Optical line broadening mechanisms in rare-earth doped oxide nanocrystals

Rare-earth ions in nanocrystals are promising solid-state platforms for quantum light-matter interfaces, combining millisecond-long nuclear spin ������2 with kHz-range optical homogeneous linewidths. Coupled to optical micro-cavities these materials can enable scalable quantum processors, nanoscale quantum memories and single photon sources. Here, we report on optical dephasing in Eu3+:Y2O3 nanocrystals of 100 nm diameter through photon echo and spectral hole burning techniques as a function of temperature and magnetic field. We identify two-level systems (TLS) and electric noise as major line broadening mechanisms below 2 K and demonstrate that material-processing strategies such as high-power oxygen plasma can partially counteract these effects. Through modeling we show that these results are compatible with a reduction in the number of oxygen vacancy pairs in the nanocrystals by the oxygen plasma processing. These pairs, even at very low concentrations (hundreds of ppb), appear detrimental to optical coherence due to their ability to generate fluctuating electric fields.