Can ZnO transform Cavity-QED?

Transparent conductive oxides (TCO) routinely enable confinement of charge-sensitive quantum systems proximal to dielectric structures including waveguides and photon detectors. However, high optical loss precludes their use on the surfaces of high-finesse optical cavities. Here we apply a 30 nm thick ZnO film to one mirror of a Fabry–Pérot cavity. Using ring-down spectroscopy, we observe a finesse of 22,000 at 1650 nm, corresponding to a loss three orders of magnitude lower than that of indium tin oxide (ITO). The same film exhibits a surface resistivity of 0.01 Ω·cm, well below the thresholds required to mitigate charging effects that degrade atom-cavity interaction and high-fidelity gates in trapped ions and Rydberg atoms. We anticipate that ZnO could enable a step change in cavity-QED systems by reducing the perturbations caused by dielectric surfaces.