Abstract
Recent studies have shown that parasitic two-level systems (TLS) in superconducting qubits, which are a leading source of decoherence, can have relaxation times longer than the qubits themselves. However, the standard techniques used to characterize qubit relaxation is only valid for measuring T1 under the Born-Markov approximation and could mask environmental memory effects in practice. Here, we introduce two-timescale relaxometry, a technique to probe the qubit and environment relaxation simultaneously and efficiently. We apply it to high-coherence fluxonium qubits over a frequency range of 0.1-0.4 GHz, and reveal a discrete spectrum of TLS with millisecond lifetimes. Our analysis of the spectrum is consistent with a random distribution of TLS in the aluminum oxide tunnel barrier of the Josephson junction chain of the fluxonium, with a spectral and volumetric density and average electric dipole similar to previous TLS studies at much higher frequencies. Our study suggests that investigating and mitigating TLS in the junction chain is crucial to the development of various types of noise-protected qubits in circuit QED.
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The data used to reproduce the plots within the paper have been deposited in the figshare database under accession code doi.org/10.6084/m9.figshare.30727316.
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Acknowledgements
We thank E. Dogan for experimental assistance and Y.-Y. Wang for helpful discussions. This research is supported by the US Army Research Office, QC-S5 Program (No. W911-NF-23-10093; Z.-T. Z., D. R., and C. W.) and the US Army Research Office, HiPS Program (No. W911-NF-18-10146; A. S. and V. E. M.). Data analysis is partially supported by the US Department of Energy, Office of Science, National Quantum Information Science Research Centers, Co-design Center for Quantum Advantage under contract DE-SC0012704 (Z.-T. Z., B.-J. L., and C. W.). The planar fluxonium device is designed by T. A. Masum, and it is fabricated and provided by the SQUILL Foundry at MIT Lincoln Laboratory, with funding from the Laboratory for Physical Sciences (LPS) Qubit Collaboratory.
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Z.-T. Z. carried out the measurements, collected the data, and performed the analysis with the assistance of D. R. and B.-J. L. The experimental protocol was initially developed by D. R. C. W., who conceived and supervised the experiment. The 3D device was fabricated by A. S. under the supervision of V. E. M. Z.-T. Z., B.-J. L. and C. W. wrote the manuscript with input from all authors.
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Zhuang, ZT., Rosenstock, D., Liu, BJ. et al. Non-Markovian relaxation rpectroscopy of fluxonium qubits. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69910-2
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DOI: https://doi.org/10.1038/s41467-026-69910-2
