Single Er³⁺ addressing in isotopically enriched Ca¹⁸⁶WO₄thin film

Rare earth ion (REI)-doped solid-state systems can be used as a quantum memory due to their stable and highly coherent optical transitions. In thin film form, oxide hosts are attractive because of their ability to be precisely doped, flexible lattice structure through strain, dimensionality control and scalability for integration with other photonic devices. Moreover, thin films synthesized by Molecular Beam Epitaxy (MBE) can have low chemical impurity levels because of the distillation process during the growth. Also, with MBE, we are able to grow isotopically enriched thin films with limited amount of source material and further increase the spin coherence time.

We have shown that Er³⁺ doped CaWO₄ thin films exhibit 210MHz inhomogeneous linewidth, indicating the thin film has high crystalline quality. In this talk, we’ll discuss the growth of isotopically enriched CaWO₄ thin films. Time of flight secondary ion mass spectrometry (Tof-SIMS) measured the concentration of ¹⁸³W isotope, which can be a limiting factor of spin decoherence, has been reduced by a factor of 10. Measurements of Photoluminescence Excitation (PLE) from a single Er³⁺ ion show a 10 MHz wide spectral diffusion that makes it difficult to conduct spin coherence measurements. We hypothesize that excess WO₃ inclusions embedded in the film, as observed using TEM, can introduce strain and defects. These defects break the S₄ symmetry of the host site and make the ion more sensitive to nearby charge noise and induce the spectral diffusion. This result paves the way for pushing the spin coherence time T₂ to the longitudinal limit and high-performance quantum devices based on REIs in CaWO₄ thin films.