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

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.

9 pages, 5 figures

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Cited by in corpus (11)

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• Autler-Townes splitting in a three-dimensional transmon superconducting qubit
• Stabilizing and improving qubit coherence by engineering noise spectrum of two-level systems
• Role of the quasi-particles in an electric circuit with Josephson junctions
• High-Order Qubit Dephasing at Sweet Spots by Non-Gaussian Fluctuators: Symmetry Breaking and Floquet Protection
• Experimentally revealing anomalously large dipoles in a quantum-circuit dielectric
• Observation of discrete charge states of a coherent two-level system in a superconducting qubit
• Decoherence in a Pair of Long-Lived Cooper-Pair Boxes
• Neural Network Based Qubit Environment Characterization
• Review of Superconducting Qubit Devices and Their Large-Scale Integration