Microwave-to-optical quantum transduction utilizing the topological Faraday effect of topological-insulator heterostructures

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 as small as $\eta \sim {10}^{-8}-{10}^{-15}$ 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 $\eta \sim {10}^{-4}$ by utilizing the heterostructures consisting of topological insulator thin films, such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ferromagnetic insulator thin films, such as YIG.