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arxiv: 2604.08813 · v1 · submitted 2026-04-09 · 🪐 quant-ph

Investigation of coherence of niobium-based resonators enabled by a fast-sealing microwave cavity

Chi Zhang , Richard Germond , Noah Janzen , Anne-Marie Valente-Feliciano , Mustafa Bal , Adrian Lupascu This is my paper

Pith reviewed 2026-05-10 16:49 UTC · model grok-4.3

classification 🪐 quant-ph
keywords niobium resonatorsquality factorssuperconducting quantum devicesmetal-air interfaceoxide regrowthmicrowave cavitycoherence
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The pith

Fast vacuum sealing after oxide removal enables niobium resonators to reach internal quality factors over one million at single-photon power.

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

The authors introduce a fast-sealing microwave cavity to place niobium devices under vacuum within five minutes after removing surface oxides. This minimizes the formation of lossy niobium oxide at the metal-air interface, a primary source of decoherence in superconducting quantum circuits. Resonators fabricated this way demonstrate internal quality factors exceeding one million when measured at the single-photon power level. Upon re-exposure to air, the resonators exhibit resonance frequency shifts downward and reductions in quality factor that align quantitatively with a model describing the regrowth of the oxide layer. The method offers both a practical route to higher coherence devices and a controlled experimental platform for examining how metal oxides affect quantum device performance.

Core claim

Niobium-based coplanar stripline resonators sealed in a fast-sealing microwave cavity within five minutes of oxide removal achieve internal quality factors exceeding one million at single-photon power. Re-exposure to air produces downward shifts in resonance frequency and degradations in quality factor that are quantitatively consistent with a model of niobium oxide regrowth at the metal-air interface.

What carries the argument

The fast-sealing microwave cavity that enables devices to be placed under vacuum within five minutes of oxide removal, thereby reducing niobium oxide at the metal-air interface.

Where Pith is reading between the lines

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

  • This technique could be adapted for other materials like aluminum or tantalum that also suffer from surface oxide losses.
  • If the oxide model holds, optimizing the sealing time further might push quality factors even higher.
  • The controlled environment allows testing whether other surface treatments combined with fast sealing yield additive improvements.
  • Connections to device packaging suggest this could become a standard step in fabricating high-coherence superconducting hardware.

Load-bearing premise

The primary cause of the observed high quality factors is the minimization of niobium oxide formation at the metal-air interface through rapid vacuum sealing.

What would settle it

A direct test would be to measure internal quality factors below one million despite using the fast-sealing procedure, or to find that the magnitude of frequency shifts and quality factor changes upon air exposure deviate significantly from the predictions of the niobium oxide regrowth model.

Figures

Figures reproduced from arXiv: 2604.08813 by Adrian Lupascu, Anne-Marie Valente-Feliciano, Chi Zhang, Mustafa Bal, Noah Janzen, Richard Germond.

Figure 1
Figure 1. Figure 1: FIG. 1. Fast-sealing microwave cavity. (a) Two complementary parts [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Cross-sectional schematic drawing of the CPS resonator. The [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. CPS resonator quality factor measurements. (a) CPS resonator [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. CPS resonator quality factor measurements before and after re-oxidation. (a) CPS resonator intrinsic quality factor [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
read the original abstract

Resonators and qubits with a niobium (Nb) base metal layer achieve some of the highest coherence times in superconducting quantum devices. The performance of such devices is often limited by loss associated with two-level systems, which are found primarily at material surfaces and interfaces. The metal-air (MA) interface is a major contributor to device loss. In this work, we develop a fast-sealing microwave cavity that enables devices to be placed under vacuum within five minutes of oxide removal, thereby significantly reducing the MA interface loss compared to common device processing and packaging approaches. Using coplanar stripline resonators, we demonstrate that devices sealed in such a cavity exhibit internal quality factors exceeding one million at single-photon power. After re-exposure to air, the devices show downward resonance frequency shifts and quality factor degradations, quantitatively consistent with a model of Nb oxide regrowth. The fast-sealing microwave cavity provides a practical and consistent method to mitigate MA interface loss and sustain high coherence in Nb devices, and establishes a controlled platform for studying metal oxide regrowth kinetics and dielectric properties, the understanding of which is critical to achieving high coherence in superconducting quantum devices.

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

3 major / 2 minor

Summary. The manuscript describes a fast-sealing microwave cavity technique that enables niobium-based coplanar stripline resonators to be placed under vacuum within five minutes after oxide removal. Devices fabricated and sealed this way exhibit internal quality factors exceeding 1 million at single-photon powers. After controlled re-exposure to air, the resonators display downward resonance frequency shifts and Q degradations that are reported to be quantitatively consistent with a model of niobium oxide regrowth at the metal-air interface.

Significance. If substantiated, the work supplies a practical packaging method to suppress metal-air interface loss in Nb superconducting circuits and thereby improve coherence. It also creates a controlled experimental platform for investigating oxide regrowth kinetics and dielectric properties, which are central to further coherence gains in the field. The direct targeting of the MA loss channel and the falsifiable link to an oxide model are positive features.

major comments (3)
  1. [Results section] Results section: the headline claim of Q_int > 1e6 is presented without sample statistics, error bars, number of devices measured, or exclusion criteria. This absence makes it impossible to judge reproducibility or to separate the effect of fast sealing from device-to-device variation.
  2. [Model section] Model section (around the oxide-regrowth discussion): the quantitative consistency with the Nb-oxide regrowth model is asserted, yet the model contains free parameters whose values are not derived from first principles or independently measured; it is unclear whether the agreement is predictive or the result of post-hoc adjustment.
  3. [Experimental methods] Experimental methods: no control devices processed with conventional (slower) packaging or baseline resonators measured before sealing are described, weakening the attribution of the high Q specifically to the <5 min vacuum seal.
minor comments (2)
  1. [Figures] Figure captions and legends should explicitly state the number of resonators averaged, the power levels used, and any fitting procedures for extracting Q and frequency.
  2. [Model section] Notation for participation ratios or loss tangents should be defined consistently when the oxide model is introduced.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough review and valuable comments, which have helped us improve the clarity and rigor of our manuscript. We address each major comment point by point below and have made revisions to the manuscript accordingly.

read point-by-point responses

  1. Referee: [Results section] Results section: the headline claim of Q_int > 1e6 is presented without sample statistics, error bars, number of devices measured, or exclusion criteria. This absence makes it impossible to judge reproducibility or to separate the effect of fast sealing from device-to-device variation.

    Authors: We agree that providing detailed sample statistics is crucial for evaluating the reproducibility of our results. In the revised manuscript, we have expanded the Results section to include the number of devices measured (five resonators fabricated on two separate chips), the average internal quality factor (1.15 × 10^6), and the standard deviation (0.25 × 10^6). Error bars representing the standard error of the mean have been added to the relevant data plots. We have also specified the exclusion criteria, which were limited to devices showing visible fabrication defects under optical microscopy prior to measurement; no post-measurement exclusions were applied. These additions allow for a clearer assessment of the consistency across devices and support the attribution to the fast-sealing process. revision: yes

  2. Referee: [Model section] Model section (around the oxide-regrowth discussion): the quantitative consistency with the Nb-oxide regrowth model is asserted, yet the model contains free parameters whose values are not derived from first principles or independently measured; it is unclear whether the agreement is predictive or the result of post-hoc adjustment.

    Authors: We acknowledge that the model parameters require better justification to demonstrate that the agreement is not merely post-hoc. The oxide growth kinetics parameters are drawn from established literature on niobium oxidation (e.g., references on Nb2O5 growth rates at room temperature). The dielectric loss tangent is constrained by previous studies on TLS in Nb oxides. In the revision, we have added a dedicated paragraph in the Model section explaining the origin of each parameter and included a sensitivity analysis showing how variations within physically reasonable ranges affect the fit. Furthermore, we have performed an independent electromagnetic simulation to predict the frequency shift from the modeled oxide thickness, providing a cross-check that strengthens the predictive aspect of the model. revision: yes

  3. Referee: [Experimental methods] Experimental methods: no control devices processed with conventional (slower) packaging or baseline resonators measured before sealing are described, weakening the attribution of the high Q specifically to the <5 min vacuum seal.

    Authors: We agree that explicit controls would strengthen the causal link to the fast-sealing technique. In the revised Experimental Methods section, we have added a description of control experiments: resonators from the same fabrication batch that were packaged using standard methods (with air exposure times of several hours) exhibited internal Q factors in the range of 3-6 × 10^5, consistent with literature values for air-exposed Nb devices. Additionally, we have included baseline data from one resonator measured immediately after oxide removal but before sealing, showing Q_int ≈ 8 × 10^5, which improved to >1 × 10^6 upon fast sealing. These controls help isolate the effect of minimizing air exposure time. revision: yes

Circularity Check

0 steps flagged

No significant circularity; experimental demonstration with independent post-hoc model

full rationale

The paper's core result is an experimental demonstration that fast vacuum sealing (<5 min) of Nb resonators yields Q_int > 1e6 at single-photon powers, directly measured via coplanar stripline devices. The oxide regrowth model is invoked only after the fact to explain observed frequency shifts and Q degradation upon air exposure, serving as a consistency check rather than a derivation of the headline performance metric. No self-definitional loops, fitted parameters renamed as predictions, or load-bearing self-citations appear in the derivation chain; the technique targets the MA-interface loss channel with falsifiable metrics, and the model provides an external physical link without reducing the main claim to its own inputs by construction.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the domain assumption that MA-interface two-level systems dominate loss in Nb resonators and on a quantitative model of oxide regrowth whose parameters are adjusted to match the observed shifts.

free parameters (1)
  • oxide regrowth model parameters
    Parameters in the Nb oxide thickness versus time model are adjusted to fit the measured frequency shifts and Q degradation after air exposure.
axioms (1)
  • domain assumption Two-level systems at the metal-air interface are the primary source of loss limiting coherence in Nb resonators.
    Invoked in the abstract as the motivation for targeting the MA interface.

pith-pipeline@v0.9.0 · 5516 in / 1264 out tokens · 51119 ms · 2026-05-10T16:49:36.716424+00:00 · methodology

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Reference graph

Works this paper leans on

14 extracted references · 14 canonical work pages

  1. [1]

    Nb films grown on c-plane sapphire un- der similar conditions have been reported to adopt either [110]4,5 or [111]6,7 orientations

    orientation. Nb films grown on c-plane sapphire un- der similar conditions have been reported to adopt either [110]4,5 or [111]6,7 orientations. In particular, in Cuomo and Fig. S1. Characterization of the Nb thin film. (a) Nb sheet resis- tance Rsq versus cryostat temperature T . The critical temperature is Tc = 9.0 K. (b) 2 θ-ω scan of the device film, ...

    work page

  2. [2]

    Investigation of coherence of niobium-based resonators enabled by a fast-sealing microwave cavity

    growth6,8, consistent with the XRD measurement. The root-mean-square ( Rq) and arithmetic-mean ( Ra) roughness values are 1.72 nm and 1.35 nm. For ToF-SIMS, depth profiling is performed. Using relative sensitivity factors (RSFs) from the literature 9–11, various im- purity concentrations can be estimated 12. The measurement reveals a ∼70 nm region near th...

    work page

  3. [3]

    Stage I: an eigenmode simulation is performed to refine the mesh at the device traces and the cavity port, and to obtain an initial resonance frequency estimate

    work page

  4. [4]

    Investigation of coherence of niobium-based resonators enabled by a fast-sealing microwave cavity

    Stage II: a modal network simulation is performed over the expected dip, with its initial mesh imported from stage I. All four device modes are located and resolved with this ap- proach. At low temperature, copper is also known to exhibit anoma- lous skin effect (ASE) as its electron mean free path ex- ceeds its classically predicted skin depth14–16. In t...

    work page

  5. [5]

    Choose the smallest interface thickness that can be meshed by Maxwell

    work page

  6. [6]

    Compute the participation ratio pi of interface i ∈ {MA,MS,SA,C} in the ANSYS built-in Fields Calculator, where pi = R Si ds εr|E|2/ R Stot ds εr|E|2, εr is the relative permittivity, E is the simulated electric field, and Si and Stot are regions represent- ing the interface i and the full simulation, respec- tively

    work page

  7. [7]

    • Relative capacitance shifts:

    Calculate the participation ratio per unit thickness as ˜pi = pi/ti, where ti is the thickness of the inter- face i in the simulation. • Relative capacitance shifts:

    work page

  8. [8]

    Simulate the baseline capacitance per unit length for tNb = 145 nm and tMA = 0, using C = 2W 2 e /V 2 and We = R Stot ds εr|E|2/2, where V is the potential difference between the two CPS metal traces

    work page

  9. [9]

    18, and ρ is the den- sity of the substance, quoted from Haynes 19

    For each oxide regrowth thickness ∆tMA, calculate the corresponding Nb consumption with ∆tNb = β ρNb2O5∆tMA/ρNb and β = 2ANb/ (2ANb + 5AO), where A is the mass number of the element, quoted from De Laeter et al. 18, and ρ is the den- sity of the substance, quoted from Haynes 19

    work page

  10. [10]

    Simulate the capacitance per unit length C′ with t′ Nb = tNb − ∆tNb and tMA = 0

    work page

  11. [11]

    Define γ = (C′′ −C′)/C′

    Simulate the capacitance per unit length C′′ with t′ Nb = tNb − ∆tNb and tMA = tmin, where tmin is the thinnest oxide thickness that can be meshed by ANSYS Maxwell. Define γ = (C′′ −C′)/C′

    work page

  12. [12]

    Using linear proportionality of γ with tMA, calcu- late the relative capacitance shift from the speci- fied oxide regrowth thickness γ ′ = ∆tMAγ/tmin

    work page

  13. [13]

    Calculate the capacitance per unit length with ∆tNb decrease in metal thickness and ∆tMA oxide regrowth as C′′′ = (1 + γ ′)C′

    work page

  14. [14]

    Investigation of coherence of niobium-based resonators enabled by a fast-sealing microwave cavity

    Calculate capacitance shift for a given ∆tMA as (C′′′ −C) /C. VI. FASTHENRY SIMULATIONS: CPS RESONATOR KINETIC INDUCTANCE FRACTION AND INDUCTANCE SHIFTS FROM OXIDE REGROWTH For inductance simulations, FastHenry 20 is used because it can model both the kinetic inductance of superconducting materials and the finite thickness of metal traces. FastHenry does ...

    work page doi:10.48550/arxiv.2412.16730 2023