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arxiv: 2606.22292 · v1 · pith:PBGD2G6Qnew · submitted 2026-06-21 · 🪐 quant-ph

Probing Single-Particle Spatial Extent With Helical Neutron Wavefronts

D. Sarenac , O. Lailey , C. W. Clark , D. G. Cory , H. Ekinci , D. V. Garrad , M. G. Huber , L. Matthews
show 3 more authors
N. Shentevski P. R. Vadnere D. A. Pushin
This is my paper

Pith reviewed 2026-06-26 10:54 UTC · model grok-4.3

classification 🪐 quant-ph
keywords neutron interferometrytransverse coherence lengthwavepacket spatial extenthelical wavefrontsannular intensity profilessingle-particle extentbeam divergence
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The pith

Helical neutron wavefronts distinguish single-particle wavepacket extent from transverse coherence length.

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

The paper shows that helical neutron wavefronts create annular intensity profiles in which the radius of the intensity peak depends only on the spatial extent of individual neutron wavepackets, while transverse coherence length affects only the width of the profile. This separation is not possible with conventional intensity measurements, where both quantities produce similar broadening. The experiment measures a beam divergence of about 1.1 milliradians, corresponding to a coherence length of roughly 180 nanometers. The same data place a lower bound of at least 2 micrometers on the wavepacket extent, more than ten times larger. The results demonstrate that the two quantities are physically distinct.

Core claim

Helical neutron states produce annular intensity profiles whose peak radius depends on the transverse wavepacket extent while coherence length contributes only to profile broadening. In the reported geometry the measured divergence of 1.1 mrad yields a transverse coherence length of 180 nm, yet the identical data require the individual neutron wavepackets to extend at least 2 micrometers.

What carries the argument

Helical neutron wavefronts that generate annular intensity profiles whose peak radius isolates wavepacket extent from coherence length.

If this is right

  • Transverse coherence length and single-particle wavepacket extent can be measured independently in neutron beams.
  • The wavepacket extent in the reported setup exceeds the coherence length by more than an order of magnitude.
  • The helical-wavefront method resolves the confusion that both quantities produce identical intensity broadening in ordinary measurements.

Where Pith is reading between the lines

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

  • The same helical-state technique could be adapted to other neutral particles such as atoms or molecules to test whether their coherence and packet extents are likewise distinct.
  • If wavepacket extent remains much larger than coherence length across different beam preparations, neutron optics models may need to treat the two lengths as separate parameters rather than interchangeable.
  • Direct imaging of the annular profiles offers a route to calibrate wavepacket size in neutron interferometers without relying on divergence alone.

Load-bearing premise

The peak radius of the annular profile is determined solely by wavepacket extent while coherence length affects only the broadening, with no other propagation effects present.

What would settle it

An experiment in which the measured peak radius of the annular profile changes when coherence length is varied while wavepacket extent is held fixed, or fails to match the radius predicted from independent wavepacket measurements.

Figures

Figures reproduced from arXiv: 2606.22292 by C. W. Clark, D. A. Pushin, D. G. Cory, D. Sarenac, D. V. Garrad, H. Ekinci, L. Matthews, M. G. Huber, N. Shentevski, O. Lailey, P. R. Vadnere.

Figure 1
Figure 1. Figure 1: FIG. 1. Illustration of two interpretations of transverse co [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Relationship between helical mode [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Experimental diffraction pattern showing [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Experimental and simulated azimuthally integrated intensity profiles for the [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. (a) Error between simulation and experiment for the [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
read the original abstract

Distinguishing transverse coherence length from single-particle wavepacket extent is fundamentally challenging, as both manifest through spatial broadening of observed intensity profiles in conventional experiments. Here we introduce a method based on helical neutron wavefronts that enables this separation. Helical neutron states produce annular intensity profiles whose peak radius depends on the transverse wavepacket extent, while coherence length only contributes to profile broadening. In our experimental geometry we measure a beam divergence of ~1.1 mrad, corresponding to a transverse coherence length of ~180 nm. In contrast, the same measurement places a lower bound of >= 2 um on the spatial extent of the individual neutron wavepackets, more than an order of magnitude larger than the coherence length. These results provide direct experimental evidence that transverse coherence length and single-particle wavepacket extent are distinct physical quantities, resolving a longstanding source of confusion in the neutron literature.

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 / 1 minor

Summary. The manuscript introduces a method based on helical neutron wavefronts to separate transverse coherence length from single-particle wavepacket extent. Helical states produce annular intensity profiles where the peak radius is claimed to depend only on wavepacket extent while coherence length affects only broadening. From a measured beam divergence of ~1.1 mrad the authors report a coherence length of ~180 nm and a lower bound of >=2 um on wavepacket extent, claiming this provides direct experimental evidence that the two quantities are distinct.

Significance. If the modeling isolation holds, the result would clarify a persistent point of confusion in neutron optics by showing that coherence length and wavepacket extent can differ by more than an order of magnitude. The helical-wavefront approach is a potentially useful addition to the experimental toolkit for neutron interferometry and coherence studies.

major comments (2)
  1. [Abstract] Abstract: the central separation rests on the unverified modeling assumption that the annular peak radius is set solely by the single-particle transverse extent while coherence length contributes exclusively to radial broadening. No derivation, propagation simulation, or test for coupling from partial coherence, mode dispersion, or diffraction is supplied; if any such effect shifts the peak location the reported distinction and the >=2 um lower bound do not follow from the 1.1 mrad datum.
  2. [Experimental section] Experimental section (implied by numerical claims): the reported values are given without error bars, raw intensity profiles, fitting procedures, or exclusion criteria, making it impossible to assess the statistical support for the 180 nm and >=2 um figures or the claimed separation.
minor comments (1)
  1. The abstract states numerical results directly but supplies no supporting equations or figures; adding at least one schematic of the helical-state propagation and the intensity annulus would improve clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive feedback. The comments highlight the need for explicit verification of the modeling assumptions and fuller documentation of the experimental analysis. We address each point below and will revise the manuscript accordingly to strengthen the presentation while preserving the core claims.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the central separation rests on the unverified modeling assumption that the annular peak radius is set solely by the single-particle transverse extent while coherence length contributes exclusively to radial broadening. No derivation, propagation simulation, or test for coupling from partial coherence, mode dispersion, or diffraction is supplied; if any such effect shifts the peak location the reported distinction and the >=2 um lower bound do not follow from the 1.1 mrad datum.

    Authors: We agree that the abstract states the separation without supplying the supporting derivation or simulations. The underlying physical model is that a helical wavefront with azimuthal phase winding produces a radial intensity maximum whose location is fixed by the single-particle transverse wave-packet extent (via the Fourier relationship between the helical phase gradient and the transverse momentum distribution), while the mutual coherence function enters only as a radial convolution that broadens the ring without shifting its peak. In the paraxial regime relevant to our beam divergence (~1.1 mrad), standard propagation calculations show that partial coherence, mode dispersion, and diffraction contribute negligibly to peak displacement. We will add a dedicated theoretical section containing the analytic derivation, numerical wave-packet propagation simulations, and explicit checks for coupling effects. The coherence length follows directly from the measured divergence via the standard relation l_c ≈ λ/(2π θ), and the >=2 μm lower bound on wave-packet extent is set by the observed ring radius being incompatible with smaller extents under the same model. revision: yes

  2. Referee: [Experimental section] Experimental section (implied by numerical claims): the reported values are given without error bars, raw intensity profiles, fitting procedures, or exclusion criteria, making it impossible to assess the statistical support for the 180 nm and >=2 um figures or the claimed separation.

    Authors: The numerical results quoted in the abstract are derived from the full data set presented in the experimental section, but we acknowledge that error bars, representative raw profiles, and the precise fitting/exclusion protocol are not summarized in the abstract itself. In the revised manuscript we will (i) attach error bars to the reported 180 nm and >=2 μm values, (ii) include example raw annular intensity profiles together with the fitted curves, (iii) describe the fitting procedure (radial Gaussian or Airy-function convolution) and any data-selection criteria, and (iv) state the statistical uncertainties and goodness-of-fit metrics. These additions will allow direct evaluation of the support for the quoted figures and the separation claim. revision: yes

Circularity Check

0 steps flagged

No significant circularity; reported bounds derive from measured divergence and profile geometry under stated modeling assumption

full rationale

The paper measures beam divergence (~1.1 mrad) to obtain coherence length (~180 nm) and uses the same annular profile data to place a lower bound (>=2 um) on wavepacket extent. The separation rests on the modeling claim that peak radius isolates extent while coherence affects only broadening, but this is presented as an input assumption rather than a quantity fitted to the target distinction or reduced by construction to the measured inputs. No self-citation load-bearing steps, no fitted parameters renamed as predictions, and no equations shown that equate the claimed distinction to its own data by definition. The result is therefore self-contained against external benchmarks (measured divergence and geometry) and receives only a minor score for the unverified isolation assumption.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only the abstract is available, so the ledger is populated at the level of standard quantum-mechanical assumptions for free-particle wavefunctions and beam propagation; no explicit free parameters, ad-hoc axioms, or invented entities are stated in the provided text.

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Reference graph

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