Can TCOs Transform Cavity-QED?

Transparent conductive oxides (TCO) enable confinement of charge-sensitive ions and Rydberg atoms proximal to dielectric structures including waveguides and photon detectors. However, optical loss precludes the use of TCOs within high-finesse optical micro-resonators. Here we characterize a ZnO-based TCO that markedly reduces optical absorption. At 1650$\text{ nm}$ we observe a 22,000 finesse in a Fabry-Pérot optical cavity coated with a 30$\text{ nm}$ ZnO layer. This is a 5000 times reduction relative to indium tin oxide (ITO) at this wavelength. The same ZnO film exhibits 0.01$\text{ensuremath}\text{Ω}\text{ensuremath}\text{cdot}\text{cm}$ surface resistivity at DC. We anticipate a step change in cavity-QED systems incorporating ultra-low loss TCOs like ZnO.