Quantum key distribution (QKD) constitutes a symmetric secret key negotiation protocol capable of maintaining information-theoretic security. Given the recent advances in QKD networks, they have evolved from academic research to some preliminary applications. A QKD network consists of two or more QKD nodes interconnected by optical fiber or free space links. The secret keys are negotiated between any pair of QKD nodes, and then they can be delivered to multiple users in various areas for ensuring long-term protection and forward secrecy. We commence by introducing the QKD basics, followed by reviewing the development of QKD networks and their implementation in practice. Subsequently, we describe the general QKD network architecture, its elements, as well as its interfaces and protocols. Next, we provide an in-depth overview of the associated physical layer and network layer solutions, followed by the standardization efforts as well as the application scenarios associated with QKD networks. Finally, we discuss the potential future research directions and provide design guidelines for QKD networks.

中文翻译：

---

量子密钥分发网络的演进：在通往 Qinternet 的道路上

量子密钥分发 (QKD) 构成了一种能够维护信息论安全性的对称密钥协商协议。鉴于 QKD 网络的最新进展，它们已经从学术研究发展到一些初步应用。一个 QKD 网络由两个或多个 QKD 节点组成，这些节点通过光纤或自由空间链路相互连接。密钥在任意一对 QKD 节点之间协商，然后可以传递给各个领域的多个用户，以确保长期保护和前向保密。我们首先介绍 QKD 基础知识，然后回顾 QKD 网络的发展及其在实践中的实施。随后，我们描述了一般的 QKD 网络架构、它的元素以及它的接口和协议。下一个，我们提供了相关物理层和网络层解决方案的深入概述，随后是标准化工作以及与 QKD 网络相关的应用场景。最后，我们讨论了潜在的未来研究方向，并为 QKD 网络提供了设计指南。