Erbium-doped crystals offer a versatile platform for hybrid quantum devices because they combine magnetically-sensitive electron-spin transitions with telecom-wavelength optical transitions. At the high doping concentrations necessary for many quantum applications, however, strong magnetic interactions of the electron-spin bath lead to excess spectral diffusion and rapid decoherence. Here we lithographically fabricate a 4.4 GHz superconducting planar micro-resonator on a CaWO4 crystal doped with Er ions at a concentration of twenty parts per million relative to Ca. Using the microwave resonator, we characterize the spectral diffusion processes that limit the electron-spin coherence of Er ions at millikelvin temperatures by applying 2- and 3-pulse echo sequences. The coherence time shows a strong temperature dependence, reaching 1.3 ms at 23 mK for an electron-spin transition of 167Er.
@article{arxiv.2203.15012,
title = {Electron-spin spectral diffusion in an erbium doped crystal at millikelvin temperatures},
author = {Milos Rančić and Marianne Le Dantec and Sen Lin and Sylvain Bertaina and Thierry Chanelière and Diana Serrano and Philippe Goldner and Ren Bao Liu and Emmanuel Flurin and Daniel Estève and Denis Vion and Patrice Bertet},
journal= {arXiv preprint arXiv:2203.15012},
year = {2022}
}