Realization of a multi-node quantum network of remote solid-state qubits

Abstract: The distribution of entangled states across the nodes of a future quantum internet will unlock fundamentally new technologies. Here we report on the experimental realization of a three-node entanglement-based quantum network. We combine remote quantum nodes based on diamond communication qubits into a scalable phase-stabilized architecture, supplemented with a robust memory qubit and local quantum logic. In addition, we achieve real-time communication and feed-forward gate operations across the network. We capitalize on the novel capabilities of this network to realize two canonical protocols without post-selection: the distribution of genuine multipartite entangled states across the three nodes and entanglement swapping through an intermediary node. Our work establishes a key platform for exploring, testing and developing multi-node quantum network protocols and a quantum network control stack.

• The paper demonstrates a scalable three-node quantum network using NV center qubits, achieving Bell state fidelities over 0.8 via entanglement protocols.
• The study integrates robust memory qubits that maintain coherence for around 1800 entanglement attempts and enables real-time feed-forward operations.
• The implementation of hybrid phase stabilization and single-photon protocols establishes a practical foundation for advanced quantum network applications.

Analysis of a Multi-Node Quantum Network of Remote Solid-State Qubits

The paper presents a significant development in quantum networking, specifically through the realization of a three-node entanglement-based quantum network. This network leverages remote quantum nodes based on diamond communication qubits, creating a scalable, phase-stabilized architecture inclusive of robust memory qubits and local quantum logic. Additionally, the study achieves real-time communication and feed-forward gate operations across the network, allowing for two key network protocols to be performed without post-selection: the distribution of multipartite entangled states across three nodes and entanglement swapping through an intermediary node.

Technical Overview and Methodology

Quantum Node Design:

The quantum network comprises three spatially separated nodes, each housing an NV center electronic spin as the communication qubit. The middle node, Bob, is further equipped with a Carbon-13 nuclear spin as a memory qubit. Real-time heralding and feed-forward operations facilitate the execution of quantum network protocols, underscoring the network's practical capabilities.

Entangling Protocols and Architecture:

A single-photon protocol enables the generation of remote entanglement between pairs of nodes, where the process involves preparing communication qubits in superposition states followed by the interference of emitted photons on a beam-splitter. Achieving indistinguishability between photons hinges on DC Stark tuning, while scaling this entanglement scheme necessitates independent phase stabilization for each elementary link.

Phase Stabilization:

The phase stabilization employs a hybrid scheme splitting the effective interferometer into local and global components, each independently addressable and optimized for bandwidth and noise rejection. This scalable approach ensures the individual interferometers can be stabilized separately, thus facilitating multi-node operation.

Key Findings and Contributions

1. Remote Entanglement Performance:
   • Successful generation of Bell states with fidelities exceeding 0.8 for both the Alice-Bob and Bob-Charlie links reflects the efficacy of the hybrid phase stabilization and entanglement protocols.
   • The entangling rates reported are higher compared to previous implementations, achieved without sacrificing state fidelity.
2. Robust Memory Qubit and Real-Time Feed-forward:
   • The memory qubit demonstrates robustness under network activity, with stored states remaining coherent for approximately 1800 entanglement attempts. This performance benchmark highlights an intrinsic resistance to dephasing induced by environmental factors.
   • The real-time asynchronous communication across nodes enables heralded delivery of multipartite states, eliminating the need for post-selection and enhancing operational efficiency.
3. Demonstration of Quantum Network Protocols:
   • The paper showcases multipartite entanglement distribution achieving a fidelity of 0.538, verifying genuine multipartite entanglement.
   • Entanglement swapping achieved a fidelity of 0.587 for the heralded $\Phi^+$ state, supporting any-to-any connectivity within the network.
4. Scalability and Future Prospects:
   • The network's architecture sets the stage for larger, more complex quantum networks by incorporating advanced control layers and extending local registers at nodes.
   • Suggestions for near-term enhancements include improving entangling rates, refining memory qubits, and advancing photonic interfaces, all building towards a more robust quantum networking framework.

Implications and Forward-Looking Considerations

The research signifies a key advancement in quantum networking, offering a practical platform for testing multi-node network protocols and enhancing the quantum network control stack. The ability to stabilize and operate a multi-node quantum network with entanglement distribution and swapping capabilities is crucial for future quantum internet applications. Looking ahead, efforts to increase fidelity and entangling rates, along with integrating more comprehensive error correction and decoherence protection methods, will be vital to realizing large-scale quantum networks. The methodologies and results presented will serve as critical references for ongoing and future developments in quantum communication technologies.