  The quantum transduction between microwave and optical photons is essential
for realizing scalable quantum computers with superconducting qubits. Due to
the large frequency difference between microwave and optical ranges, the
transduction needs to be done via intermediate bosonic modes or nonlinear
processes. So far, the transduction efficiency $\eta$ via the magneto-optic
Faraday effect (i.e., the light-magnon interaction) in the ferromagnet YIG has
been demonstrated to be small as $\eta\sim 10^{-8} \mathrm{-} 10^{-15}$ due to
the weak magneto-optic coupling. Here, we take advantage of the fact that
three-dimensional topological insulator thin films exhibit a topological
Faraday effect that is independent of the sample thickness in the terahertz
regime. This leads to a large Faraday rotation angle and therefore enhanced
light-magnon interaction in the thin film limit. We show theoretically that the
transduction efficiency between microwave and terahertz photons can be greatly
improved to $\eta\sim10^{-4}$ by utilizing the heterostructures consisting of
topological insulator thin films such as Bi$_2$Se$_3$ and ferromagnetic
insulator thin films such as YIG.
