intercontinental scale scenario [36; 37]. Moreover, the involved research groups conducted several teleportation experiments between the GSs and the satellite [38]. In addition, in [39], an Italian research group has verified the applicability of many quantum cryptography protocols from a satellite to a GS considering four different polarization states for the sent qubits. Free Space Optics (FSO) technology has been applied in several studies that also involve the area of quantum communications. In particular, it has already been used to perform quantum communication on Earth between the sites of La Palma and Tenerife, directly connecting a GS with another, separated by a distance of 143 km [40]. Furthermore, as reported in [41], FSO has also been used to connect two satellites through an inter-satellite link. Nevertheless, even though the beam wandering and the atmospheric problems limit the performance of FSO, especially in the case of G2S and Satellite-to-Ground (S2G) links,adequate performance can be achieved in favorable meteorological conditions with the 1550 nm wavelength [42]. In [43], several Quantum Key Distribution (QKD) schemes, which utilize entanglement as a resource, were compared on a simple QN composed of three satellites. Moreover, in [44], a multiple-layer architecture of QSNs has been proposed, and a solution that involves GEO and LEO satellites with a specific solution, which uses an algorithm dedicated to the E2E key distribution. Numerous configurations of satellites have been studied with the aim of balancing the total number of involved satellites and the dissemination of the entangled states trying to maximize the entanglement rate [45]. The SDN paradigm is widely employed for the control of traditional satellite networks, including interplanetary networks [46]. However, it is gradually finding application in the control of QSNs as well. In [22], SDN was considered to be an indispensable instrument allowing easy control of the future QNs. Many of the experiments conducted so far have concerned couples of G2S links towards single satellites and require the realization of a dedicated Data Link Layer protocol such as that defined in [47]; however, in order to deploy a constellation, an effective Network Layer protocol should also be designed. An architecture that integrates a single SDN Controller on Earth managing an entire constellation composed of multiple QSRs has been introduced in [20]. Specifically, the study analyzes the performance of several path selection algorithms. Furthermore, ref. [16] examined the performance of some constellations composed of QSRs. Many studies also consider the interaction between multiple domains. Specifically, in [21], an architecture with a modular two-tier CP in which some controllers are deployed in the constellation is proposed. Moreover, a similar approach is described in [48], in which different entanglement generation strategies are also studied. In [49] several types of architectures that integrate the DT for network management are highlighted.