Spectroscopy of a Cooper-Pair Box Coupled to a Two-Level System Via Charge and Critical Current

TL;DR

This study observes a quadrupled spectrum of a Cooper-pair box qubit coupled to two two-level systems, revealing significant critical current fluctuations and providing a detailed microscopic model of the fluctuators.

Contribution

The paper introduces a detailed model linking two-level systems to critical current fluctuations in a Cooper-pair box, with experimental validation and parameter extraction.

Findings

Observed spectrum quadrupling in the qubit.

Identified large critical current fluctuations (~30-40%).

Modeled TLS effects with microscopic parameters.

Abstract

We report on the quadrupling of the transition spectrum of an Al/AlOx/Al Cooper-pair box (CPB) charge qubit in the 4.0-7.3 GHz frequency range. The qubit was coupled to a quasi-lumped element Al superconducting resonator and measured at a temperature of 25 mK. We obtained good matches between the observed spectrum and the spectra calculated from a model Hamiltonian containing two distinct low excitation energy two-level systems (TLS) coupled to the CPB. In our model, each TLS has a charge that tunnels between two sites in a local potential and induces a change in the CPB critical current. By fitting the model to the spectrum, we have extracted microscopic parameters of the fluctuators including the well asymmetry, tunneling rate, and a surprisingly large fractional change (30-40%) in the critical current (12 nA). This large change is consistent with a Josephson junction with a non-uniform tunnel barrier containing a few dominant conduction channels and a TLS that modulates one of them.