Pith. sign in

REVIEW 3 major objections 5 minor 14 references

CCAT: Optical Responsivity, Noise, and Readout Optimization of KIDs for Prime-Cam

T0 review · 3 major / 5 minor · reviewed 2026-08-04 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗

Pith's one-line read A fixed readout tone on a kinetic inductance detector loses 1.5–2x in sensitivity from a 0.2 pW optical-power drift, so Prime-Cam must periodically reset its tones.

desk verdict Useful, honest KID characterization for Prime-Cam; the relative readout trends are solid, but the absolute power scale needs a light-leak check before the calibration numbers are trusted. read the letter →

arxiv 2510.12162 v2 pith:SO27YCRU submitted 2025-10-14 astro-ph.IM

Eliza Gazda , Quintin Meyers , James R. Burgoyne , Scott Chapman , Steve K. Choi , Cody J. Duell , Anthony I. Huber , Inchara Jagadeesh
show 12 more authors
David Faulkner Katz Ben Keller Lawrence T. Lin Paul Malachuk Michael D. Niemack Darshan A. Patel Gordon J. Stacey Benjamin J. Vaughan Eve M. Vavagiakis Samantha Walker Yuhan Wang Ruixuan (Matt) Xie
This is my paper
classification astro-ph.IM
keywords kineticinductancedetectorssubmillimeterastronomyreadoutoptimizationnoiseequivalentpoweropticalresponsivitytonedetuningdetectorcalibrationPrime-Cam
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

This paper reports laboratory measurements of 280 GHz superconducting microwave resonators (kinetic inductance detectors) destined for the Prime-Cam receiver, using a temperature-controlled blackbody cold load to simulate the sky. The goal is to establish how readout tone power and tone placement, together with varying optical loading, set the detectors' noise equivalent power (NEP), and to use that understanding to plan calibration and retuning for an array of more than 100,000 detectors. The central quantitative finding is that a shift in optical loading of only about 0.2 pW, which moves a fixed readout tone by half a linewidth off resonance, degrades NEP by a factor of 1.5–2. That means the readout system must periodically reset tone positions as atmospheric loading drifts, or the array spends much of its time in a degraded-noise state. The measurements also show that aluminum KIDs have a nonlinear response and that suboptimal readout power can inflate NEP by as much as 200%, reinforcing the need for per-pixel calibration and careful tone optimization.

What carries the argument

The load-bearing mechanism is the fixed microwave readout tone parked on a KID resonance. Incoming optical power changes the resonator's kinetic inductance, shifting its frequency and therefore the tone's position relative to the resonance; the resulting amplitude and phase modulation is the signal. The paper's metric is NEP(f) = sqrt(S_xx(f))/R, where S_xx is the power spectral density of fractional frequency fluctuations and R is the optical responsivity (fractional frequency shift per unit optical power). Because R and the noise level both depend on where the tone sits on the resonance, the detuning distance in linewidths becomes the controlling variable, and the paper calibrates how much

What would settle it

At fixed cold-load temperature, shift a readout tone by half a linewidth and measure NEP; the paper's claim predicts a 1.5–2x degradation, so a substantially smaller penalty would indicate overestimated detuning sensitivity. Separately, resealing the detector enclosure to eliminate stray light and remeasuring responsivity would settle whether the absolute optical calibration is inflated.

Watch

Extended reading notes

Core claim

The paper's claim, stated on its own terms, is that for 280 GHz TiN and Al KIDs the attainable noise-equivalent power depends strongly on both the power and the frequency placement of the fixed readout tone, and that this dependence is sharp enough to matter under real observing conditions. Using a cold load as a blackbody, the authors measure optical responsivity as the slope of fractional frequency shift versus incident optical power, map phase timestreams to frequency fluctuations, and compute NEP as the ratio of fractional-frequency noise spectral density to responsivity. They find that a tone detuned by half a linewidth, corresponding to roughly 0.2 pW of optical-power change, shows a 1

Load-bearing premise

The results rest on the assumption that the optical power incident on the detectors is known from the cold-load temperature, an assumed emissivity of one, known filter transmission, and a light-tight detector enclosure; if stray light enters, the inferred responsivities are too high and NEPs too low.

Editorial extensions

If this is right

  • Prime-Cam will need a tone-retuning plan: a 0.2 pW optical loading drift multiplies NEP by about 1.5–2 at half-linewidth detuning.
  • Operating readout tones at lower power is costly: 10 dB below the optimum degrades NEP by up to 200%.
  • Aluminum KIDs require a nonlinear responsivity model (lookup tables or fitted curves); a single linear calibration will bias photometry.
  • TiN and Al arrays will need distinct calibration and optimal tone settings because their responsivity and nonlinearity differ.
  • The cold-load characterization procedure will be used to set tone powers and reset cadence during Prime-Cam commissioning.

Reading between the lines

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

  • Beyond the paper, the 0.2 pW / 1.5–2x threshold provides a concrete input for a tone-reset control loop: any detected loading change of that size warrants a retune. The paper documents the penalty but does not propose a control algorithm.
  • A testable extension: if the readout continuously re-centers tones using a resonance-tracking loop, the drift penalty should disappear; the lab data here give the target performance such a loop must meet.
  • If the stray-light interpretation is correct, the reported absolute responsivities are upper limits and the NEPs are lower limits, which may change how the TiN and Al arrays compare and how the array sensitivity is projected.
Share X Bluesky LinkedIn Reddit HN

Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 5 minor

Summary. The paper presents laboratory characterization of sample 280 GHz TiN and Al kinetic inductance detectors (KIDs) for the Prime-Cam instrument on CCAT/FYST. Using a temperature-controlled cold load, the authors measure optical responsivity (Eq. 1), frequency noise PSDs (Eq. 3), and NEP (Eq. 4) as functions of optical loading, readout tone power, and tone placement. A separate drift test fixes the tone at the 9 K resonance and steps the cold load from 8 K to 10 K, yielding NEP increases of 1.5–2× for approximately 0.2 pW optical-power changes. The paper concludes that readout tone placement and power must be optimized and that periodic tone resetting is needed to avoid noise penalties under varying atmospheric loading. The authors explicitly acknowledge in Sec. III.C that preliminary optical efficiency analysis indicates excess stray light, which may inflate responsivity and reduce NEP.

Significance. If the absolute calibration is correct, the paper provides useful quantitative guidance for operating >100,000 KIDs on Prime-Cam, especially the relationship between detuning and NEP degradation and the need for tone-reset cadence. The comparison of TiN versus Al nonlinearity and noise behavior is valuable for array design. The paper is honest about its main limitation, which is a strength: Sec. III.C openly flags possible stray light and states that further measurements are underway. However, because the central quantitative claims—the 0.2 pW mapping and the absolute NEP values—rest on an optical-power model that the authors themselves suspect is contaminated, the significance of those specific numbers is currently conditional. The relative degradation factor (1.5–2× at 0.5 linewidth detuning) is less sensitive to a constant calibration error and supports the operational conclusion even if the absolute power scale shifts.

major comments (3)
  1. [Sec. III.C and Eqs. (1), (4)] The absolute responsivity R and NEP scale directly with the assumed incident optical power ΔP_opt. Sec. III.C states that preliminary optical efficiency analysis indicates excess stray light, possibly from the custom aluminum box not being light-tight. If stray light is present, the change in optical power for a given cold-load temperature step is not known, so R from Eq. (1) is overestimated and NEP from Eq. (4) is underestimated. The quoted "0.2 pW" mapping in Sec. IV is therefore not established. The authors should either perform a light-leak check and report a corrected optical-power model, or explicitly reframe the quantitative claims as relative (linewidth detuning) and remove or strongly caveat the pW conversions and absolute NEP values. At minimum, a quantitative error budget for the optical-power estimate is needed.
  2. [Sec. III.A, III.B; Figs. 3, 4] No error bars or uncertainty estimates are provided for R, NEP, or the quoted detuning-to-power mapping. The figures show representative pixels without statistical scatter or pixel-to-pixel variation. The claim "NEP degraded by a factor of 1.5 to 2" needs a statement of how many pixels were measured, the spread across pixels, and how the uncertainty in the cubic phase-to-frequency fit (Sec. III.A) and the Welch PSD estimate propagate into NEP. Without this, the precision of the central result cannot be assessed.
  3. [Sec. III.C] The discrepancy with previous measurements [6,7] is attributed to a combination of wafer/edge effects and possible stray light, but the preliminary optical efficiency analysis is not described. The reader cannot tell how large the inferred inflation is or how it would change R and NEP. Please specify the analysis that indicates stray light, the estimated magnitude of the effect, and how it affects the conclusions. This is load-bearing for the absolute numbers, not merely a caveat.
minor comments (5)
  1. [Sec. IV] Typo: "demonsrate" should be "demonstrate".
  2. [Fig. 3 caption] Typo: "bifuraction" should be "bifurcation" (also in Fig. 2 caption).
  3. [Sec. III.A] The assumption of emissivity 1 for the Eccosorb cold load is stated but not justified with a reference or uncertainty. A brief justification or citation would help.
  4. [Fig. 4 caption] The caption cites "0.2 pW optical loading changes" without indicating that this value depends on the optical-power model flagged in Sec. III.C. Add a qualifier such as "estimated optical power."
  5. [Sec. II] The phrase "photon-noise-limited performance" in the introduction cites [6]–[8], but the text does not specify which of the presented measurements are photon-noise-limited. Clarify whether the NEP values in this work are limited by photon noise or by readout noise.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: NEP and responsivity are inferred through a standard calibration chain; the self-flagged stray-light issue is an absolute-calibration limitation, not a circular step.

full rationale

The paper's derivation chain is not circular. Optical responsivity R is extracted from the measured fractional frequency shift versus estimated optical power (Eq. 1, Sec. III.A); NEP is computed as sqrt(Sxx)/R (Eq. 4) using an independently measured frequency-noise PSD obtained from phase timestreams converted via a phase-frequency fit (Eqs. 2-3). This is a conventional laboratory calibration procedure, not a prediction that reduces to a fitted parameter by construction. The claim that '0.5 linewidth detuning' corresponds to 'approximately 0.2 pW' is a calibration mapping based on the measured frequency shift and the responsivity, not a tautology. The NEP degradation factor of 1.5-2 is a relative comparison of measured noise under fixed-tone drift conditions and does not depend on the absolute optical-power scale. The paper explicitly flags a limitation in Sec. III.C and Sec. IV: 'preliminary analysis of the optical efficiency also indicates an inflation of responsivity and a reduction in NEP, consistent with excess stray light entering the setup... one possible contributor is variation in the experimental configuration, such as the custom aluminum detector box not being fully light-tight.' This affects the absolute Popt and NEP values but is a stated uncertainty, not a circular derivation. Self-citations (e.g., [6], [7], [14]) support prior detector performance and atmospheric fluctuation context; they are not load-bearing for the present measurements or conclusions. No equation reduces to its own input, and no fitted quantity is renamed as a prediction. Therefore, no significant circularity is present.

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

No new physical entities or free theoretical parameters are introduced; the paper is a measurement. The numbers that matter are the fitted responsivity slope and the calibration-polynomial coefficients, plus the assumed optical-power model. The most fragile input is the light-tight/emissivity-1 assumption that the paper itself questions.

free parameters (2)
  • Optical responsivity R (slope of Δf/f0 vs Popt) = Not quoted in text (Fig. 3)
    R is fit from the measured frequency-shift vs optical-power curve (Eq. 1) and used in Eq. 4 to scale frequency-noise PSDs into NEP; the fitted value sets the absolute NEP scale.
  • Cubic phase-to-frequency polynomial coefficients = Not quoted
    Fit to the measured phase-frequency relation near resonance (Sec. III.A) to convert phase timestreams to frequency timestreams; if inaccurate, Sxx and NEP shift.
assumptions (5)
  • domain assumption The cold load is a blackbody with emissivity of 1 within the detector passbands
    Optical loading Popt is derived from cold-load temperature and filter transmission with emissivity 1 (Sec. III.A). If emissivity is less than 1 or stray light adds power, responsivity and NEP change.
  • domain assumption The custom aluminum detector enclosure is light-tight except for the defined optical path
    Sec. III.C names 'not being fully light-tight' as a possible source of excess stray light; this assumption is load-bearing for absolute responsivity.
  • domain assumption The low-pass filter transmission curves are known accurately
    Optical power estimate uses filter transmission (Secs. II, III.A); no uncertainty is quoted.
  • domain assumption The |S21| minimum defines the resonant frequency and the phase-frequency relation is well approximated by a cubic polynomial
    Timestreams are converted to frequency via a cubic phase fit (Sec. III.A); no validation of the fit is shown.
  • domain assumption Witness pixels at the wafer edge represent the full 280 GHz arrays
    Sec. III.C notes edge pixels may have reduced inductor volumes and therefore higher responsivity; this limits how results generalize.

how reviews work

0 comments
Cite this review

Pith. "Pith review of CCAT: Optical Responsivity, Noise, and Readout Optimization of KIDs for Prime-Cam." pith.science (2026). https://pith.science/paper/SO27YCRU

@misc{pith2026251012162,
  author       = {Pith},
  title        = {Pith review of: CCAT: Optical Responsivity, Noise, and Readout Optimization of KIDs for Prime-Cam},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SO27YCRU}},
  note         = {Machine review of arXiv:2510.12162}
}
abstract

The Prime-Cam instrument of the Fred Young Submillimeter Telescope (FYST) at the CCAT Observatory will conduct sensitive millimeter to submillimeter surveys for a range of astrophysical and cosmological sciences. Prime-Cam will use kinetic inductance detectors (KIDs) sensitive to multiple frequency bands spanning 280--850 GHz. With over 100,000 sensors under development, these KID arrays will soon form the largest submillimeter focal plane ever built. With fixed microwave tones probing amplitude and phase modulations in the KIDs due to incoming radiation, challenges arise in determining the optimal readout settings, especially under varying atmospheric loading. Realizing the science goals of FYST requires operating the detectors at optimal performance and determining accurate responsivities, which depend on readout tone placement and power. To address these challenges, we present laboratory measurements of sample pixels from the 280 GHz TiN and Al arrays using a blackbody cold load to simulate observing conditions. These measurements probe detector responsivity and noise across varying optical loading, tone power, and tone placement, providing the foundation to guide in situ calibration and operation of the $>$100,000 KIDs. We characterize detector sensitivity via the Noise Equivalent Power (NEP) as a function of readout tone power and placement, and measure the impact of detuning due to varying optical power on the NEP. Our test setup and methodology will inform the commissioning of Prime-Cam, in situ detector calibration procedures, the cadence of probe tone resetting, and potential design refinements for future arrays, supporting FYST's planned first light in 2026.

Figures

Figures reproduced from arXiv: 2510.12162 by the authors.

Figure 1
Figure 1. Experimental setup. (a) Three TiN (280 GHz) sample pixels. The central cross and plus shaped features are inductors, which double as absorbers, [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Example |S21| magnitude responses of KID resonators under multiple optical loading levels. The left plot shows a resonator from the TiN sample array at a readout power of 1 dB from bifurcation at 4K, and the right plot shows a resonator from an Al array at a readout power of 1 dB from bifurcation at 4K. Red stars mark the tone placements used during data acquisition. Increasing optical power shifts the resonator fre… view at source ↗
Figure 3
Figure 3. Top: Fractional frequency shift (∆f /f0) as a function of optical loading for a representative set of 280 GHz KIDs, used to extract optical responsivity. Readout powers are relative to bifurcation levels for resonators. TiN array bifuraction occurs at +4 dB at 4 K and Al array bifuraction occurs at +1 dB at 4 K. Middle Left: Noise Equivalent Power (NEP) for a TiN KID with the readout power at 1 dB from bifurcation a… view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: (top) shows an example of the response of a resonator |S21| and phase under these conditions, illustrating how de￾tuning develops as the resonance changes with optical power. Amplitude and phase variations are recorded throughout the measurement and analyzed following …

Discussion (0). Sign in to comment.

Reference graph

Works this paper leans on

14 extracted references · 3 canonical work pages

  1. [6]

    Ccat: Comparisons of 280ghz tin and al kinetic inductance detector arrays,

    C. J. Duell and et al., “Ccat: Comparisons of 280ghz tin and al kinetic inductance detector arrays,”arXiv e-prints, p. arXiv:2406.06828, 2024

    arXiv 2024

  2. [7]

    280-ghz aluminum mkid arrays for the fred young submillimeter telescope,

    A. K. Vaskuri and et al., “280-ghz aluminum mkid arrays for the fred young submillimeter telescope,”J. Astron. Telesc. Instrum. Syst., 2025, photon-noise-limited performance at white noise down to 1Hz under pW optical loading

    2025

  3. [1]

    Ccat-prime collaboration: Science goals and forecasts with prime-cam on the fred young submillimeter telescope,

    C. Collaboration and et al., “Ccat-prime collaboration: Science goals and forecasts with prime-cam on the fred young submillimeter telescope,” The Astrophysical Journal Supplement Series, vol. 264, p. 7, 12 2022

    2022

  4. [2]

    Ccat-prime: a novel telescope for sub- millimeter astronomy,

    S. C. Parshley and et al., “Ccat-prime: a novel telescope for sub- millimeter astronomy,”https://doi.org/10.1117/12.2314046, vol. 10700, pp. 1744–1758, 7 2018

    work page doi:10.1117/12.2314046 2018

  5. [3]

    Ccat-prime: the fred young submillimeter telescope (fyst) final design and fabrication,

    C. Collaboration and et al., “Ccat-prime: the fred young submillimeter telescope (fyst) final design and fabrication,” https://doi.org/10.1117/12.2630515, vol. 12182, pp. 543–550, 8 2022

    work page doi:10.1117/12.2630515 2022

  6. [4]

    Prime-cam: a first-light instrument for the ccat-prime telescope,

    E. Vavagiakis and et al., “Prime-cam: a first-light instrument for the ccat-prime telescope,”https://doi.org/10.1117/12.2313868, vol. 10708, pp. 375–390, 7 2018

    doi:10.1117/12.2313868 2018

  7. [5]

    Sensitivity of the prime-cam instrument on the ccat-prime telescope,

    S. K. Choi and et al., “Sensitivity of the prime-cam instrument on the ccat-prime telescope,”Journal of Low Temperature Physics, vol. 199, pp. 1089–1097, 5 2020

    2020

  8. [8]

    Photon-noise limited sensitivity in titanium nitride kinetic inductance detectors,

    J. Hubmayr and et al., “Photon-noise limited sensitivity in titanium nitride kinetic inductance detectors,”Applied Physics Letters, vol. 106, no. 7, p. 073505, 2015

    2015

Show all 14 references
  1. [9]

    Ccat-prime: Designs and status of the first light 280 ghz mkid array and mod-cam receiver,

    C. J. Duell and et al, “Ccat-prime: Designs and status of the first light 280 ghz mkid array and mod-cam receiver,” https://doi.org/10.1117/12.2562757, vol. 11453, pp. 235–243, 12 2020

    2020 doi

  2. [10]

    A broadband superconducting detector suitable for use in large arrays

    P. Day and et al., “A broadband superconducting detector suitable for use in large arrays.”Nature, vol. 425, 817-, pp. 817–821, 2003

    2003

  3. [11]

    Ccat-prime: Rfsoc based readout for frequency multiplexed kinetic inductance detectors,

    A. K. Sinclair, R. C. Stephenson, and C. A. Roberson, “Ccat-prime: Rfsoc based readout for frequency multiplexed kinetic inductance detectors,” 2022. [Online]. Available: https://arxiv.org/abs/2208.07465 6

    2022 arXiv

  4. [12]

    Submillimeter and millimeter wave characterization of absorbing materials,

    H. Hemmati, J. C. Mather, and W. L. Eichhorn, “Submillimeter and millimeter wave characterization of absorbing materials,”Appl. Opt., vol. 24, no. 24, pp. 4489–4492, Dec 1985. [Online]. Available: https://opg.optica.org/ao/abstract.cfm?URI=ao-24-24-4489

    1985

  5. [13]

    Characterization of the Mid- Frequency Arrays for Advanced ACTPol,

    S. K. Choi, J. Austermann, J. A. Beall, K. T. Crowley, R. Datta, S. M. Duff, P. A. Gallardo, S. P. Ho, J. Hubmayr, B. J. Koopman, Y . Li, F. Nati, M. D. Niemack, L. A. Page, M. Salatino, S. M. Simon, S. T. Staggs, J. Stevens, J. Ullom, and E. J. Wollack, “Characterization of t...

    2018

  6. [14]

    The atacama cosmology telescope: Quantifying atmospheric emission above cerro toco,

    T. W. Morris, E. Battistelli, R. Bustos, S. K. Choi, A. J. Duivenvoor- den, J. Dunkley, R. D ¨unner, M. Halpern, Y . Guan, J. van Marrewijk, T. Mroczkowski, S. Naess, M. D. Niemack, L. A. Page, B. Partridge, R. Puddu, M. Salatino, C. Sif ´on, Y . Wang, and E. J. Wollack, “The ...

    2025

Pith tools

Reviewed August 4, 2026 · model on record in the stance chip above.