  Neutral shallow donors in zinc oxide (ZnO) are spin qubits with optical
access via the donor-bound exciton. This spin-photon interface enables
applications in quantum networking, memories and transduction. Essential
optical parameters which impact the spin-photon interface include radiative
lifetime, optical inhomogeneous and homogeneous linewidth and optical depth. We
study the donor-bound exciton optical linewidth properties of Al, Ga, and In
donors in single-crystal ZnO. The ensemble photoluminescence linewidth ranges
from 4-11 GHz, less than two orders of magnitude larger than the expected
lifetime-limited linewidth. The ensemble linewidth remains narrow in absorption
through samples with an estimated optical depth up to several hundred. The
primary thermal relaxation mechanism is identified and found to have a
negligible contribution to the total linewidth at 2 K. We find that
inhomogeneous broadening due to the disordered isotopic environment in natural
ZnO is significant, contributing 2 GHz. Two-laser spectral hole burning
measurements, indicate the dominant mechanism, however, is homogeneous. Despite
this broadening, the high homogeneity, large optical depth and potential for
isotope purification indicate that the optical properties of the ZnO
donor-bound exciton are promising for a wide range of quantum technologies and
motivate a need to improve the isotope and chemical purity of ZnO for quantum
technologies.
