Advanced Quantum Communication and Quantum Networks -- From basic research to future applications

Classical communication is the basis for many of our current and future technologies, such as mobile phones, video conferences, autonomous vehicles and particularly the internet. In contrast, quantum communication is governed by the laws of quantum mechanics. Due to this fundamental difference, it might offer enormous benefits for security applications, more precise measurements, faster computations, and many other fields of application by interconnecting different quantum devices, such as quantum sensors, quantum computers, or quantum memories. This review provides an overview of the specific properties of quantum information networks. This includes the interfaces between the classical and the quantum regime, the transmission of the quantum information by physical implementations, and potential future applications of quantum networks. We aim to provide a starting point based on fundamental concepts of quantum information processing for further research on a future quantum internet.

💡 Research Summary

**
The paper presents a comprehensive review of quantum communication and quantum networks, tracing the field from fundamental concepts to prospective large‑scale applications. It begins by contrasting classical communication, which underpins today’s internet, with quantum communication that exploits superposition, entanglement, and other uniquely quantum phenomena. The authors outline how classical information can be encoded into quantum states using basic (orthogonal‑basis) encoding and superposition encoding, and they discuss the theoretical limits of retrieving that information, emphasizing the Holevo bound and the impossibility of partial reconstruction as captured by quantum random‑access codes.

A dedicated section on Quantum Secure Direct Communication (QSDC) illustrates how entanglement can be harnessed for direct transmission of secret messages, describing both bipartite and tripartite protocols, the requirements for high‑fidelity entanglement generation, distribution, and measurement, and the associated error‑correction strategies.

The second major part of the review focuses on the physical carriers and channels for quantum information. Photons (massless wave excitations) and massive particles (electrons, ions) are compared in terms of storage time, manipulation fidelity, and suitability for long‑distance transmission. Various transmission media—free‑space propagation, atmospheric links (ground‑to‑satellite and inter‑satellite), underwater channels, and guided media such as optical fibers and microwave waveguides—are examined, with a discussion of loss mechanisms, environmental noise, and the need for frequency conversion when interfacing stationary qubits with telecom‑band photons. Hybrid approaches that combine multiple platforms (e.g., superconducting qubits coupled to microwave photons, which are then up‑converted to optical photons for fiber transmission) are highlighted as a promising route to overcome platform‑specific limitations.

The authors then survey current quantum‑node technologies, including superconducting circuits, trapped ions, color centers in diamond, and nitrogen‑vacancy centers, and they describe quantum interconnects such as photonic‑microwave converters, cavity‑QED interfaces, and quantum repeaters. A concrete experimental demonstration—microwave quantum‑state transfer between two network nodes—is presented in detail, showing how a single‑photon‑like microwave excitation can be emitted, transmitted, and faithfully re‑absorbed, thereby validating a full quantum‑network link in the laboratory. Recent advances in multi‑frequency conversion, loss‑reduction techniques, and quantum error‑correction codes for long‑distance links are also discussed.

The third part of the paper turns to applications. The authors sketch a vision of a global quantum internet, identifying essential components such as quantum routers, repeaters, memories, and entanglement‑distribution protocols. They explore concrete use cases: (1) quantum‑enhanced security, including quantum key distribution (QKD), quantum key cards, and quantum‑based authentication; (2) distributed quantum computing, with a focus on blind quantum computing where users can outsource computations to untrusted servers without revealing inputs or results; (3) secure database access via oblivious transfer and private information retrieval; (4) quantum money and quantum voting, where the unforgeability of quantum states guarantees tamper‑proof transactions and election integrity. The traveling‑ballot quantum voting protocol is examined in depth, outlining the technical requirements—high‑coherence quantum memories, reliable entanglement distribution, and robust error correction—to make such a scheme practical.

In the concluding section, the paper reviews the current state of commercial QKD deployments and early‑stage quantum‑network testbeds, then outlines a roadmap toward a fully functional quantum internet. Key milestones include the development of high‑efficiency, long‑lived quantum memories; real‑time entanglement swapping and repeater stations; seamless multi‑wavelength conversion for interfacing heterogeneous qubit platforms; and the establishment of standardized quantum‑protocol stacks. By integrating theoretical foundations, experimental progress, and application scenarios, the review offers a valuable guide for researchers and engineers aiming to navigate the complex landscape of quantum communication and to contribute to the realization of future quantum networks.