Isotopically enriched epitaxial CaWO₄ thin films for Er³⁺ spin-photon quantum interfaces

Rare earth ion (REI)-doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability^1-3. Among them, Er³⁺ doped CaWO₄ is promising because it possesses narrow optical linewidth transitions and a long spin coherence time^4-6. The electron spin coherence is limited at high temperatures by paramagnetic impurities and by the presence of the 14.3% ¹⁸³W nuclear spin. To further increase the spin coherence time at millikelvin temperatures, where the paramagnetic impurities are frozen out, our approach is to synthesize chemically and isotopically purified thin films as a host material. We first grow non-isotopically enriched Er³⁺ doped CaWO₄ thin films, which exhibit a 214(13) MHz photoluminescence (PL) inhomogeneous linewidth, indicating the thin film has high crystalline quality. We then grow isotopically enriched CaWO₄ thin films using an isotopically purified ¹⁸⁶WO₃ source. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to measure the relative concentration of W isotopes. ¹⁸³W, the only W isotope that has a net nuclear spin and is the major cause of spin decoherence, was at a relative abundance of 1.2%, a factor of 10 lower than natural abundance. We also observed PL emission from single ions after integrating nano-photonic devices with the thin film. These results establish isotopically engineered CaWO₄ thin films as a promising platform for future studies of nuclear-spin-limited coherence and for scalable rare-earth-ion-based quantum nanophotonic devices.