Microscopic model of critical current noise in Josephson-junction qubits: Subgap resonances and Andreev bound states

TL;DR

This paper introduces a microscopic model explaining critical current noise in Josephson-junction qubits, highlighting the role of Andreev bound states and subgap resonances, which could clarify observed two-level systems.

Contribution

The paper develops a microscopic model linking trapping-centers and Andreev bound states to noise, providing a new understanding of qubit decoherence sources.

Findings

Noise spectrum dominated by sharp subgap resonances

Each trapping-center causes a dielectric resonance in the superconducting phase

Derived an effective Hamiltonian connecting phenomenological and microscopic models

Abstract

We propose a microscopic model of critical current noise in Josephson-junctions based on individual trapping-centers in the tunnel barrier hybridized with electrons in the superconducting leads. We calculate the noise exactly in the limit of no on-site Coulomb repulsion. Our result reveals a noise spectrum that is dramatically different from the usual Lorentzian assumed in simple models. We show that the noise is dominated by sharp subgap resonances associated to the formation of pairs of Andreev bound states, thus providing a possible explanation for the spurious two-level systems (microresonators) observed in Josephson junction qubits [R.W. Simmonds et al., Phys. Rev. Lett. 93, 077003 (2004)]. Another implication of our model is that each trapping-center will contribute a sharp dielectric resonance only in the superconducting phase, providing an effective way to validate our results experimentally. We derive an effective Hamiltonian for a qubit interacting with Andreev bound states, establishing a direct connection between phenomenological models and the microscopic parameters of a Fermionic bath.