Architectures and Protocols Design for Non-Terrestrial Quantum Networks

Quantum mechanics has developed during the course of the first thirty years of the twentieth century by some of the greatest minds of that time, bringing to fruition a transformation in the vision of things in the physical world. However, for a long time there was not much emphasis on the physical ways in which a quantum computing device could have been built. Richard Feynman began to think about the possibility of creating a real Quantum Computer (QC), trying to conceive a working machine based on the principles of quantum physics. In his work of 1982, he demonstrated that no classic Turing Machine could simulate certain physical phenomena without incurring in an exponential slowdown of its performance, whereas a quantum machine could have performed the simulation more efficiently. In 1985, David Deutsch of the University of Oxford described the first quantum machine by formalizing Feynman’s ideas. Afterward, David Deutsch and other American scientists built models of QCs to study the differences between them and the classical ones. During the last three decades, classical telecommunications networks have had an extraordinary development, and in recent years research is increasingly focusing on the creation of networks consisting of heterogeneous Quantum Devices (QDs). In particular, many researchers focused on the study of terrestrial quantum communications over typical Optical Fibers (OFs) links. However, this technology is affected by extremely high losses that can be faced only through the deployment of several repeaters, which in turn involve impractical costs for end-to-end (E2E) route management. Quantum Satellite Networks (QSNs) can overcome the limitations of terrestrial optical networks, such as remarkable signal attenuation over long distances and difficulty of intercontinental communications. The recent studies on quantum satellite communications motivated our research towards a Low Earth Orbit (LEO) quantum satellite backbone for interconnecting quantum on Earth Servers in order to achieve an unprecedented computational capacity. Specifically, this thesis proposes a near optimum E2E path evaluation procedure allowing an efficient switching in order to maximize the entanglement generation rate. Indeed, this is one of the main issues that involve the Data Link Layer and the Network Layer of the Quantum Internet (QI) protocol stack, which is in its early standardization phase. In particular, the architectures presented in this thesis consider the use of the Software- Defined Networking (SDN) paradigm with the aim of minimizing the number of hops for E2E connection and maximizing network capacity. Therefore, the thesis compares distributed and centralized approaches in order to achieve a trade-off between performance and cost. Furthermore, the performance of different constellations with flight plans based on existing constellations is compared, spanning from LEO up to micro-satellites, to properly derive some guidelines for designing an efficient backbone. Specifically, the focus is on evaluating the impact of different path selection and satellite deployment solutions on the E2E capacity to achieve a trade-off between performance and cost. In addition, the recent technological developments in terms of quantum satellite communications and the application of SDN to the satellite case, motivated our investigation on an ad hoc quantum satellite backbone design based on the SDN paradigm with a Control Plane (CP) directly integrated into the constellation itself. As a matter of fact, the aim is to outline some guidelines by comparing several options. Specifically, the focus is to analyze different architectural solutions making some considerations on their feasibility, possible benefits, and costs. Finally, we performed some simulations on the architectures we considered the most promising, concluding that the integration of the CP in the constellation itself is the most appropriate solution. Moreover, this thesis considers the design of an ad hoc quantum satellite backbone based on the SDN paradigm with a modular two-tier CP. The first tier of the CP is embedded into a Master Control Station (MCS) on the ground, which coordinates the entire constellation and performs the management of the CP integrated into the constellation itself. This second tier is responsible for entanglement generation and management on the selected path. In addition to defining the SDN architecture in all its components, we present a possible protocol to generate entanglement on the E2E path. Furthermore, we evaluate the performance of the developed protocol in terms of the latency required to establish entanglement between two Ground Stations (GSs) connected via the quantum satellite backbone. Finally, this thesis also considers scenarios related to metropolitan quantum networks that make use of drone technology, which are also widely used in the 5G and 6G contexts. Specifically, swarms of drones are utilized in a wide range of applications, considering that they can be deployed ondemand and are very economically affordable. Therefore, they can also have a significant role in the creation of future Quantum Networks (QNs). As a matter of fact, the use of drones allows deploying a non-terrestrial Quantum Metropolitan Area Network (QMAN), overcoming OFs’ limits, due to the large percentage of photons that scatter before reaching the receiver. However, since random fluctuations concerning drones’ positions and atmospheric turbulence, the quality of the Free Space Optics (FSO) link can be affected with a significant impact on performance. Considering that Quantum Drone Networks (QDNs) require significant control, SDN technology can play a key role in their provisioning. Specifically, an SDN Controller is responsible for managing the global strategies for the distribution of E2E entangled pairs. Therefore, this thesis provides an SDN-based architecture for supporting high-performance Metropolitan Quantum Drone Networks (MQDNs) with a specific protocol that allows creating entanglement between two GSs through the swarm of drones. The proposed architecture can be employed for distributed quantum computing applications and entanglement-based Quantum Key Distribution (QKD) services. Moreover, an objective function to optimize the planning and operation of the swarm mission has been proposed. Finally, this thesis provides a performance evaluation considering the most relevant metrics, such as fidelity, entanglement rate, and the overhead of the proposed protocol.