Optical coherence of 166 Er: 7 LiYF 4 crystal below 1 Kelvin

Weexplore optical coherence and spin dynamics of an isotopically purified (166) Er:(LiYF4)-Li-7 crystal below 1K and at weak magnetic fields < 0.3T. Crystals were grown in our lab and demonstrate narrow inhomogeneous optical broadening down to 16. MHz. Solid-state atomic ensembles with such narrow linewidths are very attractive for implementing of off-resonant Raman quantum memory and for the interfacing of superconducting quantum circuits and telecom C-band optical photons. Both applications require a low magnetic field of similar to 10 mT. However, at conventional experimental temperatures T > 1.5. K, optical coherence of Er:LYF crystal attains. 10 mu s time scale only at strong magnetic fields above 1.5 T. In the present work, we demonstrate that the deep freezing of Er:LYF crystal below 1 K results in the increase of optical coherence time to similar or equal to 100 mu s at weak fields.