State Preparation by Optical Pumping in Erbium Doped Solids using   Stimulated Emission and Spin Mixing
  Erbium doped solids are potential candidates for the realization of a quantum memory for photons at telecommunication wavelengths. The implementation of quantum memory proposals in rare earth ion doped solids require spectral tailoring of the inhomogeneous absorption profile by efficient population transfer between ground state levels (spin polarization) using optical pumping. In this article we investigate the limiting factors of efficient optical pumping between ground state Zeeman levels in an erbium doped Y2SiO5 crystal. We introduce two methods to overcome these limiting factors: stimulated emission using a second laser and spin mixing using radio frequency excitation. Both methods significantly improve the degree of spin polarization. Population transfer between two Zeeman levels with less than 10 % of the total population in the initial ground state is achieved, corresponding to a spin polarization greater than 90 %. In addition, we demonstrate spectral tailoring by isolating a narrow absorption peak within a large transparency window. 
B. Lauritzen, S. R. Hastings-Simon, H. de Riedmatten, M. Afzelius, and   N. Gisin