Quantum communications infrastructure architecture: theoretical background, network structure and technologies. A review of recent studies from a European public infrastructure perspective
Progress in the development of techniques for the construction of multiuser quantum communications networks is reviewed in light of the plans for an EU quantum communications infrastructure (EU QCI). Quantum key distribution networks may be classified as trusted node; quantum repeater-entanglement-swapping based; point-to-multipoint based on passive beam splitters, active optical switchers or WDM; high dimensional-multipartite entanglement and flexible reconfigurable multipartite networks. Both satellite and terrestrial implementations are possible and ore both planned for EU QCI; their relative merits are discussed. Current technology falls short in several respects of what is required to address use cases well. Bit rates as a function of distance are currently limited by the characteristics of available devices and are an order of magnitude or more below the theoretical bounds. Non-ideal device behaviour can open loopholes for side-channel attacks. Limited range constrains network geometry. New techniques are being developed to overcome these problems. The more sophisticated schemes depend on ongoing advances in the generation and distribution of entanglement. Particularly promising are the measurement device independent protocol, which eliminates photon-detector related security weaknesses, and the twin-field protocol, which uses similar techniques to extend range, in principle even beyond previously accepted theoretical bounds. Agreement of keys between more than two users can be accomplished classically after pairwise quantum key exchange but direct quantum multipartite agreement using may offer advantages. The same applies to secret sharing. Practical quantum repeaters, to eliminate the need for trusted nodes, are closer to being realised due to recent progress in optical memories.
Progress in the development of techniques for the construction of multiuser quantum communications networks is reviewed in light of the plans for an EU quantum communications infrastructure (EU QCI). Quantum key distribution networks may be classified as trusted node; quantum repeater-entanglement-swapping based; point-to-multipoint based on passive beam splitters, active optical switchers or WDM; high dimensional-multipartite entanglement and flexible reconfigurable multipartite networks. Both satellite and terrestrial implementations are possible and ore both planned for EU QCI; their relative merits are discussed. Current technology falls short in several respects of what is required to address use cases well. Bit rates as a function of distance are currently limited by the characteristics of available devices and are an order of magnitude or more below the theoretical bounds. Non-ideal device behaviour can open loopholes for side-channel attacks. Limited range constrains network geometry. New techniques are being developed to overcome these problems. The more sophisticated schemes depend on ongoing advances in the generation and distribution of entanglement. Particularly promising are the measurement device independent protocol, which eliminates photon-detector related security weaknesses, and the twin-field protocol, which uses similar techniques to extend range, in principle even beyond previously accepted theoretical bounds. Agreement of keys between more than two users can be accomplished classically after pairwise quantum key exchange but direct quantum multipartite agreement using may offer advantages. The same applies to secret sharing. Practical quantum repeaters, to eliminate the need for trusted nodes, are closer to being realised due to recent progress in optical memories.
With the current development of quantum computing, some existing cryptographic protocols may be broken in the future, such as RSA with Shor's algorithm. To secure the future secret communications, but also the current ones from retrospective decryption, Pre-Shared Keys (PSK) can be used today, and two types of complementary solutions are currently being studied: Post Quantum Cryptography (PQC) and Quantum Key Distribution (QKD). The objective of this paper is to describe the latest, which relies on the laws of quantum physics to secure the sharing of secret keys. Among the current QKD systems, some are already commercially available. Their distance range is limited, generally around 100 km. To overcome this distance limitation, a first solution is currently being deployed in field trials using trusted nodes, forming the so-called Quantum Communication Infrastructure (QCI). The drawback is that users must trust QCI nodes, in which quantum communication are stopped, going back to classical data processing that can be eavesdropped. To overcome this problem, researchers are investigating solutions to build future quantum networks and Quantum Internet (QI), in which the processing in the networks remains quantum, using quantum buffers and entanglement swapping in quantum repeaters to increase the distance range of QKD. We briefly discuss the advantages and disadvantages of QKD versus PQC, with the two actually being complementary.
Entanglement distribution is a fundamental building block of quantum communication networks, enabling the secure and reliable exchange of quantum information over long distances. Unlike classical communication, quantum networks exploit the unique properties of quantum entanglement to support applications such as quantum key distribution, quantum teleportation, distributed quantum computing, and networked quantum sensing. Although significant progress has been made in recent years, the efficient generation, transmission, and preservation of entangled states remain major research challenges due to photon loss, decoherence, and limitations in current communication infrastructure.This review provides a comprehensive overview of entanglement distribution in quantum communication networks, focusing on the technologies that enable practical implementation and the latest experimental developments. It discusses key approaches for generating and distributing entangled photons through optical fibers, free-space links, and satellite-based communication, together with supporting technologies such as quantum repeaters, quantum memories, and photonic devices. The review also examines recent experimental demonstrations that have advanced long-distance quantum communication while highlighting the technical barriers that must be overcome for large-scale deployment. Finally, emerging applications and future research directions are presented, including the development of scalable quantum networks, the quantum internet, and secure global communication systems. This review aims to provide researchers and students with a clear understanding of the current state of entanglement distribution and its growing role in next-generation quantum communication technologies.
Quantum Key Distribution (QKD) constitutes one of the most profound practical applications of quantum mechanics, enabling two parties to exchange cryptographic keys whose security is guaranteed by the fundamental laws of physics rather than the assumed computational hardness of mathematical problems. As quantum computing capabilities continue to advance, classical public-key infrastructure faces an existential threat, rendering QKD a critical component of post-quantum secure communication architectures. This paper presents a comprehensive survey of QKD spanning foundational protocols, rigorous security proofs, hardware implementation challenges, deployment experiences across metropolitan fiber networks, and recent satellite-based experiments. We analyze the BB84, E91, B92, SARG04, continuous-variable, measurement-device-independent (MDI-QKD), and twin-field (TF-QKD) protocols in depth, examining their security assumptions, achievable key rates, and practical limitations. Particular attention is given to side-channel vulnerabilities in realistic implementations, decoy-state methods for mitigating photon-number-splitting attacks, and finite-key security analysis. We review key performance benchmarks from commercial systems and discuss integration with classical network infrastructure. The paper concludes by identifying open challenges in quantum repeater development, satellite QKD scalability, and standardization efforts essential for large-scale deployment.
Abstract - Quantum networking represents one of the most transformative developments in modern secure communication. Unlike classical networks that rely on mathematical encryption, quantum networks exploit the physical properties of qubits—superposition, entanglement, and the no-cloning theorem—to establish communication channels that are inherently tamper-evident. As quantum technologies mature, applications such as quantum key distribution (QKD), quantum repeaters, quantum teleportation, quantum memory systems, and quantum internet protocols are gaining global traction. This paper consolidates current progress in quantum networking technologies and provides an extended discussion of recent advancements in protocols, hardware components, integrated photonic systems, and machine-learning-enhanced architectures. Additionally, results synthesized from established literature, illustrate performance trends, security gains, and practical deployment challenges in quantum communication networks. The paper also outlines future directions toward scalable quantum internets, hybrid quantum-classical infrastructures, error-corrected qubit networks, satellite-based QKD, and intelligent routing systems. Overall, quantum networking is positioned to redefine the future of secure data transfer, distributed computing, and global communication infrastructures. Key Words: Quantum networking, Quantum Key Distribution (QKD), Entanglement Distribution, Integrated Photonic Chips, Machine Learning (ML)
A first quantum revolution has already brought quantum technologies into our everyday life for decades: in fact, electronics and optics are based on the quantum mechanical principles. Today, a second quantum revolution is underway, leveraging the quantum principles of superposition, entanglement and measurement, which were not fully exploited yet. International innovation activities and standardization bodies have identified four main application areas for quantum technologies and services: quantum secure communications, quantum computing, quantum simulation, and quantum sensing and metrology. This paper focuses on quantum secure communications by addressing the evolution of Quantum Key Distribution (QKD) networks (under early exploitation today) towards the Quantum-ready networks and the Quantum Internet based also on entanglement distribution. Assuming that management and control of quantum nodes is a key challenge under definition, today, a main obstacle in exploiting long-range QKD and Quantum-ready networks concerns the inherent losses due to the optical transmission channels. Currently, it is assumed that a most promising way for overcoming this limitation, while avoiding the presence of costly trusted nodes, it is to distribute entangled states by means of Quantum Repeaters. In this respect, the paper provides an overview of current methods and systems for end-to-end entanglement generation, with some simulations and a discussion of capacity upper bounds and their impact of secret key rate in QKD systems.
Recent progress in quantum technologies has brought quantum communication and networking from a largely theoretical idea to an emerging practical reality. By exploiting fundamental quantum phenomena such as superposition and entanglement, quantum communication promises capabilities that are fundamentally different and out of reach for classical communication systems, particularly in terms of security and information sharing. This review aims to present the essential theoretical concepts required to understand quantum communication, alongside an experimental and engineering perspective that highlights how these ideas are realized using real hardware. We explain core principles and protocols in a manner that connects the physics to practical system design, allowing readers to see how quantum theory and communication technology jointly enable quantum networking. The paper further surveys recent experimental and technological advances that have made practical quantum communication possible, emphasizing the role of photonic hardware, sources, detectors, and integrated platforms in enabling key operations, now leading to deployment of multi-node long-distance quantum networks. Finally, we also present the current developments in quantum-secure communication and quantum networking efforts undergoing in India, placing them in the context of the global progress.