  Quantum transduction between microwave and optical frequencies is important
for connecting superconducting quantum platforms in a quantum network.
Ensembles of rare-earth ions are promising candidates to achieve this
conversion due to their collective coherent properties at microwave and optical
frequencies. Erbium ions are of particular interest because of their telecom
wavelength optical transitions that are compatible with fiber communication
networks. Here, we report the optical and electron spin properties of erbium
doped yttrium orthovanadate (Er$^{3+}$:YVO$_{4}$), including high-resolution
optical spectroscopy, electron paramagnetic resonance studies and an initial
demonstration of microwave to optical conversion of classical fields. The
highly absorptive optical transitions and narrow ensemble linewidths make
Er$^{3+}$:YVO$_{4}$ promising for magneto-optic quantum transduction.
