Microwave-to-Optical Quantum Transduction Utilizing the Topological Magnetoelectric Effect

The quantum transduction (or equivalently quantum frequency conversion) 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. In this study, we focus on the transduction via the magneto-optic Faraday effect, i.e., the light-magnon interaction. Previous experimental studies have demonstrated the transduction efficiency of O(10^-8 - 10^-10) by using the ferromagnet YIG and have shown that the transduction efficiency is essentially determined by the light-magnon interaction strength. Here, we take advantage of the fact that three-dimensional topological insulator thin films exhibit a universal Faraday rotation angle arising from the topological magnetoelectric effect. 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 can be greatly improved to O(10^-4) by utilizing a heterostructure of a few dozen of layers consisting of topological insulator thin films such as Bi₂Se₃ and ferromagnetic insulator thin films such as YIG.