Material defects give rise to parasitic two-level systems (TLS) which present a major source of decoherence in superconducting qubits. Here, we study a strongly coupled TLS that resides in the tunnel barrier of transmon qubit. We use multi-photon spectroscopy and TLS strain tuning to explore the rich spectrum of the interacting three-partite system consisting of TLS, qubit, and its readout resonator. This reveals a strong effective resonant coupling between the TLS and the qubit's readout resonator which dresses the resonator states and results in a resonance frequency shift that spoils the readout signal. Our finding presents yet another way how material defects can interfere with qubit operation and hinder the realization of solid-state quantum processors.
@article{arxiv.2605.02755,
title = {Readout failures in superconducting qubits due to TLS-defects in tunnel junctions},
author = {J. Lisenfeld and A. K. Händel and A. Bilmes and A. V. Ustinov},
journal= {arXiv preprint arXiv:2605.02755},
year = {2026}
}