Optically Addressable Donor Qubits in ZnO: Single-Donor Isolation and Coherent Microwave Control

Abstract

Neutral shallow donors in zinc oxide (ZnO) are promising optically addressable spin qubits because they combine a donor-bound electron spin with a donor-bound-exciton optical interface in a direct-band-gap semiconductor, weak spin-orbit coupling, and a dilute natural nuclear-spin bath. Among these donors, indium is especially attractive because the coupled $^{115}\mathrm{In}$ nuclear spin, (I=9/2), provides a hyperfine-resolved auxiliary degree of freedom that could serve as a long-lived quantum memory. Advancing In donors in ZnO toward scalable spin-photon technologies requires two key capabilities: isolation of individual optically active donors and high-fidelity coherent control of the donor electron spin. This thesis addresses these requirements through two complementary experiments. First, single In donors are isolated from commercial ZnO using $\mathrm{Xe}^+$ plasma focused ion beam milling to reduce the active optical volume. Resonant optical spectroscopy, two-electron-satellite emission, magnetic-field dependence, and lifetime measurements identify localized emitters as In donors and show that narrow, stable donor-bound-exciton transitions can survive focused-ion-beam processing and annealing. The measured linewidths approach the lifetime limit of the processed emitters, demonstrating the robustness of In-donor optical properties in fabricated ZnO structures. At the same time, shortened excited-state lifetimes indicate that near-surface non-radiative recombination remains an important limitation, motivating future work on surface passivation and photonic integration. Second, coherent microwave control is demonstrated in implanted In donor ensembles. A gap-coupled microstrip resonator is developed to generate strong in-plane microwave magnetic fields while preserving optical access and the Faraday-geometry selection rules required for spin-selective initialization and readout. Resonant optical pumping initializes the donor electron spin with fidelities of (86\%) and (85\%), and pulsed optically detected magnetic resonance resolves the ten hyperfine transitions associated with the coupled ($^{115}\mathrm{In}$) nuclear spin. These spectra reveal optical-pumping-induced nuclear spin polarization together with a reproducible, angle-dependent bowl-shaped amplitude envelope. Although similar envelope shapes have been reported in high-spin Mn defects in ZnO, the In donor system lacks the same high-spin and zero-field-splitting structure, leaving the microscopic origin of the envelope an open question. Microwave pulse sequences demonstrate coherent control of the donor electron spin through Rabi oscillations, Ramsey interference, Hahn echo, and dynamical decoupling. A maximum Rabi frequency of $\Omega_R/2\pi = 36.2 \pm 0.7$\,MHz is achieved, corresponding to a pi-pulse time of $13.8 \pm 0.3$\,ns. Ramsey measurements give $T_2^* = 17 \pm 1$\,ns, while Hahn echo extends the driven-ensemble coherence to approximately $200$\,ns, with further improvement under CPMG decoupling. Additional measurements indicate that the shortened echo coherence is not dominated by steady-state microwave heating, instantaneous diffusion from the addressed In ensemble, or the In implantation profile alone, pointing instead to magnetic noise from the broader local spin environment as an important limitation. Together, these results establish single-donor isolation and coherent microwave control as essential building blocks for In donor qubits in ZnO. They provide a path toward future devices that combine efficient optical access, nanosecond-scale electron-spin control, and nuclear-spin memory in individual donor systems.