As quantum networking hardware remains costly and not yet widely accessible, simulation tools are essential for the design and evaluation of quantum network architectures and protocols. However, designing a scalable and computationally efficient quantum network simulator is intrinsically challenging: i) quantum dynamics must be emulated on classical computing platforms while capturing the stateful and non-local nature of entanglement, a quantum resource without any classical networking analog; ii) quantum networking is inherently hybrid, as protocol execution also fundamentally depends on classical signaling. This makes a tight and faithful co-simulation of quantum operations and classical message exchanges a core requirement. In this light, we present Q2NS, a modular and extensible quantum network simulator, built on top of ns-3, designed to seamlessly integrate quantum-network primitives with ns-3's established classical protocol stack. Q2NS adopts a modular architecture that decouples protocol control logic from node- and channel-level operations, enabling rapid prototyping and adaptation across heterogeneous and evolving Quantum Internet scenarios. Q2NS natively supports multiple quantum state representations through a unified interface, allowing interchangeable state-vector, density-matrix, and stabilizer backends. We validate Q2NS through realistic use-case studies and comprehensive benchmarks, demonstrating superior computational efficiency over representative state-of-the-art alternatives, while preserving modeling flexibility. Finally, we provide a dedicated visualization tool that jointly captures physical and entanglement-enabled connectivity and supports entangled-state manipulations, facilitating an intuitive interpretation of entanglement dynamics and protocol behavior. Q2NS offers a flexible, open, and scalable simulation platform for advancing Quantum Internet research.

翻译：由于量子网络硬件成本高昂且尚未广泛普及，模拟工具对于量子网络架构与协议的设计和评估至关重要。然而，设计一个可扩展且计算高效的量子网络模拟器本质上面临挑战：i）必须在经典计算平台上模拟量子动力学，同时捕捉纠缠（一种无经典网络对应物的量子资源）的状态性和非局域特性；ii）量子网络本质上是混合的，因为协议执行也从根本上依赖于经典信令。这使得量子操作与经典消息交换的紧密且精确的协同模拟成为一个核心需求。鉴于此，我们提出了Q2NS——一个基于ns-3构建的模块化、可扩展的量子网络模拟器，旨在将量子网络原语与ns-3成熟的经典协议栈无缝集成。Q2NS采用模块化架构，将协议控制逻辑与节点级和信道级操作解耦，从而支持在异构且不断演进的量子互联网场景中进行快速原型设计和适配。Q2NS通过统一接口原生支持多种量子态表示，允许互换使用态矢量、密度矩阵和稳定子后端。我们通过实际用例研究和综合基准测试验证了Q2NS，证明其在保持建模灵活性的同时，相比代表性的先进替代方案具有更优的计算效率。最后，我们提供了一个专用的可视化工具，该工具能联合捕获物理连接与纠缠使能的连接，并支持纠缠态操作，从而促进对纠缠动力学和协议行为的直观理解。Q2NS为推进量子互联网研究提供了一个灵活、开放且可扩展的模拟平台。