Solid-state quantum defects in wide band-gap two-dimensional silica bilayer

Two-dimensional (2D) oxides stand out as one of the most promising host materials for solid-state quantum defects, combining the advantages of easier fabrication from 2D systems and long coherence time from oxide materials. Here, we systematically screen the single-point defect families for promising quantum defects in two-dimensional (2D) silica bilayer $({\mathrm{SiO}}_{2})$ by analyzing the electronic, magnetic, and optical properties from theoretical simulations. The zero-phonon transition energies $({E}_{\mathrm{ZPL}})$ range from 0.22 eV to 6.35 eV, offering wide application potential. The X substitution for Si $({X}_{\mathrm{Si}}, \mathrm{X}=\mathrm{Cr}, \mathrm{Mo}, \mathrm{W})$ defect family with a tetrahedral crystal field provides 1.77 to 2.91 eV ${E}_{\mathrm{ZPL}}$ due to the large energy gap between ${e}_{g}$ and ${t}_{2g}$ orbitals, while the ${E}_{\mathrm{ZPL}}$ of X substitution for O $({X}_{O}, \mathrm{X}=\mathrm{C}, \mathrm{N}, \mathrm{O}, \mathrm{Ti}, \mathrm{Zr}, \mathrm{Hf})$ defect family highly depends on the substitution element. We have identified the ${\mathrm{Ti}}_{\mathrm{O}}$ defect as a single photon emitter operating in the telecom band and spin-defect qubits $({\mathrm{Cr}}_{\mathrm{Si}} \text{and} {\mathrm{Mo}}_{\mathrm{Si}})$ with desirable spin-triplet ground state, ${E}_{\mathrm{ZPL}}$ (2.08 eV for ${\mathrm{Cr}}_{\mathrm{Si}}$; 1.77 eV for ${\mathrm{Mo}}_{\mathrm{Si}}$), moderate zero-field splitting, and fast intersystem crossing. The 2D ${\mathrm{SiO}}_{2}$ bilayer host would benefit the integration of quantum networks and conventional silicon-based semiconductor electronic devices.