Quasiparticle tunneling and 1/f charge noise in ultrastrongly coupled superconducting qubit and resonator

TL;DR

This paper investigates the causes of anomalous spectral splitting and charge noise in an ultrastrongly coupled superconducting qubit-resonator system, attributing effects to quasiparticle tunneling and background charge fluctuations.

Contribution

It provides a detailed analysis linking spectral anomalies to quasiparticle tunneling and 1/f charge noise, supported by experimental data and theoretical models.

Findings

Doubly split spectrum observed with fluctuations over 70 hours

Spectral split width fluctuation linked to quasiparticle tunneling

Background charge noise exhibits 1/f characteristics

Abstract

We report an experimentally observed anomalous doubly split spectrum and its split-width fluctuation in an ultrastrongly coupled superconducting qubit and resonator. From an analysis of Rabimodel and circuit model Hamiltonians, we found that the doubly split spectrum and split-width fluctuation are caused by discrete charge hops due to quasiparticle tunnelings and a continuous background charge fluctuation in islands of a flux qubit. During 70 hours in the spectrum measurement, split width fluctuates but the middle frequency of the split is constant. This result indicates that quasiparticles in our device seem mainly tunnel one particular junction. The background offsetcharge obtained from split width has the 1/f noise characteristic.