Electron-spin spectral diffusion in an erbium doped crystal at millikelvin temperatures

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 electronspin bath lead to excess spectral diffusion and rapid decoherence. Here we lithographically fabricate a 4.4 GHz superconducting planar microresonator on a CaWO4 crystal doped with Er ions at a concentration of 20 ppm 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 two- and three-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.

Automated summary

In this study, a 4.4 GHz superconducting planar microresonator was lithographically fabricated on a CaWO4 crystal doped with Er ions at a concentration of 20 ppm relative to Ca. The spectral diffusion processes that limit the electron-spin coherence of Er ions at millikelvin temperatures were characterized using two- and three-pulse echo sequences, showing a strong temperature dependence on the coherence time.