Superconducting transmon qubits lose coherence and suffer low-frequency parameter instability mainly through microscopic two-level systems (TLS) — atomic-scale defects in amorphous dielectrics — and their thermally activated fluctuator (TLF) counterparts, whose interaction causes spectral diffusion and telegraph noise. A 2019 simultaneous correlation study of relaxation, dephasing, and frequency noise (Schlör et al., Phys. Rev. Lett. 123, 190502) showed that individual TLS near superconducting-film edges can suppress qubit coherence by up to an order of magnitude, and posed a set of linked open questions: what are these defects microscopically, what mechanism drives abrupt changes in decoherence dynamics, and how can such single-defect-dominated decoherence be eliminated or passivated. Localization (junction leads and lift-off residues), statistics (structure–density correlations over thousands of junctions), and mitigation (annealing, phononic bandgaps, tantalum-on-silicon, in-situ electric-field tuning) have advanced since 2019, but the precise atomic/chemical identity of the individually resolved defects remains unidentified and TLS remain the dominant loss channel in state-of-the-art devices.

Background

A transmon is the workhorse superconducting qubit: a Josephson junction (an Al/AlOx_x/Al tunnel barrier between two superconducting electrodes) shunted by a large capacitance, making its transition frequency first-order insensitive to charge noise. Its performance is characterized by the energy-relaxation time T1T_1 and the (pure) dephasing time TΦT_\Phi. Since the 2000s it has been understood that a leading limit on both is set not by the junction design itself but by parasitic microscopic defects in the surrounding disordered dielectrics — the tunnel barrier, native surface oxides, fabrication residues, and substrate interfaces. In the standard tunneling model (Anderson–Halperin–Varma and Phillips), a two-level system (TLS) is an atom or group of atoms tunneling between two nearly degenerate configurations in an amorphous solid; by its electric dipole moment it couples to the qubit's microwave electric field. A TLS whose transition frequency is near the qubit frequency can resonantly absorb energy (shortening T1T_1) or shift the qubit frequency; a TLS with excitation energy near or below the thermal energy kBTk_B T switches state stochastically and is called a thermally activated fluctuator (TLF). Interactions between many TLFs and a near-resonant TLS make the TLS frequency wander in time — spectral diffusion — which manifests as telegraph-like and 1/f1/f-type low-frequency fluctuations of the qubit frequency and coherence times, and forces continuous recalibration of qubit processors.

Schlör et al. (Phys. Rev. Lett. 123, 190502 (2019)) measured T1T_1, TΦT_\Phi, and the transition frequency of a single fixed-frequency transmon simultaneously and continuously over hours to days and analyzed their correlations. They concluded that a sparse ensemble of TLS located near the edges of the superconducting film (where electric fields are enhanced, scaling as 1/x1/\sqrt{x} from the edge) dominates dephasing, that a single such defect can reduce coherence by up to an order of magnitude, and that a power spectral density (PSD) analysis of the frequency noise — fitting a single Lorentzian (switching rate ≈1\approx 1 mHz) on a 1/f1.11/f^{1.1} background — is consistent with one thermal TLF driving spectral diffusion of near-resonant surface TLS. They estimated a dominant-TLS surface density of about 0.3 per GHz per μ\mum2^2 from the statistics of a few events. In the paper the authors leave open: (i) the precise microscopic nature and atomic structure of these dominant TLS and of the TLFs; (ii) the mechanism of the observed abrupt changes in decoherence dynamics — slow thermalization of amorphous dielectrics, logarithmically slow TLS relaxation (expected from the broad distribution of tunneling barriers in the standard model), or background ionizing radiation; (iii) better measurement sensitivity, noting that a quantum-limited amplifier (one adding the minimum noise allowed by quantum mechanics) would extend the PSD bandwidth and allow detection of additional individual fluctuators; (iv) a systematic mapping of the TLS density and energy distribution, for which their fixed-frequency qubit was unsuited and which would require tunable qubits or direct TLS spectroscopy; and (v) long-term strategies — novel materials, surface treatments, or other processing — for eliminating or passivating single-defect-dominated decoherence, which they call a necessity for useful many-qubit systems.

Problem Statement

The problem posed by Schlör et al. (2019) and still standing is the microscopic understanding and control of single-defect-dominated decoherence in superconducting transmon qubits. Its linked elements share one objective — replacing phenomenological TLS/TLF models with an identified defect population that can be engineered away: (1) Identify the precise microscopic nature and atomic structure of the individual TLS that dominate qubit decoherence (located by the source near superconducting-film edges and surfaces, and by later work near junction leads and lift-off residues) and of the thermally activated fluctuators (TLF) whose interaction with them causes spectral diffusion — distinguishing structural tunneling defects in amorphous dielectrics (AlOx_x barrier, surface oxides, adsorbates, residues) from quasiparticle-induced or other fluctuators, including their chemical identity, spatial distribution, and the fabrication steps that create them. (2) Attribute the abrupt changes in decoherence dynamics (sudden shifts of T1T_1, TΦT_\Phi, and transition frequency) to a specific mechanism — slow thermalization in amorphous materials, logarithmically slow TLS relaxation, or background radiation — sufficiently to predict qubit parameter stability over timescales relevant to quantum error correction. (3) On that basis, establish long-term strategies that eliminate or passivate this decoherence channel — naming the defect class they target, the material, fabrication, or defect-environment change and the physical mechanism by which it removes or decouples that class, and the expected quantitative reduction of defect-limited decoherence — so that single defects no longer impose continuous recalibration on scaled superconducting quantum processors. All three elements are jointly required for a complete resolution of the problem, and an element counts as established only when the answer names a specific candidate — a defect class for (1), one of the three named mechanisms for (2), a targeted change for (3) — and states the observable measurement that would confirm or refute it together with the expected effect on defect-limited decoherence; fully establishing one element while the others remain open resolves only that part of the problem, not the problem as a whole. Subsidiary methodology goals named in the source — sufficient measurement sensitivity to resolve additional individual fluctuators and their relaxation rates (e.g. quantum-limited amplification) and systematic TLS density/distribution mapping via tunable qubits or direct TLS spectroscopy beyond the indirect ~0.3/(GHz·μ\mum2^2) estimate — are instruments toward elements (1)–(3), not independent ends.

References

  1. S. Schlör, J. Lisenfeld, C. Müller, A. Bilmes, A. Schneider, D. P. Pappas, A. V. Ustinov, M. Weides, 'Correlating decoherence in transmon qubits: Low frequency noise by single fluctuators', Phys. Rev. Lett. 123, 190502 (2019), DOI: 10.1103/PhysRevLett.123.190502, https://arxiv.org/abs/1901.05352
  2. J. Lisenfeld, A. Bilmes, A. Megrant, R. Barends, J. Kelly, P. Klimov, G. Weiss, J. M. Martinis, A. V. Ustinov, 'Electric field spectroscopy of material defects in transmon qubits', npj Quantum Information 5, 105 (2019), DOI: 10.1038/s41534-019-0224-1, https://arxiv.org/abs/1909.09749
  3. A. Bilmes, A. Megrant, P. Klimov, G. Weiss, J. M. Martinis, A. V. Ustinov, J. Lisenfeld, 'Resolving the positions of defects in superconducting quantum bits', Scientific Reports 10, 3090 (2020), DOI: 10.1038/s41598-020-59749-y, https://arxiv.org/abs/1911.08246
  4. S. E. de Graaf, L. Faoro, L. B. Ioffe, S. Mahashabde, J. J. Burnett, T. Lindström, S. E. Kubatkin, A. V. Danilov, A. Ya. Tzalenchuk, 'Two-level systems in superconducting quantum devices due to trapped quasiparticles', Science Advances 6, eabc5055 (2020), DOI: 10.1126/sciadv.abc5055, https://arxiv.org/abs/2004.02485
  5. A. Premkumar, C. Weiland, S. Hwang, B. Jaeck, A. P. M. Place, I. Waluyo, A. Hunt, V. Bisogni, J. Pelliciari, A. Barbour, M. S. Miller, P. Russo, F. Camino, K. Kisslinger, X. Tong, M. S. Hybertsen, A. A. Houck, I. Jarrige, 'Microscopic Relaxation Channels in Materials for Superconducting Qubits' (2020), arXiv:2004.02908, https://arxiv.org/abs/2004.02908
  6. J. H. Béjanin, C. T. Earnest, A. S. Sharafeldin, M. Mariantoni, 'Interacting Defects Generate Stochastic Fluctuations in Superconducting Qubits', Phys. Rev. B 104, 094106 (2021), DOI: 10.1103/PhysRevB.104.094106, https://arxiv.org/abs/2106.15748
  7. M. Carroll, S. Rosenblatt, P. Jurcevic, I. Lauer, A. Kandala, 'Dynamics of superconducting qubit relaxation times' (2021), arXiv:2105.15201, https://arxiv.org/abs/2105.15201
  8. A. Bilmes, S. Volosheniuk, A. V. Ustinov, J. Lisenfeld, 'Probing defect densities at the edges and inside Josephson junctions of superconducting qubits', npj Quantum Information 8, 24 (2022), DOI: 10.1038/s41534-022-00532-4, https://arxiv.org/abs/2108.06555
  9. C.-C. Hung, L. Yu, N. Foroozani, S. Fritz, D. Gerthsen, K. D. Osborn, 'Probing hundreds of individual quantum defects in polycrystalline and amorphous alumina', Phys. Rev. Applied 17, 034025 (2022), DOI: 10.1103/PhysRevApplied.17.034025, https://arxiv.org/abs/2107.04131
  10. S. E. de Graaf, S. Un, A. G. Shard, T. Lindström, 'Chemical and structural identification of material defects in superconducting quantum circuits', Materials for Quantum Technology 2, 042001 (2022), DOI: 10.1088/2633-4356/ac78ba, https://doi.org/10.1088/2633-4356/ac78ba
  11. T. Thorbeck, A. Eddins, I. Lauer, D. T. McClure, M. Carroll, 'Two-Level-System Dynamics in a Superconducting Qubit Due to Background Ionizing Radiation', PRX Quantum 4, 020356 (2023), DOI: 10.1103/PRXQuantum.4.020356, https://arxiv.org/abs/2210.04780
  12. J. Lisenfeld, A. Bilmes, A. V. Ustinov, 'Enhancing the Coherence of Superconducting Quantum Bits with Electric Fields', npj Quantum Information 9, 8 (2023), DOI: 10.1038/s41534-023-00678-9, https://arxiv.org/abs/2208.01570
  13. C.-C. Hung, T. Kohler, K. D. Osborn, 'Quantum defects from a single surface exhibit strong mutual interactions', Phys. Rev. Applied 21, 044021 (2024), DOI: 10.1103/PhysRevApplied.21.044021, https://arxiv.org/abs/2302.00318
  14. B.-J. Liu, Y.-Y. Wang, T. Sheffer, C. Wang, 'Observation of discrete charge states of a coherent two-level system in a superconducting qubit', Phys. Rev. Lett. 133, 160602 (2024), DOI: 10.1103/PhysRevLett.133.160602, https://arxiv.org/abs/2401.12183
  15. D. P. Pappas, M. Field, C. J. Kopas, J. A. Howard, X. Wang, E. Lachman, J. Oh, L. Zhou, A. Gold, G. M. Stiehl, K. Yadavalli, E. A. Sete, A. Bestwick, M. J. Kramer, J. Y. Mutus, 'Alternating-bias assisted annealing of amorphous oxide tunnel junctions', Communications Materials 5, 150 (2024), DOI: 10.1038/s43246-024-00596-z, https://arxiv.org/abs/2401.07415
  16. M. Chen, J. C. Owens, H. Putterman, M. Schäfer, O. Painter, 'Phonon engineering of atomic-scale defects in superconducting quantum circuits', Science Advances 10, eado6240 (2024), DOI: 10.1126/sciadv.ado6240, https://arxiv.org/abs/2310.03929
  17. O. Mansikkamäki, A. Tyner, A. Bilmes, I. Drozdov, A. Balatsky, 'Two-tone spectroscopy for the detection of two-level systems in superconducting qubits' (2024), arXiv:2404.14039, https://arxiv.org/abs/2404.14039
  18. S. Zhu, X. You, U. Alyanak, M. Bal, F. Crisa, S. Garattoni, A. Lunin, R. Pilipenko, A. Murthy, A. Romanenko, A. Grassellino, 'Disentangling the impact of quasiparticles and two-level systems on the statistics of superconducting-qubit lifetime', Phys. Rev. Applied 24, 034002 (2025), DOI: 10.1103/x4ny-17lk, https://arxiv.org/abs/2409.09926
  19. Y. Reiss, M. Schechter, 'Thermal cycling: Evidence for a generalized tunneling model and a tool to distinguish noise sources in quantum circuits', Phys. Rev. B 111, L201403 (2025), DOI: 10.1103/PhysRevB.111.L201403, https://arxiv.org/abs/2410.19930
  20. J. Lisenfeld, A. K. Händel, E. Daum, B. Berlitz, A. Bilmes, A. V. Ustinov, 'Mapping the positions of Two-Level-Systems on the surface of a superconducting transmon qubit' (2025), arXiv:2511.05365, https://arxiv.org/abs/2511.05365
  21. S. Weeden, D. C. Harrison, S. Patel, M. Snyder, E. J. Blackwell, G. Spahn, S. Abdullah, Y. Takeda, B. L. T. Plourde, J. M. Martinis, R. McDermott, 'Statistics of Strongly Coupled Defects in Superconducting Qubits' (2025), arXiv:2506.00193, https://arxiv.org/abs/2506.00193
  22. M. P. Bland, F. Bahrami, J. G. C. Martinez, P. H. Prestegaard, B. M. Smitham, A. Joshi, E. Hedrick, A. Pakpour-Tabrizi, S. Kumar, A. Jindal, R. D. Chang, A. Yang, G. Cheng, N. Yao, R. J. Cava, N. P. de Leon, A. A. Houck, '2D transmons with lifetimes and coherence times exceeding 1 millisecond' (2025), arXiv:2503.14798, https://arxiv.org/abs/2503.14798
  23. O. F. Wolff, H. Mantry, R. Raja, W.-H. Peng, K. Singirikonda, S. Lee, S. Sudhaman, R. Goncalves, P. Y. Huang, A. Kou, W. Pfaff, 'Structural control of two-level defect density revealed by high-throughput correlative measurements of Josephson junctions' (2026), arXiv:2602.11469, https://arxiv.org/abs/2602.11469
  24. F. Berritta, D. Pahl, L. Pahl, W. P. Banner, G. Cutter, J. A. Krzywda, S. Weeden, S. Patel, P. Buttles, S. Eilhart, M. Gingras, B. M. Niedzielski, R. McDermott, M. E. Schwartz, K. Serniak, M. Hays, J. A. Grover, W. D. Oliver, 'Adaptive Spectroscopy of Fast Two-Level-System Dynamics in Superconducting Qubits' (2026), arXiv:2608.02086, https://arxiv.org/abs/2608.02086
  25. A. Agarwal, M. Uddin, S. Vaidya, L. P. Lindoy, E. Daghigh Ahmadi, T. Lindström, S. E. de Graaf, I. Rungger, 'Long-lived memory effects in the defect bath of superconducting qubits' (2026), arXiv:2608.10087, https://arxiv.org/abs/2608.10087