Three-Dimensional Niobium Coaxial Cavity with $\sim0.1\,$second Lifetime

Abstract: We report on the internal quality factor of a three-dimensional niobium quarter-wave coaxial cavity, with mid-temperature annealing, exhibiting $Q_{\rm int} \gtrsim 3\times10<sup>9$ at the single-photon level below 20\,mK, which corresponds to an internal photon lifetime of $\tau_{\rm int}\sim90\,\mathrm{ms}$. Moreover, $Q_{\rm int}$ of the mid-temperature annealed cavities remains almost unchanged even after several cooldown cycles and air exposure. These results suggest that stable low-loss niobium oxides might be formed by mid-temperature annealing on the surface of three-dimensional niobium cavity. This surface treatment could be applicable to the fabrication of 2D superconducting circuits and help improve the lifetime of Nb-based superconducting qubits.