arxiv: 2604.09050 · v1 · submitted 2026-04-10 · 🪐 quant-ph · cond-mat.mtrl-sci

Recognition: no theorem link

Tantalum-Encapsulated Niobium Superconducting Resonators: High Internal Quality Factor and Improved Temporal Stability via Surface Passivation

Anas Alkhazaleh , Juan Villegas , Florent Ravaux , Alexey Zharinov

Authors on Pith no claims yet

Pith reviewed 2026-05-10 17:32 UTC · model grok-4.3

classification 🪐 quant-ph cond-mat.mtrl-sci

keywords superconducting resonatorsniobium tantalum bilayerinternal quality factorsurface passivationtwo-level systemstemporal stabilityquantum processors

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The pith

A thin tantalum layer on niobium resonators raises internal quality factor to 2.4 million and improves long-term stability by reducing surface oxide losses.

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

This paper shows that capping niobium films with a thin tantalum layer during fabrication suppresses the formation of lossy niobium oxide on the surface. The resulting devices achieve higher internal quality factors at low powers and maintain better performance after months of storage compared to standard niobium resonators. A reader would care because these resonators limit how well quantum bits can hold information, so better ones could help build more reliable quantum computers. The approach fits into existing manufacturing steps without major changes.

Core claim

The authors establish that in situ tantalum encapsulation of niobium prevents Nb2O5 formation at the metal-air interface and substitutes a Ta-based oxide, leading to reduced two-level system losses. This yields internal quality factors reaching 2.4 × 10^6 in the near-single-photon regime. Ageing tests show only moderate decline after six months, remaining superior to fresh niobium devices fabricated identically.

What carries the argument

The thin tantalum capping layer deposited in situ on the niobium film, which passivates the surface against niobium oxide growth.

If this is right

Resonators with higher internal quality factors allow for longer coherence times in coupled qubits.
Improved temporal stability means devices can be stored or used over longer periods without significant performance loss.
The method integrates with standard niobium fabrication processes, enabling straightforward adoption in quantum device production.
Power dependence of quality factor indicates the loss reduction occurs primarily 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.

Extending this passivation to other niobium-based superconducting components could reduce losses in qubits or amplifiers.
Future work might test these resonators in full quantum processor environments to measure impacts on gate operations.
Comparing with other surface treatments like different metals or cleaning methods could identify optimal passivation strategies.

Load-bearing premise

That the performance gains result from the tantalum layer's effect on the surface oxide rather than variations in fabrication, etching, or testing conditions between the different resonator types.

What would settle it

Fabricating multiple batches of both Nb/Ta and Nb resonators under strictly controlled identical conditions and observing if the quality factor and stability differences consistently appear, or directly imaging the surface oxides to verify the absence of Nb2O5 in capped devices.

Figures

Figures reproduced from arXiv: 2604.09050 by Alexey Zharinov, Anas Alkhazaleh, Florent Ravaux, Juan Villegas.

**Figure 2.** Figure 2: FIG. 2. Fabrication steps of CPW Resonators, (a) substrate cleaning, (b–c) Nb/Ta bilayer sputtering, (d–f) photolithography [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗

**Figure 3.** Figure 3: FIG. 3. SEM images showing the effect of etching-parameter [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

**Figure 5.** Figure 5: FIG. 5. Oblique-view SEM image : sidewall profile and degree [PITH_FULL_IMAGE:figures/full_fig_p004_5.png] view at source ↗

**Figure 7.** Figure 7: FIG. 7. S21 fitting of Resonator R5 at -80 dBm [VNA Power]. [PITH_FULL_IMAGE:figures/full_fig_p005_7.png] view at source ↗

**Figure 6.** Figure 6: FIG. 6. Wiring scheme of the tested chips. [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗

**Figure 8.** Figure 8: FIG. 8. Internal quality factor [PITH_FULL_IMAGE:figures/full_fig_p006_8.png] view at source ↗

**Figure 9.** Figure 9: FIG. 9. Internal quality factor [PITH_FULL_IMAGE:figures/full_fig_p007_9.png] view at source ↗

**Figure 11.** Figure 11: FIG. 11. Effective internal loss 1 [PITH_FULL_IMAGE:figures/full_fig_p008_11.png] view at source ↗

**Figure 12.** Figure 12: FIG. 12. Internal loss decomposition of resonators R0–R7 of Chip A. The effective internal loss 1 [PITH_FULL_IMAGE:figures/full_fig_p009_12.png] view at source ↗

**Figure 14.** Figure 14: FIG. 14. Extracted TLS loss tangent (1 [PITH_FULL_IMAGE:figures/full_fig_p009_14.png] view at source ↗

read the original abstract

Superconducting coplanar waveguide resonators are essential components in quantum processors, where their internal quality factor (Qi) constrains qubit coherence and readout fidelity. In niobium devices, microwave losses at millikelvin temperatures are strongly influenced by two-level systems (TLS) associated with the complex NbOx surface oxide. To mitigate these losses, we investigate a surface-engineering approach in which Nb films are capped in situ with a thin tantalum layer to suppress Nb2O5 formation and replace the native NbOx interface with a Ta-based oxide. We fabricate Nb/Ta bilayer and reference Nb resonators on high-resistivity silicon using identical DC sputtering and wet etching conditions, and characterize their performance at millikelvin temperatures. Fresh Ta-encapsulated devices exhibit internal quality factors up to 2.4 x 10^6 in the near-single-photon regime, with power dependence consistent with reduced TLS-related loss at the metal-air interface. A control Nb device fabricated under the same process shows comparatively lower Q_TLS, consistent with the beneficial effect of the Ta capping layer. Furthermore, ageing tests performed on Nb/Ta resonators after six months reveal a moderate reduction in Q_TLS relative to their initial values, yet the performance remains superior to newly fabricated Nb-only devices. These results suggest that thin Ta encapsulation enhances interface quality and contributes to improved temporal stability while remaining compatible with Nb-based fabrication workflows.

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.

Desk Editor's Note private letter to a colleague

Ta capping on Nb resonators reaches Qi of 2.4e6 and holds up better after six months, but the evidence tying gains specifically to the cap is still loose.

read the letter

The key result is that a thin tantalum cap on niobium resonators can deliver internal quality factors up to 2.4 million and better stability after six months than plain niobium ones. They fabricated the bilayer and reference devices with the same sputtering and etching steps. The cold measurements indicate less TLS loss for the capped version, and the aging data shows it holds an edge over new Nb devices. The practical angle is that this fits into existing Nb workflows without major changes, which is helpful for real hardware builds. Where it is thinner is the causal evidence. Without error bars, stats on multiple devices, or detailed process logs, it's possible other fab variations explain the gains instead of the Ta layer blocking the oxide. That concern from the stress test looks reasonable on the available details. This will interest people doing superconducting resonator and qubit fabrication who want to improve surface quality and aging behavior. Send it for review. The numbers are concrete and the topic timely enough that referees can sort out the methods and ask for more controls if needed.

Referee Report

2 major / 1 minor

Summary. The manuscript investigates tantalum-encapsulated niobium superconducting coplanar waveguide resonators fabricated on high-resistivity silicon. It claims that in-situ Ta capping suppresses Nb2O5 formation at the metal-air interface, replacing it with a Ta-based oxide and thereby reducing TLS losses. Experimental results report fresh Nb/Ta devices reaching internal quality factors up to 2.4 × 10^6 in the near-single-photon regime with power dependence consistent with lower TLS loss, outperforming reference Nb resonators made under identical DC sputtering and wet-etch conditions; six-month aging tests show moderate Q_TLS reduction yet still superior performance to new Nb-only devices.

Significance. If the observed Qi gains and stability improvements are causally attributable to the Ta passivation layer, the work provides a fabrication-compatible route to higher-performance Nb resonators for quantum processors, potentially improving qubit coherence and readout without requiring major process changes.

major comments (2)

[Abstract] Abstract: The central claim of improved performance rests on comparative Qi and Q_TLS values (e.g., 'comparatively lower Q_TLS' for the Nb control and 'moderate reduction' after aging), yet no quantitative values for the Nb reference, error bars, number of devices measured, or statistical details are supplied. This leaves the magnitude and reliability of the reported gains (up to 2.4 × 10^6) difficult to evaluate.
[Abstract] Abstract and fabrication description: The manuscript states that Nb/Ta and Nb devices were fabricated using 'identical DC sputtering and wet etching conditions,' but supplies no quantitative process logs, run-to-run statistics, or explicit controls for potential confounders such as etch depth, surface roughness, or residual gas exposure. Without these, the attribution of TLS reduction and stability gains specifically to suppressed Nb2O5 via the Ta layer (rather than uncontrolled process variations) is not fully supported and is load-bearing for the causal claim.

minor comments (1)

[Abstract] The power-dependence statement ('consistent with reduced TLS-related loss') would benefit from explicit reference to the relevant figure or data set showing the photon-number dependence.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful review and constructive comments on our manuscript. We address each major comment below and have revised the manuscript to improve clarity and support for our claims where possible.

read point-by-point responses

Referee: [Abstract] Abstract: The central claim of improved performance rests on comparative Qi and Q_TLS values (e.g., 'comparatively lower Q_TLS' for the Nb control and 'moderate reduction' after aging), yet no quantitative values for the Nb reference, error bars, number of devices measured, or statistical details are supplied. This leaves the magnitude and reliability of the reported gains (up to 2.4 × 10^6) difficult to evaluate.

Authors: We agree that the abstract would be strengthened by including more quantitative details on the reference devices. In the revised manuscript, we will update the abstract to report the specific internal quality factor values measured for the Nb-only control devices, the number of devices tested in each category, and a brief note on the observed device-to-device variability. These values are already presented with supporting figures and error analysis in the main text and supplementary information; the abstract revision will make the comparison self-contained without altering the reported maximum Qi of 2.4 × 10^6 for the Nb/Ta devices. revision: yes
Referee: [Abstract] Abstract and fabrication description: The manuscript states that Nb/Ta and Nb devices were fabricated using 'identical DC sputtering and wet etching conditions,' but supplies no quantitative process logs, run-to-run statistics, or explicit controls for potential confounders such as etch depth, surface roughness, or residual gas exposure. Without these, the attribution of TLS reduction and stability gains specifically to suppressed Nb2O5 via the Ta layer (rather than uncontrolled process variations) is not fully supported and is load-bearing for the causal claim.

Authors: We acknowledge that additional fabrication details would help readers evaluate potential process variations. We have expanded the methods section to include the specific DC sputtering parameters (power, Ar pressure, deposition rate, and base pressure) and wet-etch recipe used for both device types. All Nb/Ta and Nb devices were processed in the same deposition run and etch batch to ensure identical conditions apart from the in-situ Ta capping step. We will also add a short discussion of possible confounders and why the consistent performance difference points to the Ta layer. However, dedicated post-fabrication metrology for surface roughness or residual gas exposure was not performed in this study. revision: partial

Circularity Check

0 steps flagged

No circularity: direct experimental comparison with no derivations or self-referential steps

full rationale

The manuscript is a purely experimental study that fabricates Nb/Ta and reference Nb resonators under stated identical DC sputtering and wet-etch conditions, then reports measured internal quality factors (up to 2.4e6) and six-month ageing data. No equations, first-principles derivations, fitted parameters renamed as predictions, or load-bearing self-citations appear in the provided text. The central claim—that Ta encapsulation suppresses Nb2O5 and improves Qi and stability—rests on direct comparison of measured device performance rather than any reduction of outputs to inputs by construction. This matches the reader's assessment of a non-circular experimental comparison.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

As an experimental materials paper the central claim rests on the domain assumption that observed Qi differences arise from reduced TLS at a Ta-based oxide interface rather than fabrication artifacts, with no free parameters or new entities introduced.

axioms (1)

domain assumption Microwave losses in superconducting resonators at millikelvin temperatures are dominated by two-level systems at the metal-air interface.
Invoked to motivate the Ta-capping strategy and interpret the Qi improvement.

pith-pipeline@v0.9.0 · 5575 in / 1298 out tokens · 93708 ms · 2026-05-10T17:32:40.571648+00:00 · methodology

discussion (0)

Reference graph

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