Remote Entanglement of Solid-State Spin Qubits Integrated in Broadband Waveguides

Scaling quantum networks requires high-density, reliable, and reproducible quantum nodes. Traditional approaches using nanophotonic cavities often struggle with low fabrication yields and the requirement for precise spectral matching of emitters to cavity modes. This study investigates whether diamond tin-vacancy (SnV) centers integrated into broadband waveguides can serve as a scalable, high-yield alternative for generating remote entanglement.

The researchers integrated SnV centers into diamond single-mode waveguides, which provide a broadband optical environment. This architecture eliminates the need for resonant cavity tuning. The team characterized the spin-photon interface, demonstrating coherent optical and spin control, and performed two-photon interference experiments to assess the indistinguishability of photons emitted from separate waveguides. Finally, they implemented the Barrett-Kok protocol to generate heralded remote spin-spin entanglement, using real-time feedforward to ensure a consistent entangled state regardless of the heralding pattern.

The study achieved a near-unity fabrication yield for the waveguide devices. The SnV centers exhibited high photon purity and excellent spin coherence, with coherence times exceeding 2 ms under dynamical decoupling. The researchers demonstrated two-photon interference with a visibility of up to 94.8%, comparable to cavity-enhanced systems. By combining this with spin-photon entanglement, they successfully generated remote spin-spin entanglement with a state fidelity of 0.730(11). This result confirms that broadband waveguide integration is a viable, scalable platform for quantum network nodes.

This work provides a clear pathway for scaling quantum networks. By removing the reliance on resonant cavities, the platform simplifies the fabrication process and improves device yield. When combined with existing techniques for on-chip frequency tuning and quantum frequency conversion, this architecture could enable the connection of thousands of qubits on a single chip, bringing the realization of large-scale, multiplexed quantum networks closer to reality.