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arxiv: 2504.07950 · v1 · submitted 2025-04-10 · 🪐 quant-ph · cond-mat.mes-hall· cond-mat.supr-con

Localized quasiparticles in a fluxonium with quasi-two-dimensional amorphous kinetic inductors

Trevyn F. Q. Larson , Sarah Garcia Jones , Tam\'as Kalm\'ar , Pablo Aramburu Sanchez , Sai Pavan Chitta , Varun Verma , Kristen Genter , Katarina Cicak
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Sae Woo Nam Gerg\H{o} F\"ul\"op Jens Koch Ray W. Simmonds Andr\'as Gyenis
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Pith reviewed 2026-05-22 19:43 UTC · model grok-4.3

classification 🪐 quant-ph cond-mat.mes-hallcond-mat.supr-con
keywords kinetic inductancequasiparticlesdisordered filmsfluxonium qubitsmicrowave resonatorsloss mechanismstungsten silicidesuperconducting gap
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The pith

Loss in quasi-two-dimensional kinetic inductors is dominated by localized quasiparticles trapped in superconducting gap variations.

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

The paper investigates tungsten silicide wires fabricated from quasi-two-dimensional films where one dimension is below the superconducting coherence length. These wires serve as high-kinetic-inductance components in both microwave resonators and fluxonium qubits. Measurements reveal that loss grows with increasing disorder in the films and depends on frequency and geometry. The dominant source is identified as localized quasiparticles trapped in spatial variations of the superconducting gap. This matters for quantum circuit design because disordered high-inductance materials offer nonlinearity and impedance but suffer from loss that reduces coherence.

Core claim

We fabricate tungsten silicide wires from quasi-two-dimensional films with one spatial dimension smaller than the superconducting coherence length and embed them into microwave resonators and fluxonium qubits, where the kinetic inductance provides the inductive part of the circuits. We study the dependence of loss on the frequency, disorder, and geometry of the device, and find that the loss increases with the level of disorder and is dominated by the localized quasiparticles trapped in the spatial variations of the superconducting gap.

What carries the argument

Localized quasiparticles trapped in the spatial variations of the superconducting gap

If this is right

  • Loss increases with the level of disorder in the films.
  • The loss depends on frequency and geometry in a manner consistent with quasiparticle trapping.
  • The same inductive elements are used in both resonators and fluxonium qubits.
  • Disorder control is necessary to mitigate loss in these devices.

Where Pith is reading between the lines

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

  • Reducing spatial gap variations through better film uniformity may lower loss and extend coherence times.
  • The mechanism could affect other quantum devices based on thin disordered superconducting films.
  • Testing devices with engineered gap profiles would check if quasiparticle trapping can be suppressed.

Load-bearing premise

The dependence of loss on disorder, frequency, and geometry is enough to pinpoint localized quasiparticles in gap variations as the main cause instead of two-level systems or broken Cooper pairs.

What would settle it

A measurement where loss fails to increase with disorder or follows a different pattern than predicted for gap-trapped quasiparticles would disprove the claim.

Figures

Figures reproduced from arXiv: 2504.07950 by Andr\'as Gyenis, Gerg\H{o} F\"ul\"op, Jens Koch, Katarina Cicak, Kristen Genter, Pablo Aramburu Sanchez, Ray W. Simmonds, Sae Woo Nam, Sai Pavan Chitta, Sarah Garcia Jones, Tam\'as Kalm\'ar, Trevyn F. Q. Larson, Varun Verma.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) False-colored optical image of a WSi distributed res [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. (a) The internal quality factors of a set of lumped ele [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. (a) Circuit schematics of the WSi-fluxonium and (b) optical false-colored image of one of the devices. The WSi strip forms the inductor [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. (a) and (b) Measured relaxation [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. The photon number ( [PITH_FULL_IMAGE:figures/full_fig_p010_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Internal quality factors below single photon threshold as a [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Wiring diagram of the dilution fridge measurement setup. [PITH_FULL_IMAGE:figures/full_fig_p011_7.png] view at source ↗
read the original abstract

Disordered superconducting materials with high kinetic inductance are an important resource to generate nonlinearity in quantum circuits and create high-impedance environments. In thin films fabricated from these materials, the combination of disorder and the low effective dimensionality leads to increased order parameter fluctuations and enhanced kinetic inductance values. Among the challenges of harnessing these compounds in coherent devices are their proximity to the superconductor-insulator phase transition, the presence of broken Cooper pairs, and the two-level systems located in the disordered structure. In this work, we fabricate tungsten silicide wires from quasi-two-dimensional films with one spatial dimension smaller than the superconducting coherence length and embed them into microwave resonators and fluxonium qubits, where the kinetic inductance provides the inductive part of the circuits. We study the dependence of loss on the frequency, disorder, and geometry of the device, and find that the loss increases with the level of disorder and is dominated by the localized quasiparticles trapped in the spatial variations of the superconducting gap.

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 manuscript reports fabrication of tungsten silicide wires from quasi-two-dimensional amorphous films (one dimension smaller than the coherence length) and their integration into microwave resonators and fluxonium qubits to exploit high kinetic inductance. The authors measure loss as a function of frequency, disorder level, and device geometry, observing an increase in loss with disorder and concluding that the dominant mechanism is localized quasiparticles trapped in spatial variations of the superconducting gap.

Significance. If the mechanism attribution can be made quantitative and robust against alternative channels, the result would help clarify loss sources in disordered high-kinetic-inductance materials near the superconductor-insulator transition, informing material choices for coherent quantum devices. The experimental platform combining resonators and fluxonium qubits is a strength for probing these effects in situ.

major comments (2)
  1. [Abstract and conclusion] Abstract (final paragraph) and conclusion: the claim that loss 'is dominated by the localized quasiparticles trapped in the spatial variations of the superconducting gap' rests on observed trends with disorder, frequency, and geometry. However, the abstract itself notes the presence of broken Cooper pairs and two-level systems in these films; no quantitative model (e.g., predicted frequency scaling of QP loss versus TLS, or calculated QP density from gap fluctuations) is provided to discriminate or subordinate these channels. The quasi-2D geometry is expected to enhance gap fluctuations, yet the manuscript does not show an explicit calculation or functional-form comparison against the data.
  2. [Results] Results section (disorder-dependence data): the reported increase in loss with disorder level is consistent with the central claim but does not exclude post-hoc selection or unaccounted channels, as no error bars, exclusion criteria, or statistical tests for mechanism identification are described. This weakens the attribution relative to the reader's weakest assumption.
minor comments (2)
  1. [Methods] Clarify the precise definition of 'disorder level' (e.g., sheet resistance, film thickness, or resistivity ratio) and how it is controlled across samples.
  2. [Results] Add a table or figure summarizing the measured loss rates, frequencies, and geometries with uncertainties to allow direct comparison with theoretical predictions.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and constructive comments on our manuscript. We respond to each major comment below, clarifying our reasoning and indicating revisions where they will strengthen the presentation without altering the core findings.

read point-by-point responses

  1. Referee: [Abstract and conclusion] Abstract (final paragraph) and conclusion: the claim that loss 'is dominated by the localized quasiparticles trapped in the spatial variations of the superconducting gap' rests on observed trends with disorder, frequency, and geometry. However, the abstract itself notes the presence of broken Cooper pairs and two-level systems in these films; no quantitative model (e.g., predicted frequency scaling of QP loss versus TLS, or calculated QP density from gap fluctuations) is provided to discriminate or subordinate these channels. The quasi-2D geometry is expected to enhance gap fluctuations, yet the manuscript does not show an explicit calculation or functional-form comparison against the data.

    Authors: The attribution to localized quasiparticles follows from the systematic increase in loss with disorder level, which is a signature of enhanced gap fluctuations in the quasi-2D limit, together with the frequency and geometry dependences that align with theoretical expectations for QP trapping rather than uniform TLS or pair-breaking channels. While we did not include an explicit numerical model of gap-variation statistics, the observed trends are inconsistent with dominant TLS loss (which lacks the strong disorder scaling) and with uniform broken-pair contributions. We will revise the abstract and conclusion to phrase the conclusion more precisely as 'consistent with dominance by' and add a short discussion paragraph comparing the expected functional forms drawn from the literature on disordered superconductors. revision: partial

  2. Referee: [Results] Results section (disorder-dependence data): the reported increase in loss with disorder level is consistent with the central claim but does not exclude post-hoc selection or unaccounted channels, as no error bars, exclusion criteria, or statistical tests for mechanism identification are described. This weakens the attribution relative to the reader's weakest assumption.

    Authors: The disorder series comprises devices fabricated in the same run with controlled variation of film deposition parameters; loss values are extracted from resonator Q measurements on multiple devices per disorder level. Standard deviations across repeated cooldowns and devices are smaller than the plotted symbols. We will add explicit error bars to the relevant figure, include a methods paragraph describing sample inclusion criteria, and report a simple linear-regression analysis of loss versus disorder to quantify the trend. revision: yes

Circularity Check

0 steps flagged

No circularity: purely experimental claims from measured trends

full rationale

The manuscript is an experimental study fabricating tungsten silicide resonators and fluxonium qubits from quasi-2D disordered films and reporting loss dependence on frequency, disorder level, and geometry. The central conclusion attributes increasing loss to localized quasiparticles trapped in gap variations, based on observed trends. No derivation chain, equations, fitted parameters renamed as predictions, or self-citation load-bearing steps appear in the provided text or abstract. The argument relies on empirical data rather than any self-referential modeling or ansatz smuggled through citations. This satisfies the default expectation of an independent experimental result with no reduction of outputs to inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the experimental identification of quasiparticle localization as the dominant loss source; this depends on standard assumptions about disordered superconductors and the interpretation of geometry- and disorder-dependent loss data.

axioms (1)
  • domain assumption One spatial dimension of the tungsten silicide wires is smaller than the superconducting coherence length, placing the system in the quasi-two-dimensional regime.
    Stated as the fabrication condition that enables enhanced order-parameter fluctuations.

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Universal bound on microwave dissipation in superconducting circuits

    cond-mat.mes-hall 2025-07 unverdicted novelty 6.0

    Empirical scaling across materials reveals a universal bound on microwave dissipation tied to superfluid density and attributed to trapped nonequilibrium quasiparticles.

Reference graph

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