Q2NS: A Modular Framework for Quantum Network Simulation in ns-3

Quantum network simulators are essential for the design and evaluation of quantum network architectures and protocols. However, developing an effective simulator is particularly challenging due to the stateful and non-local nature of quantum entanglement, which requires persistent state awareness and tight in-network coordination. Moreover, because quantum communication protocols inherently rely on classical signaling, a realistic simulator must seamlessly integrate classical and quantum communication primitives within a unified framework. In this work, we present Q2NS, a modular and extensible quantum network simulation framework built on top of ns-3. The architectural design of Q2NS enforces a clear separation of concerns between the operational functionalities of modules and entities and the overarching control logic, enabling easy adaptation to heterogeneous and rapidly evolving Quantum Internet paradigms while leveraging ns-3's mature classical networking stack. Additionally, Q2NS includes a dedicated visualization tool to facilitate developing an intuitive understanding of entanglement-based quantum networking. This design enables accurate modeling of entanglement dynamics and protocol control logic, by providing a flexible, open, and scalable platform for advancing the Quantum Internet research.