Experimental evidence for a surface distribution of two-level systems in superconducting lithographed microwave resonators

TL;DR

This study measures how the frequency of superconducting microwave resonators shifts with temperature, supporting the theory that two-level systems are distributed on the surface, likely within a native oxide layer.

Contribution

It provides experimental evidence for a surface distribution of two-level systems affecting superconducting resonators, with quantitative analysis of TLS surface layer thickness.

Findings

Frequency shift data agree with TLS theory

TLS are distributed on the resonator surface

Estimated TLS surface layer thickness is a few nanometers

Abstract

We present measurements of the temperature-dependent frequency shift of five niobium superconducting coplanar waveguide microresonators with center strip widths ranging from 3 $\mu$m to 50 $\mu$m, taken at temperatures in the range 100-800 mK, far below the 9.2 K transition temperature of niobium. These data agree well with the two-level system (TLS) theory. Fits to this theory provide information on the number of TLS that interact with each resonator geometry. The geometrical scaling indicates a surface distribution of TLS, and the data are consistent with a TLS surface layer thickness of order a few nm, as might be expected for a native oxide layer.