Analysis of high quality superconducting resonators: consequences for TLS properties in amorphous oxides

J Burnett,T Lindström,L Faoro

doi:10.1088/0953-2048/29/4/044008

Abstract

noise caused by microscopic two-level systems (TLS) is known to be very detrimental to the performance of superconducting quantum devices but the nature of these TLS is still poorly understood. Recent experiments with superconducting resonators indicates that interaction between TLS in the oxide at the film-substrate interface is not negligible. Here we present data on the loss and frequency noise from two different Nb resonators with and without Pt capping and discuss what conclusions can be drawn regarding the properties of TLS in amorphous oxides. We also estimate the concentration and dipole moment of the TLS.

Highlights

Superconducting electronics has become a frontrunner in the race to create viable applications of solid state quantum technology
The fact that a logarithmic dependence is found for the Nb+Pt resonator implies that the interaction with the conducting electrons present in the Pt capping does not play a role in the relaxation mechanisms of the Two Level Systems (TLS) responsible for the noise
We have studied the ratio between the noise and the loss and extract order of magnitude estimates for the density of states P0, the interaction scale U0 of the thermally activated TLS in the resonators and the number of classical fluctuators that are strongly coupled to a resonant TLS and in our model are responsible for the noise and the anomalous weak power dependence of the loss of the resonators at high fields

Summary

INTRODUCTION

Superconducting electronics has become a frontrunner in the race to create viable applications of solid state quantum technology. [12] that contains two different types of TLS: “slow” classical fluctuators that can be thermally activated even at millikelvin temperatures with very long time-constants and “fast” coherent TLS with typical energy scales of GHz. In that work, data was shown that could not be fit by the conventional STM, instead pointing toward the model in Ref. Calculations carried out in Ref [12, 14] show that the interaction with strongly coupled classical fluctuators (i) results in a formula for the temperature dependent frequency shift that agrees with STM theory: δν = F tan δi ReΨ ν0 π. (ii) does not change the absorption at low powers but changes the square-root dependence of the absorption into a logarithmic one at high applied fields Where NT LS(T ) is the number of thermal TLS coupled to the resonator

RESULTS

We fit our data with

CONCLUSIONS