The implementation of a quantum network opens up a range of new opportunities for secure communication and distributed quantum computing. To achieve this, entanglement must be distributed between remote users, using photons as carriers of quantum information. However, the probability of photon absorption in optical fibers increases exponentially with distance. To address this, quantum repeaters have been proposed, dividing the total distance into shorter segments where direct transmission is more feasible. Nevertheless, for long-distance links, satellite-assisted free-space channels offer a promising near-term alternative that avoids the complexity of quantum repeaters needed to compensate for transmission losses. The distribution of quantum keys between distant users has already been demonstrated via a single satellite link over distances of up to 7000 km. In this thesis, we focus on the development and analysis of satellite-based quantum networks.

To this end, we begin by exploring how current satellite links can be made more efficient, specifically, how to increase the amount of quantum information reaching the ground stations, avoiding the requirement of long coherent time in the quantum memories and without altering the satellite hardware. We propose the use of high-dimensional encoding, showing an improvement in the rate of entanglement compared with conventional qubit encoding.

From there, the discussion expands to a full quantum network architecture using several quantum repeaters place in space, with the aim of achieving secure liks across intercontinental distances. We propose a setup based on individually trapped atoms acting as both single-photon sources and quantum memories. Incorporating hardware imperfections and modeling transmission losses through free space and the atmosphere, we estimate the hardware performance required to achieve high-fidelity entanglement at a chosen transmission rate.

Finally, we go to a more specific example of distributing quantum key (QKD) to different cities of the Iberian Peninsula. Taking into account real-time weather conditions, atmospheric effects, and propagating losses, we analyze the feasibility of deploying current use cases of satellite-based QKD. ... Read more

Satellite-based quantum repeaters are a promising means of reaching global distances in quantum networking due to the polynomial decrease of optical transmission with distance in free space, in contrast to the exponential decrease in optical fibers. We propose a satellite-based quantum repeater architecture with trapped individual atomic qubits, which can serve both as quantum memories and true single-photon sources. This hardware allows for nearly deterministic Bell measurements and exhibits long coherence times, without the need for costly cryogenic technology in space. We develop a detailed analytical model of the repeater, which includes the main imperfections of the quantum hardware and the optical link, assuming high-altitude ground stations, and consequently working in a regime of weak atmospheric turbulence. Our model allows us to estimate that high-rate and high-fidelity entanglement distribution can be achieved over intercontinental distances. In particular, we find that high-fidelity entanglement distribution over thousands of kilometres at a rate of 100 Hz can be achieved with orders of magnitude fewer memory modes than conventional architectures based on optical Bell state measurements. ... Read more