  Ensembles of erbium dopants can realize quantum memories and frequency
converters that operate in the minimal-loss wavelength band of fiber optical
communication. Their operation requires the initialization, coherent control
and readout of the electronic spin state. In this work, we use a split-ring
microwave resonator to demonstrate such control in both the ground and
optically excited state. The presented techniques can also be applied to other
combinations of dopant and host, and may facilitate the development of new
quantum memory protocols and sensing schemes.
