Quantum Communication is a groundbreaking technology that promises to revolutionize our current telecommunication systems. However, being a relatively new field, many challenges are yet to be confronted. This Doctoral thesis is dedicated to the advancement of Quantum Communications for fundamental studies and applications. Here, satellite-based, free-space and optical-fiber links are exploited to test and develop different Quantum Communication protocols. Different technologies, in particular free-space optics, fiber-based components and silicon photonic integrated circuits, are adopted to manipulate the quantum states of light. Different types of studies are performed ranging from fundamental tests of physics, to technological advancements, up till the development of fully-functioning Quantum Key Distribution systems. The work described in this thesis, I hope, represents a substantial contribution in the effort to transition Quantum Communications from the laboratories to a wide-spread deployment in future telecommunication infrastructures.
Quantum Communication is a groundbreaking technology that promises to revolutionize our current telecommunication systems. However, being a relatively new field, many challenges are yet to be confronted. This Doctoral thesis is dedicated to the advancement of Quantum Communications for fundamental studies and applications. Here, satellite-based, free-space and optical-fiber links are exploited to test and develop different Quantum Communication protocols. Different technologies, in particular free-space optics, fiber-based components and silicon photonic integrated circuits, are adopted to manipulate the quantum states of light. Different types of studies are performed ranging from fundamental tests of physics, to technological advancements, up till the development of fully-functioning Quantum Key Distribution systems. The work described in this thesis, I hope, represents a substantial contribution in the effort to transition Quantum Communications from the laboratories to a wide-spread deployment in future telecommunication infrastructures.
Quantum technologies have the potential to improve in an unprecedented way the security and efficiency of communications in network infrastructures. We discuss the current landscape in quantum communication and cryptography, and focus in particular on recent photonic implementations, using encoding in discrete or continuous properties of light, of central quantum network protocols, enabling secret key distribution, verification of multiparty entanglement and transactions of quantum money, with security guarantees impossible to achieve with only classical resources. We also describe current challenges in this field and our efforts towards the miniaturization of the developed photonic systems, their integration into telecommunication network infrastructures, including with satellite links, as well as the practical demonstration of novel protocols featuring a quantum advantage for a wide range of tasks. These advances enrich the resources and applications of the emerging quantum networks that will play a central role in the context of future global-scale quantum-safe communications.
Quantum communications represent a cutting-edge frontier in secure information exchange, with satellite networks emerging as a promising platform for their implementation. This paper provides an overview of the trends and challenges in leveraging quantum technologies for satellite communications networks. Beginning with a brief introduction to quantum communications and satellite networks, the paper delves into the fundamentals of quantum key distribution (QKD), teleportation, and entanglement, highlighting their relevance to satellite-based communication systems. The paper then explores recent advancements in quantum satellite communications, including the development and deployment of quantum satellites and the achievement of long-distance quantum communication via satellites. However, alongside these advancements come notable challenges. Technical hurdles such as atmospheric interference and signal degradation over long distances pose significant obstacles to the practical implementation of quantum communication in satellite networks. Moreover, practical challenges such as high development costs, regulatory complexities, and integration issues with existing infrastructure further complicate the adoption of quantum technologies in satellite communications. Looking ahead, the paper discusses future trends and opportunities in the field, including advancements in satellite technology, integration with 5G networks, and the expansion of quantum network applications. Despite the challenges, the potential benefits of quantum communications for satellite networks are substantial, offering unparalleled security and the ability to establish secure communication channels over vast distances. By addressing the identified challenges and leveraging emerging opportunities, quantum communications hold the promise of revolutionizing satellite networks and paving the way for a new era of secure and efficient global communication.
This thesis is focused on the development of advanced techniques for quantum communications in free-space channels. At present there are several demonstrations of quantum communication protocols at long distances, many of these exploit optical fiber as transmission channel. The optical fiber is very advantageous in that it is scarcely influenced by external conditions and allows to connect remote localities that are not in direct line of sight. On the other hand, it presents strong limitations in transmission distance because of dispersion and attenuation. It is therefore necessary to explore new transmission channels in order to allow a global spread of quantum communication. The free-space channels are good candidates, both the vertical ones (between earth and space) and the horizontal ones. The study of these is of fundamental importance for the extension of quantum communication over a global scale. Since the eighties to present day have been defined several quantum communication protocols such as Bennet-Brassard 84 (BB84), Bennet 92 (B92), Ekert 91, Decoy State, Dense Coding, etc.. These protocols present different characteristic such as reliability, security and key rate. These three parameters are very important for the characterization of a communication protocol and are directly related to each other. The development of new quantum communication protocols that maximize these parameters is very important for future progress of quantum communications. This thesis is divided into three parts. In the first part we consider the characteristics and the behaviour of a free-space horizontal quantum channel. We study the impact of atmospheric turbulence in the case of single beam propagation and twin-beam propagation. We then measure the losses of the channel and analyse the effect of turbulence on photon statistics. Finally, we propose a method that exploits the turbulence to improve the signal to noise ratio (SNR) of the channel. In the second part we take into consideration the security and efficiency of Quantum Key Distribution (QKD) protocols . We propose the experimental demonstration of the B92 protocol with non-maximally entangled states . Using this kind of states allow us to improve the security and the key rate of this protocol. Finally, in the last part, we presented a possible way to improve the quantum channel capacity exploiting hyperentangled photon pairs. We designed and experimentally tested a system that can be used to transmit hyperentangled states at long distances.
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 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 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-photons 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 the Bell’s inequality. To address the problem of the reference frame alignment using the 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 rotation of the reference frame. We implemented an experiment to realize a QKD that is independent on the relative rotation of the two user. The final goal of my thesis was devoted to the in-field demonstration of quantum 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 into real-case scenario. 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.
Quantum communication promises unconditional security by leveraging the fundamental laws of quantum physics, which make eavesdropping detectable through the disturbance it introduces in quantum states. A cornerstone of this technology is quantum key distribution (QKD), enabling the distribution of a shared secret key between distant users with information- theoretic security, rather than relying on assumptions about an eavesdropper’s computational power, as in classical cryptography. However, current QKD systems rely largely on bulk-optical solutions, which limits scalability, stability, and integration into existing telecommunication infrastructures. Photonic integrated circuits (PICs) offer a promising platform to overcome these challenges by providing compact, stable, and CMOS-compatible architectures. This PhD work investigates integrated photonic platforms for practical quantum communication at telecommunication wavelengths, with a focus on demonstrating QKD protocols across three different encoding schemes: time-bin, frequency-bin, and path. First, a time-bin QKD receiver using a borosilicate-glass integrated imbalanced Mach–Zehnder interferometer (iMZI) is developed and compared with a fiber-based receiver, showing enhanced phase stability and improved long-distance secure key rates. Next, frequency-bin entanglement-based QKD is implemented using silicon microring resonators, demonstrating high-visibility quantum interference and active phase-stabilization techniques. Finally, a chip-to-chip path-entangled QKD link is realized using silicon photonic spiral sources, representing a crucial step toward fully integrated quantum transceivers. Across these experiments, this PhD thesis establishes a unified experimental framework for integrated QKD and evaluates performance in terms of quantum bit error rate, secure key rate, and system stability under realistic operating conditions. Together, the results provide a proof-of-concept demonstration of scalable, high-performance quantum communication using PIC technology, advancing toward practical deployment in future quantum networks and laying foundations for multidimensional and fully integrated quantum communication nodes.
Satellite-based quantum communications represent a critical advancement in the pursuit of secure, global-scale quantum networks. Leveraging the principles of quantum mechanics, these systems offer unparalleled security through Quantum Key Distribution (QKD) and other quantum communication protocols. This review provides a comprehensive overview of the current state of satellite-based quantum communications, focusing on the evolution from terrestrial to space-based systems. We explore the distinct advantages and challenges of discrete-variable (DV) and continuous-variable (CV) quantum communication technologies in the context of satellite deployments. The paper also discusses key milestones such as the successful implementation of quantum communication via the Micius satellite and outlines the primary challenges, including atmospheric turbulence and the development of quantum repeaters, that must be addressed to achieve a global quantum internet. This review aims to consolidate recent advancements in the field, providing insights and perspectives on the future directions and potential innovations that will drive the continued evolution of satellite-based quantum communications.