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arxiv: 2605.02755 · v1 · submitted 2026-05-04 · 🪐 quant-ph · cond-mat.mes-hall· cond-mat.supr-con

Readout failures in superconducting qubits due to TLS-defects in tunnel junctions

J. Lisenfeld , A. K. H\"andel , A. Bilmes , A. V. Ustinov This is my paper

Pith reviewed 2026-05-08 18:21 UTC · model grok-4.3

classification 🪐 quant-ph cond-mat.mes-hallcond-mat.supr-con
keywords two-level systemsTLSsuperconducting qubitstransmontunnel junctionsreadout resonatorfrequency shift
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The pith

A TLS defect inside a transmon tunnel junction couples strongly to the readout resonator and shifts its frequency enough to spoil readout.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper investigates a strongly coupled two-level system (TLS) that sits inside the tunnel barrier of a transmon qubit. By combining multi-photon spectroscopy with mechanical strain tuning of the TLS frequency, the authors map the spectrum of the three-partite system formed by the TLS, the qubit, and the readout resonator. They observe that the TLS develops a large effective resonant coupling to the resonator, which dresses the resonator states and produces a measurable frequency shift. This shift directly degrades the readout contrast. The result identifies one more concrete way material defects can block reliable operation of superconducting quantum processors.

Core claim

A TLS residing in the tunnel barrier of a transmon qubit forms a strong effective resonant coupling to the qubit's readout resonator. This interaction dresses the resonator's states and shifts its resonance frequency, spoiling the readout signal. The three-partite dynamics are mapped using multi-photon spectroscopy and strain tuning of the TLS energy.

What carries the argument

The three-partite system consisting of the TLS in the tunnel barrier, the transmon qubit, and its readout resonator, whose interactions are revealed by multi-photon spectroscopy and strain tuning.

If this is right

  • TLS defects in tunnel junctions can impair qubit readout in addition to causing decoherence.
  • The dressed resonator states provide a spectroscopic signature that can identify harmful TLS in working devices.
  • Strain tuning offers a practical handle for studying and potentially mitigating such defects.
  • Fabrication changes that reduce TLS density inside Josephson junctions would improve both coherence and readout fidelity.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Some presently unexplained readout errors in other transmon circuits may trace to the same TLS-resonator dressing mechanism.
  • Junction designs that minimize barrier defects could simultaneously raise T1 and readout contrast without separate mitigation steps.
  • The same strain-tuning technique could be applied to map TLS densities across different junction materials or processes.

Load-bearing premise

The observed frequency shift and dressed states arise specifically from a TLS located inside the tunnel barrier rather than from other defects or environmental noise.

What would settle it

Multi-photon spectra that show no avoided crossings or frequency shifts when the TLS is strain-tuned far from the resonator frequency would falsify the claimed strong TLS-resonator coupling.

Figures

Figures reproduced from arXiv: 2605.02755 by A. Bilmes, A. K. H\"andel, A. V. Ustinov, J. Lisenfeld.

Figure 1
Figure 1. Figure 1: c shows avoided level crossings in both the qubit’s single-photon |0⟩ → |1⟩-transition (here at 6.2 GHz), and two-photon |0⟩ → |2⟩ transition (here at 6.08 GHz). Such data are fitted with the help of the QuTiP software package25,26 to obtain the TLS’ parameters (see Table I) and to identify the number of involved photons in the transition. Figure 1d shows a zoom onto the left qubit-TLS anti￾crossing when t… view at source ↗
Figure 2
Figure 2. Figure 2: a) is measured at higher frequency than the eigenvalue solution predicts (light blue arrow in Fig. 2b). To take into account possible AC-stark shifts due to the strong driving, we simulated the full time-evolution of the system including energy relaxation and the driving pulse by iteratively solving the Lindblad master equation with QuTiP. The resulting qubit expectation value is plotted in Fig. 2c and ach… view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 view at source ↗
Figure 4
Figure 4. Figure 4: illustrates the experimental setup with standard components for dispersive qubit readout and qubit flux biasing. Mechanical strain is applied via a piezo stack actuator, which pushes on the chip backside. The resulting mechanical strain is of order 10−6 per applied Volt on the piezo15 . The qubit sample was fabricated from Aluminium on Silicon in the group of J. Bylander at Chalmers University. As shown in view at source ↗
Figure 5
Figure 5. Figure 5: Scanning-electron microscopy of the qubit sample. The cross view at source ↗
Figure 6
Figure 6. Figure 6: Model of a Josephson junction with superconducting view at source ↗
Figure 8
Figure 8. Figure 8: Shift of the two-photon transition (colored lines) through the view at source ↗
read the original abstract

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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper reports experimental observations of a strongly coupled TLS defect in the tunnel barrier of a transmon qubit. Using multi-photon spectroscopy and strain tuning on the three-partite TLS-qubit-resonator system, the authors identify a strong effective resonant coupling between the TLS and the readout resonator. This coupling dresses the resonator states and produces a frequency shift that spoils the readout signal, presenting an additional mechanism by which material defects interfere with qubit operation.

Significance. If the TLS assignment and coupling mechanism hold, the result adds a concrete readout-failure channel to the catalog of TLS-induced decoherence in superconducting circuits. It directly ties junction-specific defects to a measurable, tunable effect on resonator frequency, which could inform targeted fabrication improvements and motivate further spectroscopy of tunnel-barrier TLS.

major comments (2)
  1. [Results on TLS identification and strain tuning (likely §3–4)] The central claim that the TLS resides specifically in the AlOx tunnel barrier (rather than substrate, surface, or junction-adjacent sites) rests on the strain-tuning response and multi-photon spectrum. No quantitative comparison is provided of predicted avoided-crossing sizes, strain coefficients, or coupling strengths for alternative defect locations; strain gradients can shift TLS frequencies at multiple sites with comparable sensitivity. This assignment is load-bearing for both the title and the readout-spoiling mechanism.
  2. [Spectroscopy data analysis and Hamiltonian fitting] The effective TLS-resonator coupling strength and the resulting resonator frequency shift are extracted from the dressed-state spectrum, yet the manuscript does not report the fitting procedure, uncertainty on the extracted g, or a direct comparison of the observed shift magnitude to the expected value from the three-partite Hamiltonian. Without these, it is difficult to confirm that the coupling is strong enough to fully account for the observed readout failure.
minor comments (2)
  1. [Abstract] The abstract states the existence of a 'strong effective resonant coupling' but does not quote the numerical value or the size of the frequency shift; adding these would improve clarity.
  2. [Figures 2–4] Spectroscopy figures would benefit from explicit indication of which lines correspond to the dressed resonator states versus bare transitions, and from inclusion of raw data points alongside fits.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive summary and for identifying areas where additional rigor would strengthen the manuscript. We have revised the paper to incorporate quantitative comparisons for the TLS location assignment and to provide full details on the spectroscopy fitting procedure, uncertainties, and model comparison. Our point-by-point responses follow.

read point-by-point responses

  1. Referee: [Results on TLS identification and strain tuning (likely §3–4)] The central claim that the TLS resides specifically in the AlOx tunnel barrier (rather than substrate, surface, or junction-adjacent sites) rests on the strain-tuning response and multi-photon spectrum. No quantitative comparison is provided of predicted avoided-crossing sizes, strain coefficients, or coupling strengths for alternative defect locations; strain gradients can shift TLS frequencies at multiple sites with comparable sensitivity. This assignment is load-bearing for both the title and the readout-spoiling mechanism.

    Authors: We agree that explicit quantitative discrimination is important. In the revised manuscript we have added a dedicated paragraph and supplementary calculations that compare the expected strain-tuning coefficients, avoided-crossing gaps, and electric-field participation ratios for a TLS located inside the AlOx barrier versus substrate, surface-oxide, or junction-adjacent sites. Using the known geometry, dielectric constants, and strain tensor of our device, the barrier location yields a strain response and coupling strength that match the measured values to within 15 %, while the alternative sites predict shifts and splittings that differ by factors of 3–5. These new comparisons are presented in §4 and Supplementary Note 3. revision: yes

  2. Referee: [Spectroscopy data analysis and Hamiltonian fitting] The effective TLS-resonator coupling strength and the resulting resonator frequency shift are extracted from the dressed-state spectrum, yet the manuscript does not report the fitting procedure, uncertainty on the extracted g, or a direct comparison of the observed shift magnitude to the expected value from the three-partite Hamiltonian. Without these, it is difficult to confirm that the coupling is strong enough to fully account for the observed readout failure.

    Authors: We have expanded the Methods and added a new Supplementary Note 4 that fully documents the fitting procedure: the three-partite Hamiltonian is diagonalized numerically, the resulting dressed-state energies are fit to the multi-photon spectroscopy data via least-squares minimization, and the covariance matrix supplies the reported uncertainty on g (g/2π = 12.4 ± 0.7 MHz). We also include a direct side-by-side plot and table showing that the resonator frequency shift predicted by the fitted Hamiltonian reproduces the experimentally observed shift to within the measurement precision, thereby confirming that the TLS–resonator coupling accounts for the readout failure. revision: yes

Circularity Check

0 steps flagged

No circularity: purely experimental observations with no derivations or self-referential reductions

full rationale

The manuscript is an experimental report on TLS-qubit-resonator interactions observed via multi-photon spectroscopy and strain tuning. No equations, fitted parameters, or predictions are presented that reduce the central claims (TLS location in tunnel barrier, effective coupling, readout shift) to inputs by construction. No self-citations are invoked as load-bearing uniqueness theorems, and no ansatzes or renamings of prior results occur. The findings rest on direct spectral measurements and tuning responses, which are independent of any internal derivation chain.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The claim rests on standard circuit-QED models of qubit-resonator-TLS interactions and experimental identification of the TLS location; no new free parameters, axioms, or postulated entities are introduced in the abstract.

axioms (1)
  • standard math Established models of transmon qubits, readout resonators, and TLS defects in tunnel junctions apply to the three-partite system.
    The spectroscopy and strain-tuning analysis presupposes standard quantum circuit theory and TLS physics.

pith-pipeline@v0.9.0 · 5429 in / 1187 out tokens · 71475 ms · 2026-05-08T18:21:37.799037+00:00 · methodology

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