Thesis Title: Quantum transduction with hybrid systems: electro-mechanics and electro-optics

Abstract: Quantum transduction enables quantum information transfer across disparate systems, leveraging their individual advantages, which is essential for scalable quantum networking. A scalable quantum internet requires quantum nodes linked by quantum channels which are intrinsically lossy, necessitating quantum repeaters to extend communication range via entanglement swapping. Quantum memory, which allows temporary storage of quantum states, is a key component of repeaters. Equipping quantum nodes with quantum memory also enables advanced protocols like quantum error correction. Meanwhile, most leading qubit platforms operate at microwave frequencies, whereas photonic channels – widely used for long-distance quantum communication due to low loss and robustness against noise – operate at optical frequencies. Bridging such frequency mismatch necessitates microwave-optical quantum transducers.

This dissertation presents two projects focused on quantum transduction. First, we develop one-dimensional quartz phononic crystal resonators for quantum memory applications, with millisecond-long mechanical lifetimes measured at 8 K. These piezoelectric devices are well-suited for coupling to superconducting qubits, and we estimate their parametric coupling to a transmon mediated by the nonlinear circuit element SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement). Second, we develop LiNbO₃-based microwave-optical quantum transducers. By placing the nonlinear material LiNbO₃ at the overlap of optical cavity mode and microwave cavity mode, the electro-optic effect enables the microwave-optical transduction. We demonstrate triply-resonant electro-optic transduction, validating the system’s functionality for quantum transduction.