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REVIEW 4 major objections 6 minor 54 references

Stability studies on subtractively-fabricated CMOS-compatible superconducting transmon qubits

T0 review · 4 major / 6 minor · reviewed 2026-08-03 · deepseek-v4-flash

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Pith's one-line read Subtractively fabricated CMOS-compatible transmon qubits show temporal stability on par with lift-off devices, with all T1 fluctuations following a single scaling law.

desk verdict Useful benchmark data for CMOS-compatible qubits; 'on par' is credible, but the universal scaling law depends on fragile outlier handling. read the letter →

arxiv 2512.18037 v2 pith:JAUDRRVT submitted 2025-12-19 quant-ph cond-mat.supr-con

Chawki Dhieb , Johannes Weber , Samuel Taubenberger , Carla Moran Guizan , Simon J. K. Lang , Zhen Luo , Emir Music , Alwin Maiwald
show 7 more authors
Wilfried Lerch Lars Nebrich Marc Tornow Thomas Mayer Daniela Zahn Rui N. Pereira Christoph Kutter
This is my paper
classification quant-phcond-mat.supr-con
keywords superconductingqubitstransmonCMOS-compatiblefabricationsubtractivetemporalstabilityT1fluctuationstwo-levelsystemsqubitaging
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Superconducting qubits must keep their coherence times and control fidelities stable over hours and across many thermal cycles before large error-corrected processors become practical. This paper argues that transmon qubits made with a subtractive, CMOS-compatible fabrication process—where metal is etched away rather than lifted off—are just as temporally stable as qubits made with conventional lift-off methods. Over 95 hours in a single cooldown, eight qubits showed T1 and T2* fluctuations driven mainly by two-level-system defects, and the fluctuations follow the predicted scaling σT1 = a⟨T1⟩^{3/2} with one common factor a across all fabrication methods. Over ten cooldowns spanning more than a year, two tracked qubits kept a stable T1 baseline while their frequencies drifted downward by about 61 MHz, which the paper attributes to slow aging of the Josephson junction barrier. The upshot: the industrial-grade fabrication route does not appear to cost qubit stability, a key requirement for scaling up.

What carries the argument

The central object is the transmon qubit and its two-level-system (TLS) environment. The load-bearing identity is the scaling relation σT1 = a⟨T1⟩^{3/2}, derived from an ensemble of independent TLS defects each contributing to the decay rate; variance adds in quadrature, giving standard deviation proportional to ⟨T1⟩^{3/2}. To extract σT1 and ⟨T1⟩ from skewed histograms, the paper uses an empirical mirrored-Rician fit; for long-term aging it uses the relation fq ≈ (1/h)√(8EJEC) − EC/h and EJ ∝ 1/RN to infer junction resistance drift from frequency shifts.

What would settle it

Recompute σT1 and ⟨T1⟩ from the full time traces without excluding any dropout events, using a physically motivated model for the skewed distribution; if one constant a no longer fits both subtractive and lift-off devices, the universality claim fails.

Watch

Extended reading notes

Core claim

According to the paper, subtractively fabricated transmon qubits—made by etching rather than lift-off—show the same degree of short-term T1 stability as lift-off qubits: over 95 hours in one cooldown, T1 and T2* fluctuate because two-level-system defects cross the qubit frequency, but the fluctuations follow the same σT1 = a⟨T1⟩^{3/2} curve with a single constant a for all three fabrication approaches surveyed. Long-term, two qubits tracked over 10 cooldowns and more than a year keep a stable T1 baseline, while qubit frequencies drift downward by about 61 MHz on average, attributed to a slow increase in junction resistance RN; readout resonators shift much less.

Load-bearing premise

The 'on par with lift-off' and single-a conclusions depend on excluding dropout events and fitting T1 histograms with an empirical skewed distribution; if those choices are applied differently across datasets, the fitted proportionality factor could be an artifact of selection rather than a universal property.

Editorial extensions

If this is right

  • Qubit arrays made by subtractive CMOS fabrication can be used in error-correction contexts without an extra stability penalty relative to lift-off devices.
  • Because σT1 grows as ⟨T1⟩^{3/2}, pushing qubit lifetimes higher makes T1 fluctuations larger; a QPU must budget for this.
  • Over many cooldowns, qubit frequencies drift downward by tens of MHz as junction resistance creeps up, so long-lived systems need recalibration or drift compensation.
  • Readout resonators move much less than qubit frequencies, so the dominant long-term correction is on the qubit drive frequency.
  • The observation that a single proportionality factor fits all fabrication methods points to TLS defects, not fabrication details, as the limiting source of short-term instability.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A direct test of the aging story would be to measure room-temperature junction resistance alongside each cooldown; the paper infers RN from frequency shifts, and such a measurement would settle whether the barrier-thickening model is correct.
  • If the universal scaling law holds generally, it implies that fabrication-method comparisons of T1 stability should be normalized by ⟨T1⟩, not compared raw.
  • The same TLS-ensemble variance argument predicts similar scaling for pure dephasing or correlated T2* fluctuations; extending the analysis to T2* would test the model further.
  • The exclusion of dropout events is a modeling choice; a mechanistic model that includes strong TLS interactions might explain the drops and remove the need for empirical Rician fits.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 6 minor

Summary. The paper reports a temporal-stability study of eight transmon qubits fabricated with a subtractive, CMOS-compatible process, monitored continuously for ~95 h in a single cooldown (T1, T2*, Ramsey frequency, readout fidelity, Δm, Teff), plus a multi-cooldown study of two qubits over 10 thermal cycles spanning more than a year. The central claims are: (i) T1/T2* fluctuations are dominated by TLS-qubit interactions and follow the scaling σT1 = a⟨T1⟩^{3/2} (Eq. 1) with a single proportionality factor a for both the authors' subtractive devices and literature lift-off devices; (ii) subtractively fabricated devices are therefore stable 'on par' with lift-off devices; and (iii) long-term frequency shifts are dominated by an increase in the junction normal-state resistance RN, estimated indirectly at ΔRN/RN below 3.4% over 400 days. The analysis uses a modified (mirrored, offset) Rician fit to T1 histograms, with moments computed by integration of the fitted density; the fit is explicitly acknowledged as purely empirical.

Significance. If the reported claims hold, the practical significance is real: a wafer-scale-compatible subtractive process producing qubits whose short- and long-term stability matches lift-off devices is an important data point for scalable QPUs, and the year-long, ten-cooldown tracking of fq, fr, and T1 is a useful community dataset. The paper's strengths include the detailed and transparent experimental description, the continuous 95-hour multi-parameter monitoring of 8 qubits, the explicit acknowledgment that the Rician fit is empirical (Fig. 3 caption; S.I. II.B), the inclusion of literature-comparison datasets, and a supplementary section with complete pulse sequences and a derivation of the scaling law. The verification of the σT1 ∝ ⟨T1⟩^{3/2} law across fabrication methods is, however, a fit with a free prefactor rather than a parameter-free prediction; the statistical support for a universal prefactor is not yet demonstrated, and the long-term RN aging inference is indirect. The significance is therefore conditional on the robustness checks requested below.

major comments (4)
  1. [S.I. II.B (Eq. S9); §III A 'T1 stability'] The statement that computing moments by integration of the fitted Rician density 'avoid[s] the influence of extreme and prolonged outliers (dropouts)' is not supported. Eq. S9 is fitted to the full histogram, which contains the dropout tail; the fit parameters (ν, σ, Ti,max) are therefore pulled by the same outlier points the authors intend to exclude, and the integrated ⟨T1⟩ and σT1 inherit that pull. Because dropout frequency/severity varies strongly across qubits (B.1 vs A.2 in Fig. 4), the σT1 values entering Fig. 5 are differentially censored. Please quantify the sensitivity: e.g., recompute moments after explicit censoring of points below a defined threshold, compare with the fit-based moments, and report how the fitted a changes.
  2. [§III A, Eq. (1), Fig. 5] The universal-scaling claim is under-supported statistically. The exponent 3/2 is fixed a priori and only the prefactor a is fitted; no goodness-of-fit statistic is reported, no free-exponent fit is shown, and no test is given of whether EMFT and literature qubits are consistent with a common a. This matters because the S.I. II.C.2 derivation (Eqs. S14–S15) leaves a dependent on the single-TLS decay-rate statistics, so a universal a is not a model prediction, and because Eq. S13 (delta-method) is questionable in the presence of the large fluctuations observed. Please report fit quality, residuals, a free-exponent fit, and an explicit common-a vs separate-a comparison; and soften 'confirm' (§IV) accordingly.
  3. [§III A 'T1 stability'; dataset inclusion criteria] The comparability of the literature and EMFT datasets is not established. The inclusion criteria (>500 points, >10 h, single cooldown) do not control for measurement duration, sampling cadence, or histogram binning. Since dropout probability grows with observation time and the fit-based σT1 is sensitive to dropout contamination (previous comment), longer datasets may have systematically inflated σT1. The single-a collapse in Fig. 5 could then reflect disparate censoring rather than a universal TLS limit. Please demonstrate robustness to duration/cadence (e.g., restrict all datasets to a common 10-h window) or include these as covariates.
  4. [§III B; Eq. (3)] The long-term aging result ΔRN/RN < 3.4% is an indirect estimate: RN is never measured, but reconstructed from fq shifts via Eq. (3), with Δ taken from Tc and EC from the design anharmonicity. The comparison with lift-off aging rates therefore inherits these model assumptions, and the 25%/75% split in §III B between RN-mediated and bare-resonator contributions to Δfr is stated without uncertainty. The conclusion that subtractive junctions age more slowly should be supported by direct RN measurements (or a sensitivity analysis) or presented as a preliminary inference. The authors partially acknowledge this in §III B, which mitigates the concern.
minor comments (6)
  1. [Fig. 5 caption] The units of a are given as s^{2/3}, but Eq. (1) requires [a] = s^{-1/2}. Please correct the units.
  2. [Abstract vs §III B] The abstract reports an average total downward shift of approximately 61 MHz, while §III B states 50–70 MHz; please reconcile the numbers.
  3. [§IV] The phrase 'confirm that the TLS-qubit interactions remain the limiting factor' is stronger than the evidence supports, given the empirical fit and free prefactor; recommend 'are consistent with'.
  4. [Acknowledgments; S.I. II.C.1] Typos: 'form Zurich Instruments' should read 'from Zurich Instruments'; 'which occurs When certain numbers' has an erroneous capital W.
  5. [Fig. 4(d)] The mixed logarithmic/linear axis (log scale with a linear interval ±0.1 around zero) is hard to read; please clarify the scale convention in the caption.
  6. [References [11], [44]] Quantitative claims citing an annual report and a PhD thesis should indicate where in those documents the data appear, or prefer peer-reviewed sources.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the central benchmark is an external comparison and the scaling-law prefactor is openly fitted.

full rationale

I find no step where a claimed prediction or first-principles result reduces to its own inputs by construction. The central stability benchmark compares sigma_T1 vs. <T1> across the authors' subtractive devices and external lift-off datasets. Eq. 1 is presented as a theory-motivated scaling law whose proportionality factor a is explicitly fitted (Fig. 5: "fit following Eq. 1 which gives a = (1.220 +/- 0.052) x 10^-2 s^(2/3)"), so the agreement is a fit, not a parameter-free prediction; a free fit is circular only if the fitted value is then renamed as an independent prediction, which is not done here. The 3/2 exponent is re-derived in SI S14-S15 from the external TLS model of You et al. using variance propagation; that algebraic derivation is not fitted to the data. The Rician distribution is explicitly acknowledged as empirical and non-physical ("The choice of the fit function in Eq. S9 is purely empirical and is not based on an established physical model"), so no ansatz is smuggled in as a first-principles input. The dropout handling is an estimator-robustness choice; even if it biases the benchmark, that is a statistical concern, not a definitional equivalence. The long-term aging analysis infers R_N from f_q through Eq. 3, but the paper is transparent that it "calculate[s] it based on our values of f_q"; this is an indirect measurement, not a hidden reduction used to predict the observed f_q from independently measured R_N. Self-citations support fabrication background and are not load-bearing for the stability conclusion, which is benchmarked against external datasets. Therefore the derivation chain is self-contained in the relevant circularity sense.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The central claims rest on the TLS ensemble model from prior work and on the assumption that the observed frequency drift is caused by junction resistance aging. The only genuinely free parameter in the scaling test is the global factor a, which is fitted to the data. No new physical entities are introduced.

free parameters (2)
  • a (proportionality factor in σ_T1 = a·⟨T1⟩^{3/2}) = (1.220±0.052)×10^{-2} s^{2/3} (paper; dimensionally likely s^{-1/2})
    Global scaling constant fitted to combined T1 fluctuation data across all fabrication methods (Fig. 5, Eq. 1). This is a free parameter used to test the predicted scaling.
  • Modified Rician fit parameters (ν, σ, T_i,max) per device = Not reported explicitly
    Fitted to each T1/T2* histogram (Eq. S9) and used to compute the mean and standard deviation that define the plotted data points. These are empirical fit parameters, not physical constants.
assumptions (5)
  • domain assumption TLS ensemble model: ⟨Γ1,tot⟩ = N_TLS ⟨Γ1,single⟩ and Var(Γ1,tot) = N_TLS Var(Γ1,single) (SI S10–S11)
    Imported from You et al. [30]. Assumes TLS defects are independent and dominate relaxation; the scaling law follows from this model.
  • standard math Delta-method approximation Var(T1) ≈ Var(Γ1)/⟨Γ1⟩^4 (SI S13)
    Standard first-order propagation of uncertainty, valid only for small relative fluctuations; this is used to derive the 3/2 power law.
  • domain assumption TLS defects are the dominant source of energy relaxation
    Cites [30] and earlier TLS studies. If other mechanisms (e.g., quasiparticles) contribute comparably, the scaling interpretation weakens.
  • domain assumption Long-term qubit frequency decrease is primarily due to increasing junction resistance RN (Eq. 3)
    The paper infers ΔRN/RN from f_q measurements using transmon frequency formula and Ambegaokar-Baratoff relation. This attributes the drift to barrier aging, with alternative mechanisms mentioned but not directly ruled out.
  • ad hoc to paper Modified Rician distribution is an adequate empirical fit for T1/T2* histograms
    Acknowledged in SI as purely empirical, not derived from a physical model. The mean/σ values used in Fig. 5 depend on this choice.

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Cite this review

Pith. "Pith review of Stability studies on subtractively-fabricated CMOS-compatible superconducting transmon qubits." pith.science (2026). https://pith.science/paper/JAUDRRVT

@misc{pith2026251218037,
  author       = {Pith},
  title        = {Pith review of: Stability studies on subtractively-fabricated CMOS-compatible superconducting transmon qubits},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JAUDRRVT}},
  note         = {Machine review of arXiv:2512.18037}
}
abstract

Developing fault-tolerant quantum processors with error correction demands large arrays of physical qubits whose key performance metrics (coherence times, control fidelities) must remain within specifications over both short and long timescales. Here we investigated the temporal stability of subtractively fabricated CMOS-compatible superconducting transmon qubits. During a single cooldown and over a period of 95 hours, we monitored several parameters for 8 qubits, including coherence times $T_1$ and $T_2^*$, which exhibit fluctuations originating primarily from the interaction between two-level system (TLS) defects and the host qubit. We also demonstrate that subtractively-fabricated superconducting quantum devices align with the theoretical predictions that higher mean lifetimes $T_1$ correspond to larger fluctuations. To assess long-term stability, we tracked two representative qubits over 10 cooldown cycles spanning more than one year. We observed an average total downward shift in both qubit transition frequencies of approximately 61 MHz within the thermal cycles considered. In contrast, readout resonator frequencies decreased only marginally. Meanwhile, $T_1$ exhibits fluctuations from cycle to cycle, but maintains a stable baseline value.

Figures

Figures reproduced from arXiv: 2512.18037 by the authors.

Figure 1
Figure 1. Sketch of a single-qubits chip layout with 4 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Illustration of the cryogenic measurement [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗
Figure 3
Figure 3. Results of 95-hour single-cooldown measurements for qubit A.2, with time traces on the left and [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: Temporal evolution of a) T1(t)/T1(0), b) T ∗ 2 (t)/T ∗ 2 (0), c) |fRamsey|−∆f , d) ∆m(t)−∆m(0) ∆m(0) , and e) readout fidelity F for all qubits (except Ramsey results of A.1 and ∆m results for B.3). Panel (d) uses a logarithmic scale on the y-axis with a linear interva…
Figure 6
Figure 6. Figure 6: Evolution over multiple cooldown cycles of a) [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]

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    2019 arXiv

Pith tools

Reviewed August 3, 2026 · model on record in the stance chip above.