  Developing schemes for efficient and broad-band frequency conversion of
quantum signals is an ongoing challenge in the field of modern quantum
information. Especially the coherent conversion between microwave and optical
signals is an important milestone towards long-distance quantum communication.
In this work, we propose a two-stage conversion protocol, employing a resonant
interaction between magnetic and mechanical excitations as a mediator between
microwave and optical photons. Based on estimates for the coupling strengths
under optimized conditions for yttrium iron garnet, we predict close to unity
conversion efficiency without the requirement of matching cooperativities. We
predict a conversion bandwidth in the regions of largest efficiency on the
order of magnitude of the coupling strengths which can be further increased at
the expense of reduced conversion efficiency.
