# A Survey on Satellite Communication System Security
## 7 Future Research Directions
### Full Quantum Networks  
SCSs, a crucial national asset, have become a target for eavesdropping attempts by enemies. However, traditional cryptography methods are ineffective due to technical and resource limitations [161]. One proposed possible solution is applying the quantum internet principle, where packet information becomes unreadable to eavesdroppers. According to recent research, the development of quantum networks can be outlined in five stages. The prepare and measure networks stage enables end-to-end QKD without the need for intermediary nodes, which is essential for basic quantum communication tasks. The entanglement distribution network stage involves the deterministic or heralded creation of entangled states between nodes without requiring quantum memory, crucial for enhancing the reliability and functionality of the network. Quantum memory networks introduce local quantum memory at the nodes, enabling the storage and manipulation of quantum states and supporting sophisticated operations and protocols for complex distributed quantum applications. The few-qubit fault-tolerant network stage allows for fault-tolerant quantum operations with a few qubits, significant for tasks requiring high reliability and for applications requiring prolonged quantum state coherence and manipulation. Finally, the quantum computing network stage represents a fully developed quantum internet, where quantum computers are interconnected, facilitating the unrestricted exchange of quantum information and the execution of computationally intensive quantum algorithms. This stage realizes the ultimate goal of quantum networking, enabling a vast array of quantum applications that are unfeasible with classical technologies [162; 163]. For the quantum internet, the essential components include physical quantum channels, quantum repeaters, and end nodes/quantum processors. Physical quantum channels are important for transmitting qubits over distances, where minimizing the photon loss and managing the inherently lossy nature of quantum communication are critical [164]. The quantum repeaters are needed to combat photon loss and the exponential scaling of loss with distance. They are strategically placed along the channel to effectively extend the reach of quantum communication and may also serve as long-distance routers within quantum networks. Also, end nodes/quantum processors range from simple devices capable of preparing and measuring single qubits to sophisticated large-scale quantum computers. These nodes are vital for various quantum network tasks and must have robust storage for quantum states and high-fidelity quantum-information-processing capabilities, along with compatibility with photonic communication hardware, especially for efficient interfacing with light at telecom bands [165; 166; 163]. However, currently, due to technical limitations, quantum communication in SCSs transmits only quantum cryptographic keys through quantum links, while data are sent through classical channels. Recently, research on satellite-based quantum repeaters has been initiated, but it cannot keep up with the speed and stability of classical communication networks [167]. Consequently, the data still utilize the existing communication infrastructure, sharing the same vulnerabilities inherent in current telecommunication equipment and software. Therefore, the final goal of quantum communication is to have a network entirely composed of quantum communication systems. This means a network made solely of quantum processors and quantum repeaters, but as of now, a network solely based on this communication system has not yet been developed. Recent research has shown success in a prototype city-scale quantum network where eight users, each connected in a mesh network, successfully communicated in the experiment [168]. Despite quantum networks being in the experimental phase and facing many limitations, it is expected that they will eventually evolve from QKD in satellite networks to a fully quantum mesh network.