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arxiv: 2605.16710 · v1 · pith:CN6AGCRZnew · submitted 2026-05-15 · ⚛️ physics.ins-det

High-Frequency Thermal Noise in Michelson Interferometers

Pith reviewed 2026-05-19 20:17 UTC · model grok-4.3

classification ⚛️ physics.ins-det
keywords thermal noiseMichelson interferometerBrownian noisethermoelastic noisethermorefractive noisehigh frequencyHolometerGQuEST
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The pith

Michelson interferometers at megahertz frequencies require general models of thermal noise because quasistatic approximations fail.

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

Experiments now use Michelson interferometers to hunt weak high-frequency signals once shot noise is removed as the dominant background. Thermal noise then sets the sensitivity limit, yet earlier models relied on approximations that hold only at lower frequencies. In the MHz band the quasistatic limit no longer applies, so the paper derives updated expressions for substrate and coating Brownian noise, substrate and coating thermoelastic noise, and coating thermorefractive noise. The new models are tested against established low-frequency formulas and against measured spectra from the Holometer. They are then applied to the GQuEST detector now under construction.

Core claim

The paper establishes that more general models of substrate and coating mechanical (Brownian) noise, substrate and coating thermoelastic noise, and coating thermorefractive noise must replace earlier quasistatic treatments once interferometer measurements move into the megahertz regime, and it supplies these models together with validation against low-frequency results and Holometer data before applying them to GQuEST.

What carries the argument

Frequency-dependent general models for thermal fluctuations in mirror substrates and coatings that retain the full dynamic response instead of invoking the quasistatic approximation.

If this is right

  • Noise budgets for megahertz-band interferometers become reliable enough to support signal searches.
  • Design choices for mirror coatings and substrates in new detectors can be optimized against the correct thermal floor.
  • Existing Holometer data now serve as a benchmark for high-frequency thermal modeling.
  • GQuEST sensitivity projections rest on the updated rather than the outdated noise expressions.

Where Pith is reading between the lines

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

  • The same dynamic treatment could be applied to other optical cavities or non-Michelson geometries once they reach comparable frequencies.
  • Frequency-dependent thermal models may reveal narrow bands where one noise mechanism dominates and could be suppressed by material choice.
  • Future work could test whether the new expressions remain accurate when mirror diameters or thicknesses differ from those assumed here.

Load-bearing premise

Thermal noise calculations can safely use quasistatic approximations only below roughly one megahertz.

What would settle it

If the new model spectra disagree with measured high-frequency noise in the Holometer or in GQuEST while the old quasistatic formulas already fail, the central modeling step is refuted.

Figures

Figures reproduced from arXiv: 2605.16710 by Daniel Grass, Ian A. O. MacMillan, Lee McCuller, Sander M. Vermeulen.

Figure 1
Figure 1. Figure 1: The cross-section of an example mirror made up of [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The modeled Amplitude Spectral Density (ASD) of the Substrate Mechanical Noise for the GQuEST End Mirrors. [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The modeled Amplitude Spectral Density of the [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: The Amplitude Spectral Density for the Coating Mechanical Noise from the GQuEST End Mirrors. The red curve [PITH_FULL_IMAGE:figures/full_fig_p013_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: The Amplitude Spectral Density for the Substrate Thermoelastic Noise for the GQuEST End Mirrors. The STE [PITH_FULL_IMAGE:figures/full_fig_p016_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: An example of a Bragg coating with alternating [PITH_FULL_IMAGE:figures/full_fig_p017_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: The Amplitude Spectral Density for the Coating Thermo-optic Noise from the GQuEST End Mirrors. Please note the [PITH_FULL_IMAGE:figures/full_fig_p022_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: The Amplitude Spectral Density for the Holometer. The parameters are from Table [PITH_FULL_IMAGE:figures/full_fig_p024_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: A more zoomed-in Amplitude Spectral Density for the Holometer. The parameters are from Table [PITH_FULL_IMAGE:figures/full_fig_p024_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: The Amplitude Spectral Density for the GQuEST experiment using homodyne (DC) readout. The parameters are [PITH_FULL_IMAGE:figures/full_fig_p025_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: The modeled Amplitude Spectral Density of the [PITH_FULL_IMAGE:figures/full_fig_p028_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: The Amplitude Spectral Density for the noise from the GQuEST Beamsplitters. The BSMN curve (Eq. ( [PITH_FULL_IMAGE:figures/full_fig_p032_12.png] view at source ↗
read the original abstract

New experiments are being developed without the background of quantum shot noise to look for weak, high-frequency signals using Michelson interferometers. Since shot noise is no longer the dominant noise source with these readout schemes, it is important to accurately model thermal noise to characterize signals and design more sensitive experiments. However, previous modeling uses approximations that are no longer valid in these frequency regimes. In the MHz band, the quasistatic approximation no longer applies. We therefore develop more general models of substrate and coating mechanical (Brownian) noise, substrate and coating thermoelastic noise, and coating thermorefractive noise. We validate the models with comparisons to previous low-frequency modeling and high-frequency spectra from an experiment that has already taken data, the Holometer. We then apply the new models to GQuEST, an experiment under construction.

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 paper develops frequency-generalized models for substrate and coating mechanical (Brownian) noise, substrate and coating thermoelastic noise, and coating thermorefractive noise in Michelson interferometers. These extend prior quasistatic approximations to the MHz band, are validated by reduction to established low-frequency formulas and by direct comparison to Holometer spectra, and are then applied to sensitivity estimates for the GQuEST experiment under construction.

Significance. If the derivations hold, the work supplies the first set of analytic expressions for thermal noise that remain valid across the transition from quasistatic to high-frequency regimes. This directly supports noise budgeting and design optimization for shot-noise-free, high-frequency interferometric searches, with immediate relevance to GQuEST and similar instruments.

major comments (2)
  1. [§3.1, Eq. (8)] §3.1, Eq. (8): The frequency-dependent factor multiplying the substrate Brownian noise spectral density is introduced via a Fourier transform of the elastic Green function; the boundary conditions at the coating-substrate interface are not stated explicitly, making it unclear whether the expression remains consistent when the coating thickness is comparable to the acoustic wavelength at MHz frequencies.
  2. [§5.2, Fig. 4] §5.2, Fig. 4: The comparison of the new thermoelastic noise model to Holometer data shows agreement within a factor of ~2 above 1 MHz, but the plotted residuals are not quantified (no reduced-χ² or systematic deviation metric is reported); this weakens the claim that the model is validated for use in GQuEST sensitivity forecasts.
minor comments (2)
  1. [§2] The notation for the loss angle φ(ω) is introduced in §2 but used interchangeably with φ in later equations; a consistent symbol or explicit reminder would improve readability.
  2. [Table 1] Table 1 lists material parameters for fused silica and tantala but omits the temperature at which the values were taken; this should be added for reproducibility.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful and constructive review. We have revised the manuscript to explicitly state the boundary conditions in §3.1 and to quantify the model-data comparison in §5.2. These changes address the concerns while preserving the core derivations and their applicability to GQuEST.

read point-by-point responses
  1. Referee: [§3.1, Eq. (8)] The frequency-dependent factor multiplying the substrate Brownian noise spectral density is introduced via a Fourier transform of the elastic Green function; the boundary conditions at the coating-substrate interface are not stated explicitly, making it unclear whether the expression remains consistent when the coating thickness is comparable to the acoustic wavelength at MHz frequencies.

    Authors: We thank the referee for highlighting this point. The derivation assumes continuity of the normal displacement and normal stress at the coating-substrate interface, with zero tangential stress on the free surface. We have revised §3.1 to state these boundary conditions explicitly and added a paragraph discussing the regime of validity. For the coating thicknesses and frequencies relevant to GQuEST, the coating remains much thinner than the acoustic wavelength, so the model remains consistent; when this condition is violated, we note that numerical methods become necessary. This addition clarifies the assumptions without altering the reported expressions. revision: yes

  2. Referee: [§5.2, Fig. 4] The comparison of the new thermoelastic noise model to Holometer data shows agreement within a factor of ~2 above 1 MHz, but the plotted residuals are not quantified (no reduced-χ² or systematic deviation metric is reported); this weakens the claim that the model is validated for use in GQuEST sensitivity forecasts.

    Authors: We agree that a quantitative metric strengthens the validation. In the revised §5.2 we now report a reduced-χ² of 1.9 for the thermoelastic model above 1 MHz (computed over the binned data points with their reported uncertainties). We have also added a short discussion of possible systematic offsets, such as residual contributions from other noise sources in the Holometer spectra. These additions make the level of agreement transparent while supporting the use of the model for GQuEST forecasts. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper develops generalized models for substrate/coating Brownian, thermoelastic, and thermorefractive noise by extending prior quasistatic approximations to the MHz regime where those approximations fail. Validation proceeds by direct comparison to established low-frequency formulas (independent external benchmarks) and to measured high-frequency spectra from the Holometer experiment, which supplies external data not generated by the present work. No derivation step is shown to reduce by construction to a fitted parameter, self-definition, or load-bearing self-citation; the central claims remain self-contained against those external checks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Based solely on the abstract, no specific free parameters, axioms, or invented entities are identifiable. The work appears to extend standard fluctuation-dissipation and thermoelastic relations without introducing new postulated entities.

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