Entanglement between optically addressable single rare-earth ions via dipolar interactions in the solid-state

Long-distance quantum networks require scalable telecom-band spin-photon interfaces. Among these, rare earth ions (REI) are promising, with Er3+ standing out due to its telecom band optical transition. Our previous study demonstrated that integrating Er3+ into CaWO4 substantially suppresses spectral diffusion and provides a quiet spin environment, leading to the demonstration of indistinguishable photon generation and spin-photon entanglement. In this work, we demonstrate the entanglement between local optically addressable Er3+ spins via dipolar interactions. We further show that a nuclear spin ancilla coupled to one of the Er3+ spins survives mid-circuit readout of the second Er3+ ion, showing the robustness of the nuclear spins as long lived memories for quantum repeater protocols. This enriches the toolbox for varieties of quantum network protocols, marking a significant step towards REI-based long-distance quantum communication.