# Towards the Quantum Internet: Satellite Control Plane Architectures and Protocol Design
## 1 Introduction  
The future quantum Internet (QI) is expected to interconnect quantum computers (QCs) in order to achieve unprecedented capabilities that are impossible to achieve by using only classical information [1; 2]. In a quantum network (QN), a quantum state can be teleported over an arbitrarily long distance, provided that an entangled pair of particles is exchanged through a quantum channel, and a classical communication channel is established over the same link [3; 4; 5; 6; 7]. Through the QI, remote quantum devices can communicate and cooperate to solve computational tasks by adopting a distributed computing approach. In fact, as explained in [8; 9; 10], through the interconnection of multiple QCs, it is possible to obtain a _single_ quantum device with a number of qubits that scales linearly with the number of remote QCs. Moreover, the QI could provide other benefits, such as a near optimal network security. Nevertheless, despite the significant evolution of quantum technologies, the generation rate of quantum encryption keys decreases exponentially with the distance due to the fiber attenuation; thus, obtaining an efficient entanglement distribution over long distances is still an open challenge [11; 12]. In order to mitigate the transmission losses, devices such as quantum repeaters (QRs) must be introduced [2; 13]. QRs are equipped with quantum memories to store intermediate quantum states. The QRs divide the long-distance communication channel into several segments, making it possible to generate the entanglement between adjacent nodes by transmitting photons entangled with their own memories [14]. Then, the _entanglement swapping_ procedure is performed between adjacent nodes that have acknowledged the existence of entanglement with different QRs by receiving heraldingsignals from other QRs at long distances [15]. As depicted in Figure 1, the entanglement swapping procedure consists of the conversion of two independent entangled photon pairs--for example, photons \(\alpha\) and \(\beta\), and \(\gamma\) and \(\delta\)--to a new entangled pair of photons between \(\alpha\) and \(\gamma\) that are not originally entangled by performing a Bell State Measurement (BSM) on photons \(\beta\) and \(\delta\)[16; 17; 18]. Considering that the free-space photon usually experiences negligible loss in a vacuum, free-space quantum satellite links have been considered in recent years in order to further address the limitation of optical fibers, making the transmission of photons over thousands of kilometers possible [19; 20; 21]. However, the realization of a global quantum network requires the development of specific controlled architectures and protocols [22], also considering heterogeneous technologies while ensuring their interoperability, in Service-Oriented Architecture (SOA) [23; 24; 25; 26], which also involves distributed computing. In a distributed classical system, different services are located on different machines, and the communication over the network increases the response time [27]. This can also occur in a distributed quantum system, wherein the different operations required by the quantum algorithm need to be properly allocated among the qubits of the different QCs [10]. Furthermore, as explained in [28], considering the limited lifetimes of quantum memories, it is very difficult to build completely distributed quantum network management protocols. This is one of the motivations that make Software-Defined Networking (SDN) a very attractive technology for the management of quantum networks. In fact, in a quantum network, an SDN Controller manages the global strategies for the distribution of long-distance Bell pairs. According to the previous considerations, we propose a quantum network backbone composed of a constellation of low Earth orbit (LEO) satellites controlled through a _modular two-tier_ Control Plane (CP) based on SDN.