The ability to distribute quantum resources over long distances is a key \nelement for the development of several practical applications in the field of \nquantum information. In this way, major efforts have been made in the \npast decade to extend the maximum distances for the observation of entanglement \nor for the realization of quantum cryptography. Using optical fibers \nas the quantum channel, entanglement has already been achieved between \nphotons separated by 50 km of fiber, and quantum cryptography is now a \ncommercial technology that allows quantum key distributon over distances \nup to around 100 km. Progress has also been made towards entanglement \ndistribution in free space, with the achieved distances of about 10 km \non ground atmosphere being a substantial step toward the development \nof satellite-based quantum communication.
The ability to distribute quantum resources over long distances is a key \nelement for the development of several practical applications in the field of \nquantum information. In this way, major efforts have been made in the \npast decade to extend the maximum distances for the observation of entanglement \nor for the realization of quantum cryptography. Using optical fibers \nas the quantum channel, entanglement has already been achieved between \nphotons separated by 50 km of fiber, and quantum cryptography is now a \ncommercial technology that allows quantum key distributon over distances \nup to around 100 km. Progress has also been made towards entanglement \ndistribution in free space, with the achieved distances of about 10 km \non ground atmosphere being a substantial step toward the development \nof satellite-based quantum communication.
Long-distance quantum communication is one of the most important tasks in quantum information science. It contains two main motivations: one is the fundamental interests, and the other is the practical requirements. The physical distance of quantum communication over free-space channel has been extended from several meters in the lab to 1000 km through satellite. In this mini-review, we emphasize the progress made toward the global quantum network, mainly following the world’s first satellite-to-Earth quantum communication in 2017. This includes the development of commercial small-scale quantum satellite constellations and high-orbit satellites capable of all-time coverage, as well as the feasibility of quantum communication network. On this basis, we then discussed the possibility of cross-application of free-space quantum communication technology in fundamental physics, precision measurement, and even astronomical observations. Free-space quantum communication presents both challenges and opportunities, playing a crucial role in the future global quantum network.
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.
Abstract At present, the main components of quantum communication channels are optical fibers and free space. However, whether it is a fiber channel or a free-space channel, the channel in which it propagates is affected by noise, resulting in loss or absorption of photons. Experimental studies have shown that the quantum key can basically meet the requirements of the local area network, and the safety distance is about 100km. However, if you want to achieve longer distance distribution, you can’t do without quantum repeaters. The quantum repeater can divide the long-distance communication distance into several segments, and then perform quantum key distribution in several segments respectively; then establish a longer-distance EPR pair between adjacent nodes, and establish an EPR pair to utilize entanglement switching and Entanglement purification technology; finally repeat the above steps, and finally achieve long-distance information transmission. The development of quantum repeaters can solve the communication problems facing the world and promote the replacement of global communication methods.
Quantum mechanics allows a number of communications protocols that would not be possible classically, including provably-secure messages (quantum cryptography), increased channel capacity, and the ability to transmit an unknown quantum state (quantum teleportation). This tutorial will provide an introductory description of what appear to be the most promising applications of quantum communications, including quantum cryptography and quantum repeaters. The most commonly-used methods for quantum cryptography will be described, including the advantages and disadvantages of each in practical applications. The maximum range of current quantum cryptography systems is limited by the fact that optical amplifiers would destroy the quantum coherence of single photons and cannot be used in these kinds of systems. One potential solution to this problem is the transmission of single photons in free space, which would allow a global system for quantum cryptography based on a network of satellites and ground stations. Free-space quantum cryptography systems of that kind have now been demonstrated over relatively short distances. The range of quantum cryptography systems in optical fibers can also be extended using quantum repeaters that make use of entanglement purification and quantum teleportation. These same techniques could also be used to implement a “quantum Internet” for the transmission of qubits from one quantum computer to another. The practical challenges that will have to be met in order to implement quantum repeaters and quantum teleportation over large distances will be discussed.
Quantum key distribution is now a mature quantum communication protocol which allows the verifiably secure sharing of encryption keys between two communicating parties. It seeks to address potential vulnerabilities of data transmission and storage, offering a realistic possibility to share encryption keys which are robust to eavesdropping attacks and future-proof against hacking. Fibre-optic implementations of quantum key distribution currently have a limited practical transmission distance, of less than 400 km, making commercial applications limited. Quantum-specific amplifier/repeater technology is not yet mature enough to increase the transmission distance to achieve global capabilities. Optical fiber is also impractical and expensive for applications where a remote area or moving platform are involved. In recent years, long-distance free-space quantum communications using low-Earth orbit satellites has seen an increase in interest from the academic community as well as from industrial organisations and national research institutes. The source of this new interest was a series of proof-of-principle demonstrations of satellite-based quantum key distribution in 2017. The use of free-space channels implementing airborne or satellite platforms also opens a range of new applications for quantum communications, as they allow coverage of remote areas, moving platforms, and also avoid the requirement of spooling fibre through volatile regions. This presentation will give a general introduction to satellite-based quantum communications, an overview of the field, and discuss future endeavours. The talk will also include an overview of our research into novel photonic technology, such as the use of detector array technology, for the optical ground station receiver.