Thesis title: Photonic Technologies for Free-Space Quantum Communication
Quantum information and quantum optics offer powerful, innovative tools for secure communication and cryptography, with Quantum Key Distribution (QKD) becoming a standard technology in the cryptographic world. By exploiting fundamental principles such as quantum superposition, entanglement, and the no-cloning theorem, QKD enables two users to establish a shared secret key with information-theoretic security. Quantum communication is a fundamental element for the realization of the quantum internet, a network of distant nodes connected by a quantum channel. The realization of such networks requires methods to distribute states and correlations with high fidelity and low losses. This thesis explores the use of innovative photonic technologies for the realization of an experiment aiming at improving quantum communication and quantum channels. Single photons based on QD are generated on-demand and show high purity, near-optimal indistinguishability, and low multi-photon emission. By exploiting the features of these photons, a method to realize entangled states is implemented. Using the polarization degree of freedom, these entangled states are ideal candidates for free-space QKD protocols based on Bell’s inequality. To address the problem of the reference frame alignment using polarization, hybrid states are studied and implemented. The specific combination of polarization and orbital angular momentum (OAM) can produce photons that are not modified by the rotation of the reference frame. We implemented an experiment to realize a QKD that is independent of the relative rotation of the two users. The final goal of my thesis was devoted to the in-field demonstration of quan-
tum teleportation as a fundamental building block of distributed quantum networks, enabling the reliable transmission of quantum information between remote nodes. Exploiting the free-space infrastructure in the Sapienza University of Rome, a link of around ∼ 300 m, we implemented the quantum teleportation between two different QDs, showing the possibility of using this
technology in real-case scenarios. The results obtained show not only that QDs are ideal sources for quantum communication, but also the possibility of using new states to improve current technologies, advancing the development of efficient and reliable quantum networks.