# Towards the Quantum Internet: Satellite Control Plane Architectures and Protocol Design
## 2 Related Works  
The next development in the field of quantum communications is the creation of a global network of quantum computers, which might allow computations that are beyond the reach of even the most powerful single future quantum computers. As explained in [32], this goal is now much closer thanks to developments in satellite and quantum memory. In [33], the properties of the photonic networks that can be generated by satellite-based quantum communications are compared with those of the optical fibers. In [20], a detailed analysis of quantum communication channels considering different disturbance effects is provided. The Micius satellite, which launched in 2016 and is positioned in low orbit, has been used as a trusted relay in order to distribute secure keys among different locations deployed on a continental scale scenario. Furthermore, it has also been possible to perform some experiments on quantum teleportation between the ground stations and the satellite [34; 35]. Additionally, in [21], the feasibility of the BB84 protocol [36] from satellite to ground with the qubits encoded in four different polarization states has been proved. Figure 2: Quantum SDN backbone architecture. A Bell pair is generated between GSs through operations driven by the controllers embedded in the LEO constellation. Free Space Optics (FSO) technology has been used in many experiments in the field of quantum communications. For instance, it has already been used for quantum communications on Earth, connecting a transmitter station (at the Jacobus Kapteyn Telescope (JKT) of the Isaac Newton Group on La Palma) and a receiver station (at the Optical Ground Station (OGS) of the European Space Agency on Tenerife), separated by 143 km [37]. This technology has also been used on inter-satellite links as explained in [38]. However, despite the atmospheric factors and beam wandering limiting the performance of FSO significantly [39], especially in the case of Earth-to-satellite communications, adequate performance can be achieved in fair weather conditions using the 193.415 THz frequency [40]. FSO has also been used in the experiment described in [41], where laser connections were made using the 193.415 THz frequency with High-Throughput Satellites (HTS) positioned in geostationary orbit with the aim of reaching speeds of 10 Gbps. In [42], different Quantum Key Distribution (QKD) protocols were tested on a three-satellite quantum network that utilizes entanglement as a resource. Besides this, in [43], a double-layer architecture of quantum satellite networks was proposed and a Joint GEO-LEO Routing and Key Allocation algorithm was designed to resolve the E2E key distribution problem. Many satellite configurations with continuous coverage have been studied in order to balance both the total number of satellites and entanglement distribution rates [44]. In [22], an architecture consisting of a single ground controller that manages the entire constellation was introduced. The CP consists of a single controller deployed in an MCS on the ground, whereas the DP consists of numerous QSRs. The performances of some constellations composed of QSRs were analyzed in [45]. In [46; 47], a quantum network stack and a link layer entanglement generation protocol were proposed. Moreover, in [28], the SDN was considered to be a very attractive technology in order to control future quantum networks. The experiments carried out to date have concerned individual links between a ground station and a single satellite, requiring the realization of Data Link Layer protocols such as those in [48], but when the first constellations are launched, it will also be necessary to address an efficient Network Layer protocol. Only a few studies have been conducted so far on path selection for terrestrial and satellite quantum networks [22]; however, the application of a networking protocol on quantum networks based on LEO satellites is a research direction that is worth exploring.