Effect of Two Level System Saturation on Charge Noise in Josephson Junction Qubits

TL;DR

This paper investigates how two-level systems (TLS) in substrates contribute to charge noise in Josephson qubits, revealing the impact of TLS saturation and flux ratios on noise characteristics across frequencies.

Contribution

It demonstrates that TLS saturation significantly influences charge noise behavior, providing a theoretical framework consistent with experimental observations.

Findings

Charge noise $S_Q$ arises from fluctuating TLS with electric dipoles.

High flux ratios $J/J_c$ lead to TLS saturation effects.

Charge noise exhibits $1/f$ dependence at low frequencies and linear at high frequencies.

Abstract

We show that charge noise $S_Q$ in Josephson qubits can be produced by fluctuating two level systems (TLS) with electric dipole moments in the substrate using a flat density of states. At high frequencies the frequency and temperature dependence of the charge noise depends on the ratio $J/J_c$ of the electromagnetic flux $J$ to the critical flux $J_c$. It is not widely appreciated that TLS in small qubits can easily be strongly saturated with $J/J_c\gg 1$. Our results are consistent with experimental conclusions that $S_Q\sim 1/f$ at low frequencies and $S_Q\sim f$ at high frequencies.