Exploiting potentials of satellite applications for establishing a global quantum communication network without fibre-based relays

. INTRODUCTION Quantum key distribution (QKD) is a family of protocols can provide information-theoretic security to share keys between two distant parties. Apart from the matured fibre-based QKD approaches, free-space QKD has progressed out of laboratories into real-world scenarios. Several pioneering experiments such as the Chinese Quantum Experiments at Space Scale (QUESS) missions contributing to the full in-orbit demonstrations of the satellite-based QKD approaches, provides a most feasible option to achieve an ultralong-distance QKD with today’s technology. Very recently, by integrating the fibre and free-space QKD links, a spaceto-ground QKD network has been successfully extended to a distant optical ground station (OGS) site, up to a total distance of 4,600 kilometers, which sparked a worldwide interest in design for the future global quantum network. In such an integrated space-to-ground network, the fiber-based trusted nodes have been built for extending these limited point-topoint QKD distances from one backbone node to another but are fixed locations that could be subject to constant surveillance and probes. To remove these fiber-channel risks, one could consider a hypothetical but possible network (illustrated in Fig.1) evolved from the paradigm, consisting of a space-based mobile platform (i.e., the Micius satellite) and the selected cities with OGSs (i.e., the built and planned backbone nodes for the future national network). Here, the only relay, i.e., Micius satellite, operates as either ‘trusted node’ or ‘untrusted node’ and directly mediates the distribution of secure encryption keys pairwise between these cities. The remarkable fact that, the ‘untrusted node’ configuration such as the implemented entanglement-based QKD or future space-based measurementdevice-independent (MDI) QKD is more secure but feasible only when both OGSs are within the satellite coverage simultaneously. Moreover, though a constellation of satellites that provides a continuous, on-demand entanglement distribution service to cities appears to be viable in future term, the limitation of orbit resources and the costs of construction should be considered before a widely deployment. Instead, in the current ‘trusted node’ configuration, the Micius satellite carries out QKD operations with distinct OGSs to establish independent keys with each of them, and subsequently broadcasts over a public channel the XOR hash of both delivered keys allowing thus any two cities to have a shared key. Therefore, the encrypted-communication in such a possible network could be implemented without the need for fibre-based relays. However, one major challenging bottleneck putting the network in extensive use is so far the satellite-based QKD has not been efficient enough to support the for the one-time-pad encryption. In seeking a higher key rate, there have been proposals for free-space continuous-variable (CV) QKD, asymmetric MDI-QKD, whereas, directly applying the modifications to space environment is difficult yet to be achieved. For instance, in a high-loss regime adopting CVRecent the large-scale quantum network that connects metropolitan area quantum networks between the cities is realized by integrating the free-space and fibre QKD links, yet the fiber-based trusted nodes in such network could be subject to constant surveillance and probes. To remove these fiber-channel risks, we consider a network where a space-based relay, Micius satellite, executes a sequence of keys delivery missions, allowing thus any two cities to have a shared key. In this work, we develop a comprehensive framework integrated with the precise orbital modeling and the cloud statistics model to enable a pre-assessment on the satellite-based QKD applications. Using this framework, we consider three different scheduling strategies and estimate the keys possible to deliver to the cities, respectively. Results show that the strategy of pursing the total final keys maximized significantly embodies space-based QKD advantage, while the strategy of considering different levels of missions achieves more keys delivered to a higher priority mission. Most importantly, the targeted strategy of pursuing the distribution of final keys delivered being coincident with network traffic distribution, guarantees the individual needs, further promoting the utilization of delivered keys in practice. We also provide a comparison between the total number of delivered keys by the satellite with different altitude orbits. It is demonstrated that the plans for constructing a low earth orbit (LEO) satellite constellation, are more effective than that for employing an expensive high-orbit satellite, for a realization of the potential applications. Our works not only provides a practical method in the near term but also gives the aforehand exploration to establish satellite-based quantum communication network.