pith. sign in

arxiv: 2605.15408 · v1 · pith:NL4Z5DBOnew · submitted 2026-05-14 · ⚛️ physics.optics · quant-ph

Second-order moment equivalence of twisted Gaussian Schell model beams and orbital angular momentum eigenmodes

Pith reviewed 2026-05-19 15:26 UTC · model grok-4.3

classification ⚛️ physics.optics quant-ph
keywords orbital angular momentumtwisted Gaussian Schell-model beamscovariance matrixsecond-order momentsbeam propagationABCD transformationsLaguerre-Gaussian beams
0
0 comments X

The pith

Covariance matrices of coherent OAM eigenmodes and twisted Gaussian Schell-model beams are identical

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

The paper shows that the covariance matrix of any cylindrically symmetric coherent orbital angular momentum eigenmode with quantum number ℓ depends only on the average squared radius, average squared radial wave number, and ℓ itself. This matrix has exactly the same pattern of zero and nonzero entries as the covariance matrix of a twisted Gaussian Schell-model beam, with off-diagonal blocks proportional to ℓ and the twist parameter respectively. A reader would care because the covariance matrix controls how second moments transform under lenses and free space, so the two families of beams evolve identically in beam width, far-field divergence, and beam quality factor. The result holds for arbitrary radial profiles and supplies explicit parameter mappings for Laguerre-Gaussian, Bessel-Gaussian, and perfect vortex beams.

Core claim

The covariance matrix of any cylindrically symmetric coherent orbital angular momentum eigenmode with quantum number ℓ takes a universal form depending only on ⟨r²⟩, ⟨k_r²⟩, and ℓ, independently of the radial profile. This form is identical to the covariance matrix of a twisted Gaussian Schell-model beam. Both matrices share the same pattern of zero and nonzero entries, with the off-diagonal blocks proportional to ℓ and the TGSM twist parameter u respectively. This holds for arbitrary radial profiles and provides direct term-by-term identification of parameters between the two sets of beams.

What carries the argument

The phase-space covariance matrix that encodes the second moments of transverse position and transverse wave vector for a light beam.

Load-bearing premise

That the covariance matrix alone fully governs second-moment evolution under arbitrary ABCD transformations.

What would settle it

Observing a difference in beam-width evolution or M² factor between a matched Laguerre-Gaussian beam and its corresponding twisted Gaussian Schell-model beam after passage through a thin lens would falsify the claimed equivalence.

read the original abstract

We show that the covariance matrix of any cylindrically symmetric coherent orbital angular momentum (OAM) eigenmode with quantum number $\ell$ takes a universal form depending only on $\langle r^2\rangle$, $\langle k_r^2\rangle$, and $\ell$, independently of the radial profile, and that this form is identical to the covariance matrix of a twisted Gaussian Schell-model (TGSM) beam.} More specifically, both matrices share the same pattern of zero and nonzero entries, with the off-diagonal blocks proportional to $\ell$ and the TGSM twist parameter $u$, respectively. This result holds for an arbitrary radial profile and provides direct term-by-term identification of parameters between the two sets of beams. We work out the correspondence in detail for three important families: Laguerre--Gaussian (LG), Bessel--Gaussian, and perfect vortex beams (PVBs), and derive the conditions under which each coherent OAM mode maps onto a physically realizable TGSM beam. {Because the covariance matrix governs second-moment evolution under arbitrary ABCD (symplectic) transformations, any two beams sharing the same covariance matrix are second-order indistinguishable at every propagation plane. In particular, the matched TGSM and coherent OAM beams share identical beam-width evolution, far-field divergence, and $M^2$ beam-quality factor.} In particular, the well-developed TGSM propagation toolbox applies directly to the second-order moment evolution of the three coherent families. We further show that within each beam family the covariance matrix uniquely determines the beam parameters, with exact uniqueness established for LG modes. Additional results include cross-family second-moment equivalence conditions and a proof that PVB modes form a complete orthonormal basis in the limit $w\to 0$.

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

1 major / 2 minor

Summary. The manuscript claims that the covariance matrix of any cylindrically symmetric coherent OAM eigenmode with quantum number ℓ takes a universal form depending only on ⟨r²⟩, ⟨k_r²⟩, and ℓ, independently of the radial profile. This form is identical to the covariance matrix of a TGSM beam, with matching patterns of zero and nonzero entries and off-diagonal blocks proportional to ℓ (or the TGSM twist parameter u). Explicit parameter mappings are derived for LG, Bessel-Gaussian, and perfect vortex beams, along with conditions for physical realizability, uniqueness within each family (exact for LG), cross-family equivalence, and a completeness result for PVBs as w→0. Because the covariance matrix governs second-moment evolution under ABCD transformations, matched beams are second-order indistinguishable in beam-width evolution, far-field divergence, and M² factor, allowing direct application of the TGSM propagation toolbox.

Significance. If the central equivalence holds, the work would usefully connect fully coherent OAM eigenmodes to the partially coherent TGSM framework, enabling reuse of established TGSM second-moment propagation formulas for OAM beams. The explicit mappings for three families, the uniqueness proof for LG modes, and the PVB completeness result are concrete strengths that would support the result's utility in optics.

major comments (1)
  1. [Abstract] Abstract and the derivation of the universal covariance form: the claim that the covariance matrix depends only on ⟨r²⟩, ⟨k_r²⟩, and ℓ independently of radial profile is contradicted by the structure of the transverse momentum variances. Symmetry gives ⟨k_x²⟩ = ⟨k_y²⟩ = (⟨k_r²⟩ + ℓ² ⟨1/r²⟩)/2. For fixed ⟨r²⟩ and ⟨k_r²⟩, ⟨1/r²⟩ varies across radial profiles (e.g., LG vs. Bessel-Gaussian or annular), so the momentum variances and thus the full covariance matrix differ. This prevents a profile-independent universal form and undermines the term-by-term identification with TGSM beams.
minor comments (2)
  1. The definition and normalization of ⟨k_r²⟩ should be stated explicitly with reference to the radial and azimuthal decomposition to avoid ambiguity in the momentum block.
  2. Figure captions for the three beam families could include the explicit parameter mappings derived in the text for easier cross-reference.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments on our manuscript. The major comment raises an important point about the profile dependence of certain covariance matrix elements, which we address directly below. We agree that a revision is warranted to clarify the scope of the claimed universality.

read point-by-point responses
  1. Referee: [Abstract] Abstract and the derivation of the universal covariance form: the claim that the covariance matrix depends only on ⟨r²⟩, ⟨k_r²⟩, and ℓ independently of radial profile is contradicted by the structure of the transverse momentum variances. Symmetry gives ⟨k_x²⟩ = ⟨k_y²⟩ = (⟨k_r²⟩ + ℓ² ⟨1/r²⟩)/2. For fixed ⟨r²⟩ and ⟨k_r²⟩, ⟨1/r²⟩ varies across radial profiles (e.g., LG vs. Bessel-Gaussian or annular), so the momentum variances and thus the full covariance matrix differ. This prevents a profile-independent universal form and undermines the term-by-term identification with TGSM beams.

    Authors: We appreciate the referee identifying this subtlety. We agree that ⟨k_x²⟩ and ⟨k_y²⟩ explicitly involve the profile-dependent moment ⟨1/r²⟩, which is not determined by ⟨r²⟩ and ⟨k_r²⟩ alone. Consequently, the full covariance matrix is not independent of the radial profile in the manner stated. The structural features that are universal—the zero/nonzero pattern and the off-diagonal blocks scaling with ℓ (or u)—remain valid, but the specific values of the momentum variances require additional profile-specific information. We will revise the abstract and relevant derivation sections to qualify the universality claim, explicitly noting the role of ⟨1/r²⟩ and adjusting the parameter mappings for each family (LG, Bessel-Gaussian, PVB) to incorporate it where needed. The central result on second-order equivalence under ABCD transformations is unaffected, since once all relevant moments are matched the beams remain indistinguishable at the second-moment level. This clarification strengthens rather than weakens the connection to the TGSM framework. revision: yes

Circularity Check

0 steps flagged

No circularity: covariance matrix form derived directly from OAM mode definitions and symmetry

full rationale

The paper derives the claimed universal covariance matrix form for cylindrically symmetric OAM eigenmodes by explicit computation of second moments from the mode wavefunctions (or their Fourier transforms), using cylindrical symmetry to fix the pattern of zeros and the proportionality of off-diagonal blocks to ℓ. This holds independently of radial profile because the relevant cross terms and variances are fixed by the angular momentum operator and the definitions of ⟨r²⟩ and ⟨k_r²⟩; explicit verification is provided for LG, Bessel-Gaussian, and PVB families. No parameter is fitted to data and then relabeled a prediction, no self-citation supplies a load-bearing uniqueness theorem, and no ansatz is smuggled in. The equivalence to TGSM matrices follows term-by-term from matching the computed entries, after which ABCD propagation is invoked as a standard symplectic fact. The derivation is therefore self-contained and does not reduce to its inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard properties of paraxial beam covariance matrices and the domain assumption that second moments determine ABCD propagation; no free parameters or new entities are introduced in the abstract.

axioms (1)
  • domain assumption The covariance matrix governs second-moment evolution under arbitrary ABCD (symplectic) transformations
    Invoked in the abstract to conclude that beams sharing the same covariance matrix are second-order indistinguishable at every propagation plane.

pith-pipeline@v0.9.0 · 5878 in / 1338 out tokens · 57427 ms · 2026-05-19T15:26:51.454751+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Reference graph

Works this paper leans on

44 extracted references · 44 canonical work pages · 1 internal anchor

  1. [1]

    Allen, M

    L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, Physical Review A45, 8185 (1992)

  2. [2]

    Bagini, F

    V. Bagini, F. Frezza, M. Santarsiero, G. Schettini, and G. Schirripa Spagnolo, Journal of Modern Optics43, 1155 (1996)

  3. [3]

    McGloin and K

    D. McGloin and K. Dholakia, Contemporary Physics46, 15 (2005)

  4. [4]

    Molina-Terriza, J

    G. Molina-Terriza, J. P. Torres, and L. Torner, Nature Physics3, 305 (2007)

  5. [5]

    A. E. Willner, H. Huang, Y. Yan, Y. Ren, N. Ahmed, G. Xie, C. Bao, L. Li, Y. Cao, Z. Zhao, et al., Adv. Opt. Photon.7, 66 (2015), URLhttps://opg.optica.org/ aop/abstract.cfm?URI=aop-7-1-66

  6. [6]

    M. J. Padgett, Opt. Express25, 11265 (2017), URLhttps://opg.optica.org/oe/abstract.cfm?URI= oe-25-10-11265

  7. [7]

    Rubinsztein-Dunlop, A

    H. Rubinsztein-Dunlop, A. Forbes, M. V. Berry, M. R. Dennis, D. L. Andrews, M. Mansuripur, C. Denz, C. Alp- mann, P. Banzer, T. Bauer, et al., Journal of Optics19, 013001 (2017)

  8. [8]

    A. M. Yao and M. J. Padgett, Advances in Optics and Photonics3, 161 (2011)

  9. [9]

    High-Dimensional Quantum Photonics: Roadmap

    M. Malik, M. Kues, T. Ikuta, H. Takesue, D. Bajoni, D. J. Moss, R. Morandotti, A. Forbes, S. Walborn, E. Karimi, et al.,High-dimensional quantum photonics: Roadmap(2026), 2604.06528, URLhttps://arxiv.org/ abs/2604.06528

  10. [10]

    S. P. Walborn, A. N. de Oliveira, R. S. Thebaldi, and C. H. Monken, Phys. Rev. A69, 023811 (2004)

  11. [11]

    D’Ambrosio, N

    V. D’Ambrosio, N. Spagnolo, L. Del Re, S. Slus- sarenko, Y. Li, L. C. Kwek, L. Marrucci, S. P. Wal- born, L. Aolita, and F. Sciarrino, Nature Communi- cations4, 2432 (2013), ISSN 2041-1723, URLhttps: //doi.org/10.1038/ncomms3432

  12. [12]

    Gori, Optics Communications46, 149 (1983)

    F. Gori, Optics Communications46, 149 (1983)

  13. [13]

    Starikov and E

    A. Starikov and E. Wolf, Journal of the Optical Society of America72, 923 (1982)

  14. [14]

    Simon and N

    R. Simon and N. Mukunda, J. Opt. Soc. Am. A10, 95 (1993)

  15. [15]

    F. Gori, M. Santarsiero, R. Borghi, and S. Vicalvi, Jour- nal of Modern Optics45, 539 (1998)

  16. [16]

    Gori and M

    F. Gori and M. Santarsiero, Opt. Lett.40, 1587 (2015), URLhttps://opg.optica.org/ol/abstract.cfm?URI= ol-40-7-1587

  17. [17]

    Wang and Y

    F. Wang and Y. Cai, Opt. Express18, 24661 (2010), URLhttp://www.opticsexpress.org/abstract.cfm? URI=oe-18-24-24661

  18. [18]

    Y. Liu, X. Liu, L. Liu, F. Wang, Y. Zhang, and Y. Cai, Applied Sciences9(2019), ISSN 2076-3417, URLhttps: //www.mdpi.com/2076-3417/9/15/3023

  19. [19]

    Tong and O

    Z. Tong and O. Korotkova, Opt. Lett.37, 2595 (2012), URLhttps://opg.optica.org/ol/abstract.cfm?URI= ol-37-13-2595

  20. [20]

    F. Wang, Y. Cai, H. T. Eyyubo˘ glu, and Y. Baykal, Opt. Lett.37, 184 (2012), URLhttp://opg.optica.org/ol/ abstract.cfm?URI=ol-37-2-184

  21. [21]

    C. Zhao, Y. Cai, and O. Korotkova, Opt. Express 17, 21472 (2009), URLhttps://opg.optica.org/oe/ abstract.cfm?URI=oe-17-24-21472

  22. [22]

    A. T. Friberg, E. Tervonen, and J. Turunen, J. Opt. Soc. Am. A11, 1818 (1994)

  23. [23]

    H. Wang, X. Peng, L. Liu, F. Wang, Y. Cai, and S. A. Ponomarenko, Opt. Lett.44, 3709 (2019), URLhttp: //ol.osa.org/abstract.cfm?URI=ol-44-15-3709

  24. [24]

    C. Tian, S. Zhu, H. Huang, Y. Cai, and Z. Li, Opt. Lett. 45, 5880 (2020), URLhttp://ol.osa.org/abstract. cfm?URI=ol-45-20-5880

  25. [25]

    H. Wang, X. Peng, H. Zhang, L. Liu, Y. Chen, F. Wang, and Y. Cai, Nanophotonics p. 000010151520210432 (2021), URLhttps://doi. org/10.1515/nanoph-2021-0432

  26. [26]

    Ca˜ nas, E

    G. Ca˜ nas, E. S. G´ omez, G. H. dos Santos, A. G. de Oliveira, N. R. da Silva, S. Joshi, Y. Ismail, P. H. S. Ribeiro, and S. P. Walborn, J. Opt.24, 094004 (2022)

  27. [27]

    Hutter, G

    L. Hutter, G. Lima, and S. P. Walborn, Phys. Rev. Lett. 125, 193602 (2020), URLhttps://link.aps.org/doi/ 10.1103/PhysRevLett.125.193602

  28. [28]

    Hutter, E

    L. Hutter, E. S. Gomez, G. Lima, and S. P. Walborn, AVS Quantum Science3, 031401 (2021), URLhttps: //avs.scitation.org/doi/10.1116/5.0058681

  29. [29]

    G. H. dos Santos, A. G. de Oliveira, N. R. da Silva, G. Ca˜ nas, E. S. G´ omez, S. Joshi, Y. Ismail, P. H. S. Ribeiro, and S. P. Walborn, Nanophoton- ics11, 763 (2022), URLhttps://doi.org/10.1515/ nanoph-2021-0502

  30. [30]

    M. G. de Oliveira, A. Santos, A. Barbosa, B. P. da Silva, G. dos Santos, G. Ca˜ nas, P. S. Ribeiro, S. Walborn, and A. Khoury, Optics & Laser Tech- nology176, 110983 (2024), ISSN 0030-3992, URL https://www.sciencedirect.com/science/article/ pii/S0030399224004419

  31. [31]

    G. d. Santos, A. Santos Junior, M. Gil de Oliveira, A. Barbosa, B. Pinheiro da Silva, N. Rubiano da Silva, G. Ca˜ nas, S. Walborn, A. Khoury, and P. S. Ribeiro, Phys. Rev. Appl.24, 034075 (2025), URLhttps:// link.aps.org/doi/10.1103/63bq-lq1l

  32. [32]

    C. Gao, G. Wei, and H. Weber, Science in China (Series A)43, 1306 (2000)

  33. [33]

    Serna, F

    J. Serna, F. Encinas-Sanz, and G. Nemes, Journal of the Optical Society of America A18, 1726 (2001). 16

  34. [34]

    Simon, K

    R. Simon, K. Sundar, and N. Mukunda, J. Opt. Soc. Am. A10, 2008 (1993), URLhttp://josaa.osa.org/ abstract.cfm?URI=josaa-10-9-2008

  35. [35]

    Simon and N

    R. Simon and N. Mukunda, J. Opt. Soc. Am. A15, 2373 (1998)

  36. [36]

    Ribeiro and S

    G. Vallone, G. Parisi, F. Spinello, E. Mari, F. Tam- burini, and P. Villoresi, Phys. Rev. A94, 023802 (2016), URLhttps://link.aps.org/doi/10.1103/PhysRevA. 94.023802

  37. [37]

    Rojas-Rojas, G

    S. Rojas-Rojas, G. C. nas, G. Saavedra, E. S. G´ omez, S. P. Walborn, and G. Lima, Opt. Express29, 23381 (2021), URLhttps://opg.optica.org/oe/abstract. cfm?URI=oe-29-15-23381

  38. [38]

    Villalba, C

    N. Villalba, C. Melo, S. Ayala, C. Mancilla, W. Valen- zuela, M. Figueroa, E. Baradit, R. Lin, M. Tang, S. P. Walborn, et al., Opt. Express31, 40113 (2023), URLhttps://opg.optica.org/oe/abstract.cfm?URI= oe-31-24-40113

  39. [39]

    Pinnell, V

    J. Pinnell, V. Rodr´ ıguez-Fajardo, and A. Forbes, Opt. Lett.44, 5614 (2019), URLhttps://opg.optica.org/ ol/abstract.cfm?URI=ol-44-22-5614

  40. [40]

    Ahmed, Z

    N. Ahmed, Z. Zhao, L. Li, H. Huang, M. P. J. Lavery, P. Liao, Y. Yan, Z. Wang, G. Xie, Y. Ren, et al., Scientific Reports6, 22082 (2016), ISSN 2045-2322, URLhttps: //doi.org/10.1038/srep22082

  41. [41]

    D. Deng, H. Guo, D. Han, and C. Li, Op- tics Communications238, 205 (2004), ISSN 0030- 4018, URLhttps://www.sciencedirect.com/science/ article/pii/S0030401804004675

  42. [42]

    I. S. Gradshteyn and I. M. Ryzhik,Table of Integrals, Se- ries, and Products(Elsevier Academic Press, Burlington, MA, 2007), 7th ed

  43. [43]

    A. M. Perelomov,Generalized Coherent States and Their Applications(Springer, Berlin, 1986)

  44. [44]

    G. B. Arfken, H. J. Weber, and F. E. Harris,Mathe- matical Methods for Physicists: A Comprehensive Guide (Academic Press, Waltham, MA, 2013), 7th ed