Date of Award
Fall 2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Electrical Engineering (ENAS)
First Advisor
Tang, Hong
Abstract
With superconducting circuits emerging as a promising platform for quantum computation and optical photons being the most suitable long-haul quantum information carrier, efficient bidirectional conversion between microwave and optical photons at the quantum level is in critical demand. Through high-efficiency microwave-to-optical interface, a hybrid system where quantum information is processed by superconducting circuits and distributed with photonic circuits is one of the most promising schemes to implement large-scale quantum networks. Among various approaches to realize coherent microwave-to-conversion, the cavity electro-optic (EO) system utilizes the Pockels nonlinearity to realize direct conversion between GHz microwave and optical photons without introducing an intermediate excitation. In the EO system, strong Pockels nonlinearity and high-performance optical cavity are the prerequisites for high conversion efficiency. Thin-film lithium niobate (TFLN) offers these desired characteristics. As one of the most widely used synthetic crystals, lithium niobate (LN) plays a significant role in modern telecommunication due to its favorable optical nonlinearity as well as broad transparency window. Traditional optical waveguides on LN are fabricated using titanium in-diffusion or proton exchange method, leading to large mode volume and weak optical confinement. With recent development in wafer production techniques, high-quality TFLN on insulator wafers enables low-loss integrated LN waveguides and high-Q microresonators, providing an attractive on-chip platform for various optical applications. In the thesis, we present our study on the route toward highly efficient EO microwave-to-optical conversion with TFLN. First, we develop nanofabrication techniques to achieve low-loss optical waveguides and high-Q (>1 X 10^6) microring cavities. These are not only prerequisites for high-efficiency microwave-to-optical converters but also enable various nonlinear optics applications in the integrated LN platform. Next, we study the photorefractive (PR) effect in the TFLN material platform, which is a major obstacle that limits the device's performance. The PR effect in LN causes refractive index variation of the material in the presence of light illumination, introducing unfavorable instability to the converter system. We observe a strong Bragg scattering induced by the PR effect in high-Q LN ring resonators measured at cryogenic temperature. To overcome this obstacle, We mitigate this strong PR effect by removing the commonly used dielectric cladding layer and using an air-clad device architecture instead. Then we demonstrate bidirectional EO conversion in TFLN-superconductor (niobium nitride) hybrid system, with on-chip conversion efficiency exceeding 1 %. We further improve our device design, achieving a high microwave Q in the material platform, and observe a dynamic microwave frequency shifting in the superconducting cavity resulting from its fast response to the optical pump pulse. We also design and test a helium cell that could improve the power handling of the converter device at milli-kelvin temperature. We show that unity internal conversion efficiency is achievable with proper optimization. Our study project that integrated TFLN photonics has promise for future quantum applications such as verification of non-classical correlation, generation of entangled photon pairs, etc. In the next step, quantum state transfer between microwave and optical domains will be a significant step toward the realization of hybrid quantum networks.
Recommended Citation
Xu, Yuntao, "Cavity-Enhanced Lithium Niobate Microwave-to-Optical Conversion" (2022). Yale Graduate School of Arts and Sciences Dissertations. 852.
https://elischolar.library.yale.edu/gsas_dissertations/852
