Millikelvin-temperature spin dynamics of optically addressed single Er³⁺ ions in CaWO₄

Solid-state single photon sources and quantum memories are a promising route to realizing quantum repeaters. Single Er³⁺ ions implanted in CaWO₄ interfaced with nanophotonic cavities have recently demonstrated indistinguishable photon emission [1] and spin-photon entanglement [2] directly in the telecom band, providing a promising route to long-distance quantum repeater networks. However, entanglement fidelities are largely limited by spin decoherence arising from noise in the solid-state environment. Noise from paramagnetic impurities could be reduced by increasing the material purity or decreasing the temperature to polarize the spin environment. In this presentation, we report studies of the spin coherence of implanted Er³⁺:CaWO₄ at millikelvin temperatures.