Interplay of Classical-Quantum Resources in Space-Terrestrial Integrated Networks

Quantum communication infrastructures are being built worldwide to establish a quantum secure communication shield across geographical areas to protect economies, societies, and individuals from cyberthreats. It is a first step toward interconnecting nodes with classical/quantum communication, computation, and storage capabilities. However, insufficient emphasis has been placed on the fact that such infrastructure will not operate like classical networks, even though they must operate in conjunction with classical networks. In particular, classical and quantum resources can be generated and/or consumed to realize quantum protocols (e.g., a teleportation protocol consumes two classical information bits and one entanglement bit (ebit) to deliver one quantum bit (qubit)). This fundamental paradigm shift with respect to current (Shannon) classical communication brings fundamental network design challenges. In this work, we focus on the increasingly relevant space-terrestrial integrated networks, and investigate such challenges. First, we introduce the interplay between classical and quantum network resources according to the underlying theoretical limits band, and justify why the problem is significant and needs to be addressed urgently. Second, we present two illustrative numerical analyses: trade-offs between classical-quantum and classical-entanglement capacities, both over space channels. We identify (both technical and policy/ethical) research challenges ahead for the staged development of integrated quantum-classical networks as the required quantum technology becomes available. Finally, we identify the synergies with 6G usage scenarios endorsed by ITU-R.