Coherence properties of shallow donors in ZnO*

The donor system in ZnO is a promising potential qubit for spin-based quantum information processing. It has similar advantages as the well-studied phosphorus donors in Si, i.e. high homogeneity, long coherence time, and strong donor nucleus-electron interaction, with the additional advantage that ZnO donors are optically coupled to donor-bound excitons. Here we present optical measurements of the Ga donor electron spin T₁, T₂^*, and T₂. We find the longitudinal spin relaxation time T₁ is inversely proportional to the external magnetic field with a relationship ~B^-3.5. The longest T₁ we measure is ~0.1 s at 2.5 T. Due to the B^-3.5 relationship, we expect T₁ will exceed 1 s at fields lower than 1 T. We measure a T₂^* of ~20 ns and a spin echo decoherence time T₂ of ~30 μs, which may be limited by instantaneous diffusion due to the high donor concentration (~10¹⁷ cm^-3) or the spectral diffusion due to flip-flop of the surrounding Zn⁶⁷ nuclear spins (4.1% of natural Zn). We describe these experiments as well as present a path toward single Zn donor isolation and increased coherence times.