Comment on "Angular momentum dynamics of vortex particles in accelerators''

TL;DR

This paper critiques a proposed equation for vortex particle angular momentum in accelerators, showing it lacks general validity and emphasizing the need for a detailed quantum state analysis.

Contribution

It demonstrates the limitations of a BMT-like equation for mean kinetic OAM and advocates for a mode-resolved quantum approach.

Findings

The proposed closure depends on second moments, leading to incompatible oscillations.

Assumption of negligible mixed correlators suppresses transverse OAM components.

Mean-OAM transport does not capture quantum state spectra or coherences.

Abstract

We comment on Ref.[D. Karlovets, D. Grosman, and I. Pavlov, Phys. Rev. Lett. 136, 085002 (2026)], which proposes a BMT-like equation for the mean kinetic orbital angular momentum (OAM) of vortex particles in accelerator fields and draws spin-like conclusions about depolarization, resonances, and control. We show that the proposed closure is not generally valid even at the mean-value level. In the authors' own homogeneous-field model, Eq.(8) already makes $\langle L_z\rangle$ depend on the packet second moment $\langle \rho^2\rangle(\tau)$; for an exact family of breathing Landau/LG packets this yields an explicit oscillation incompatible with Eq.(9) except in the nongeneric matched case. Moreover, the Appendix A assumption that mixed correlators are negligible suppresses the transverse kinetic-OAM components themselves, since those correlators are precisely the building blocks of $L_x$ and $L_y$. We also stress that, even if a closed equation for $\langle \hat{\mathbf L}\rangle$ were available, it would still not constitute a transport equation for a vortex quantum state. Mean-OAM transport does not determine OAM spectra, inter-mode coherences, or fidelity. State-level claims therefore require a mode-resolved density-matrix treatment rather than an Ehrenfest equation for a low-order moment.