Optically Accessible Spin Qubits in ZnO

Abstract

Quantum information science has witnessed significant advancements in the pursuit of fault-tolerant quantum computers with growing emphasis on realizing large-scale quantum networks. However, the community has yet to identify a single or composite quantum system that will enable robust and scalable quantum communication technologies. Among a plethora of candidates, the neutral shallow donor in zinc oxide (ZnO) has recently emerged as a promising platform due to its unique combination of efficient light-matter interaction and potential for long coherence times. In this dissertation, we present a comprehensive study of the spin and optical properties of aluminum (Al), gallium (Ga), and indium (In) donors in natural isotopic abundance ZnO, with an emphasis on assessing their suitability for photon-based quantum networks. Using optical spin initialization via the donor-bound exciton, we demonstrate ensemble longitudinal spin relaxation times nearing half a second, which suggests a great potential for long transverse spin relaxation (coherence) times in isotopically and chemically purified ZnO. Photoluminescence excitation spectroscopy reveals narrow, non-thermally-broadened ensemble optical linewidths (less than two orders of magnitude larger than the lifetime limit) at liquid helium temperatures, further improvable by isotopic and chemical purification. We further showcase that single In emitters can be spatially and spectrally isolated via focused ion beam milling while preserving their favorable optical properties, albeit with reduced radiative photoluminescence and increased emission inhomogeneity. Finally, we report on the progress of emission frequency tuning by employing the Stark effect, realized via the fabrication of planar capacitors on the host crystal surface. Our findings, coupled with the potential for subsequent optimization, establish the neutral shallow donor in ZnO as a compelling platform for photon-based quantum technologies. This work underscores the need for the research and development of higher purity ZnO production techniques. It ushers in future research aimed at deciphering and eliminating optical linewidth broadening mechanisms, achieving scalable and deterministic isolation of single emitters, and, ultimately, integrating this platform in both homogeneous and heterogeneous quantum networks.