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REVIEW 3 major objections 6 minor 24 references

Dielectric waveguide setup tested with a superconducting millimeter-wave Fabry-P\'erot interferometer at milli-Kelvin temperatures

T0 review · 3 major / 6 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read A cryogenic setup built on flexible plastic waveguides carries millimeter-wave signals into a 10 mK dilution refrigerator, and a superconducting Fabry–Pérot cavity measured with it shows internal quality factors up to 15 million at the…

desk verdict Solid engineering demonstration of mK dielectric waveguides for W-band, but the no-TLS and single-photon claims in the abstract outrun the data. read the letter →

arxiv 2411.15058 v3 pith:UWLCDNFW submitted 2024-11-22 physics.app-ph cond-mat.supr-conphysics.ins-det

Jakob Lenschen (1) , Rosalie Labbe (1) , Nils Drotleff (1) , Markus Fuhrmann (1) , Jürgen Lisenfeld (1) , Hannes Rotzinger (1 , 2) , Alexey V. Ustinov (1
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2) ((1) Physiklalisches Institut Karlsruher Institut für Technologie (2) Institut für Quantenmaterialien und Technologie Karlsruher Institut für Technologie)
This is my paper · ORCID
classification physics.app-phcond-mat.supr-conphysics.ins-det
keywords dielectricwaveguidemillimeter-wavecryogenicFabry-Pérotcavitysuperconductingresonatorqualityfactortwo-levelsystemsW-band
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

The paper proposes replacing rigid metallic hollow waveguides with flexible dielectric strips made of high-density polyethylene to carry millimeter-wave signals in the 75–110 GHz band from room temperature down to 10 mK. The authors show that these waveguides combine low, roughly frequency-independent photon loss with a very small heat load at each cryogenic stage, and that they can be thermally anchored, shielded against crosstalk, and attenuntated using black HDPE or copper-powder coatings. To demonstrate suitability for low-photon-number quantum measurements, they mounted a superconducting Fabry–Pérot cavity at the cold stage and measured resonances with internal quality factors up to about 15 million at photon numbers as low as a few. They interpret the flat quality factor versus power and temperature as evidence that atomic two-level systems in the cavity surfaces do not limit the resonator. If this is right, it opens a practical route to millimeter-wave quantum experiments that need low noise and low photon numbers.

What carries the argument

The dielectric waveguide itself is the central object: a rectangular strip of HDPE (relative permittivity 2.33) surrounded by polyurethane foam, guiding mm-waves by total internal reflection with an evanescent field that can be used for coupling and attenuation. Thermal anchoring is achieved by metallic transitions at each temperature stage, where tapered waveguide ends couple into WR10 rectangular waveguides. The test device is a Fabry–Pérot cavity with sputtered niobium mirrors coupled through a small sub-cutoff hole to a WR10 waveguide, and resonance parameters are extracted with a circle-fit algorithm.

What would settle it

Measure the same cavity mode's internal quality factor versus photon number with a scheme that eliminates long-term drift, for instance by interleaving a high-power reference with each low-power point or by using faster phase-locked averaging. If the quality factor still falls below 100 photons after drift correction, the claim that two-level systems do not affect the cavity would be falsified.

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Extended reading notes

Core claim

The central claim is that dielectric waveguides made from strips of HDPE embedded in low-loss foam provide a viable, low-heat-leak signal path for W-band measurements at milli-Kelvin temperatures, and that the setup is sensitive enough to resolve the intrinsic quality factor of a superconducting Fabry–Pérot cavity down to the single-photon limit. Multiple cavity modes were observed at 10 mK, with internal quality factors up to 15 million and coupling quality factors above 45 million. The authors find no evident influence of atomic two-level systems: the internal quality factor does not rise substantially with photon number above roughly 1000 photons, as it would for TLS-limited resonators; the modest decrease below 100 photons is attributed to parameter drift during multi-hour averaging. The conclusion is that the large mode volume of the cavity keeps the electric field density at the mirror surfaces low enough to avoid TLS loss.

Load-bearing premise

The no-TLS conclusion rests on attributing the drop in internal quality factor below 100 photons to slow parameter drift during hours-long averaging; if that attribution is wrong, the flatness at higher photon counts would not prove the absence of two-level-system loss.

Editorial extensions

If this is right

  • Dilution refrigerators can be fitted with many dielectric waveguide lines without exceeding thermal budgets, because each anchored line adds only nanowatts to the coldest stages.
  • Black HDPE waveguides can serve as cold attenuators that also block infrared and thermal photons, simplifying noise engineering at millimeter-wave frequencies.
  • The demonstrated single-photon sensitivity at around 100 GHz opens the door to millimeter-wave circuit quantum electrodynamics, such as coupling superconducting qubits or cavities at higher photon energies.
  • The absence of TLS loss in a large-mode-volume cavity implies that millimeter-wave cavities can reach very high coherence without special surface treatments.

Reading between the lines

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

  • If the drift explanation is correct, an improved averaging scheme—such as interleaved reference tones or faster digitization—should recover a flat internal quality factor versus photon number; such a control measurement would directly test the no-TLS conclusion.
  • The frequency-independent attenuation of transparent HDPE suggests the same waveguide technology could extend naturally to sub-THz and THz bands, where other low-loss flexible transmission lines are scarce.
  • Black HDPE as a distributed cold attenuator with infrared blocking could be combined with the copper-powder coating to realize fully absorptive, impedance-matched input lines for millimeter-wave qubit experiments.
  • Crosstalk suppression by metal-mesh shielding may scale to dense multi-channel arrays, enabling frequency-multiplexed millimeter-wave readout.
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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

3 major / 6 minor

Summary. The paper describes a cryogenic millimeter-wave setup based on dielectric waveguides (DWGs) made of high-density polyethylene (HDPE), designed to operate from room temperature to 10 mK in the 75-110 GHz band. The authors report room-temperature and cryogenic transmission measurements of transparent and black HDPE DWGs, thermal-load estimates, and an attenuation scheme using black HDPE or copper-loaded coatings. The setup is validated with a superconducting niobium-coated Fabry-Pérot cavity, for which they report internal quality factors up to about 15 million and coupling quality factors above 45 million, and they argue that atomic two-level systems have no evident influence.

Significance. If the reported results hold, the work provides a practical, low-heat-load alternative to metal waveguides for mm-wave quantum experiments, with the added capability of cold attenuation and infrared blocking. The paper includes direct S-parameter data, a thermal model, and a full system-level demonstration; these are concrete strengths. However, the headline physics claim about the absence of TLS losses is currently conditional on a drift interpretation that is not quantitatively supported, and the 'single photon limit' wording exceeds the displayed data.

major comments (3)
  1. [III.B, Fig. 10 (top)] The conclusion that two-level systems do not affect the cavity quality factor rests on attributing the drop in Qi from ~15 million to ~11 million at photon numbers below 100 (Fig. 10, top) to parameter drift during multi-hour averaging. The text offers no quantitative drift model, no repeated high-power/low-power cycles, and no independent bound on the circle-fit uncertainty from slowly varying cable phase, coupling, or background. Since a TLS-like loss mechanism would produce exactly this drop, the data as presented do not exclude a real power-dependent loss; please add a quantitative drift analysis or temper the no-TLS and single-photon-limit claims.
  2. [Abstract and Conclusion; Section III.B, Fig. 10] The Abstract and Conclusion claim 'quality factors up to 15 million in the single photon limit,' but the lowest average photon number shown in Fig. 10 is about 3, and at that point the internal quality factor has already decreased to ~11 million. The maximum Qi of ~15 million is measured at photon numbers above 100, not in the single-photon regime; the current phrasing overstates what the data support. Please state the actual photon-number range and either replace 'single photon limit' with 'down to a few photons' or provide data at n < 1.
  3. [II.D, Fig. 4] The reported cryogenic attenuation (0.6 dB/m for the transparent DWG at 2.5 K) is obtained by de-embedding from several cooldown cycles, and the text itself notes that the calibration is sensitive to non-stationary DWG-to-adapter impedance matching. No quantified error bars, repeated measurements, or alternative calibrations are provided, so the frequency-independent low-loss value is not fully established. Adding uncertainty estimates or reproducibility data would strengthen the central feasibility claim.
minor comments (6)
  1. [II.D] The sentence after the de-embedding discussion says 'Figure 2 shows the resulting transmission data'; this should reference Fig. 4, which displays the transmission data.
  2. [II.B] The phrase 'stainless steal' should read 'stainless steel'.
  3. [Figure 2 caption] The caption begins with 'T op:' which appears to be a typesetting error for 'Top:'.
  4. [III.B] The notation 'Qc(A) = 46 ± 3.5 10^6' should be typeset as (46 ± 3.5) × 10^6 to avoid ambiguity.
  5. [Figure 10 caption] The phrases 'Indicated errors factored by two' and 'Blue errors factored by 15' are ambiguous; please specify whether the displayed error bars are raw, doubled, or multiplied by the given factor.
  6. [References] The reference list appears under 'Appendix A' after the main text, but the body cites [1]-[23] as standard references; please reformat this as a conventional reference section.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: direct transmission and quality-factor measurements, standard mode formulas, and no fitted parameter presented as a prediction.

full rationale

The paper's central results are direct measurements: transmission through dielectric waveguides is obtained from calibrated S21 data, and cavity quality factors are extracted with a standard circle-fit algorithm applied to raw transmission data. Mode-frequency formulas (Eqs. 3 and 4) are textbook expressions used only for identification, not for deriving the measured quality factors. The no-TLS conclusion is an interpretation of the measured power dependence, and the attribution of the low-photon-number Qi decrease to drift is an assumption rather than a fitted-input-as-prediction step; it is a limitation of the evidence, not a circular construction. Self-citation appears only for the circle-fit routine and the QKIT package, which are analysis tools and do not carry the physical claims. No equation or parameter in the paper is defined in terms of the conclusion it supports, and no benchmark result is imported from the authors' prior work to force the outcome. Therefore no significant circularity is present.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

No new entities or free parameters. The central experimental claims are direct measurements; the main auxiliary assumptions are standard cavity theory and assumed thermal-conductivity data.

assumptions (4)
  • standard math Fabry-Pérot cavity resonance frequencies given by Eqs. (3) and (4) from Kogelnik and Li / Krupka et al.
    The paper uses standard cavity formulas without derivation; these are well-established.
  • standard math Coupling hole attenuation described by circular waveguide cutoff approximation, Eq. (5)
    Standard electromagnetic theory for an evanescent circular waveguide.
  • domain assumption Tabulated and extrapolated thermal conductivity values for HDPE and stainless steel (Table I)
    Heat-load estimates rely on literature values, some extrapolated below 1 K.
  • ad hoc to paper Observed low-photon-number Q_i decrease attributed to measurement drift rather than TLS loss (Fig. 10)
    This is a post-hoc explanation used to support the no-TLS conclusion.

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Cite this review

Pith. "Pith review of Dielectric waveguide setup tested with a superconducting millimeter-wave Fabry-P\'erot interferometer at milli-Kelvin temperatures." pith.science (2026). https://pith.science/paper/UWLCDNFW

@misc{pith2026241115058,
  author       = {Pith},
  title        = {Pith review of: Dielectric waveguide setup tested with a superconducting millimeter-wave Fabry-P\'erot interferometer at milli-Kelvin temperatures},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/UWLCDNFW}},
  note         = {Machine review of arXiv:2411.15058}
}
read the original abstract

We propose and test a cryogenic setup comprising dielectric waveguides for mm-wave frequencies in the range of 75-110 GHz and temperatures down to 10 mK. The targeted applications are quantum technologies at millimeter-wave frequencies, which require measurements at low photon numbers and noise. We show that the high density polyethylene waveguides combine a frequency independent low photon loss with a very low heat conductance. Black high density polyethylene shows a higher attenuation, which is useful to block thermal photons in a cryogenic environment. The dielectric waveguides are thermally anchored and attenuated at several stages of the cryostat. They are individually protected by additional metallic shields to suppress mutual cross-talk and external interference. With this setup, multiple superconducting resonances of a Fabry-P\'erot cavity were measured at 10 mK. We find quality factors up to 15 million in the single photon limit for resonances above 100 GHz. These results show no evident influence of atomic two-level systems in the cavity.

Figures

Figures reproduced from arXiv: 2411.15058 by the authors.

Figure 1
Figure 1. FIG. 1. Schematic of the cryogenic setup with added DWGs. [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 3
Figure 3. FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 2
Figure 2. FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figures from the paper (5 more)
Figure 4
Figure 4. Figure 4: FIG. 4. Transmission of a 50 cm long transparent and a 25 cm [PITH_FULL_IMAGE:figures/full_fig_p003_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5. Disassembled DWG with [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]
Figure 6
Figure 6. Figure 6: FIG. 6. Schematic of the Fabry-P´erot cavity (not to scale). [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]
Figure 10
Figure 10. Figure 10: FIG. 10 [PITH_FULL_IMAGE:figures/full_fig_p007_10.png]
Figure 9
Figure 9. Figure 9: FIG. 9. Transmission measurement of a resonance at different [PITH_FULL_IMAGE:figures/full_fig_p007_9.png]

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Reference graph

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Reviewed August 12, 2026 · model on record in the stance chip above.