A stable and low loss oxide layer on α-Ta (110) film for superconducting qubits

Abstract

The dielectric loss of amorphous oxide layers is a major limiting factor for the coherent time of superconducting qubits. Usually, the surface oxides of superconductor film are lossy and unstable in air. To increase the coherence time, it is essential for qubits to have stable and low dielectric loss oxides, either as barrier or passivation layers. Here, we demonstrate that a kind of amorphous tantalum oxide on α-Ta (110) film is robust and stable by means of chemical and structural analysis. Such kind of oxide layer forms in a self-limiting process on the surface of α-Ta (110) film in piranha solution, yielding stable thickness and steady chemical composition. Quarter-wavelength coplanar waveguide resonators are made to study the loss of this oxide. One fresh resonator have a Qi of 3.0×106 in the single photon region. The Qi of most devices are higher than 2.0×106. And most of them are still over one million even after exposed to air for months. Based on these findings, we propose an all-tantalum superconducting qubit utilizing such oxide as passivation layers, which possess low dielectric loss and improved stability.