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arXiv:2604.03607

Interaction of twisted light with free twisted atoms

I. Pavlov, A. Chaikovskaia, D. Karlovets

quant-phphysics.atom-phSubmitted Apr 4, 2026• Announced Apr 7, 2026• 0 citations• Revised Jul 13, 2026 (v2)
v1 · Apr 4, 2026v2 · Jul 13, 2026
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Abstract

We investigate absorption and scattering of structured light by atoms, treating the photon and the atomic center of mass as spatially localized wave packets. We show that vortex photons can transfer orbital angular momentum (OAM) to the atomic center of mass with near-perfect efficiency in head-on collisions when the impact parameter $b$ is smaller than the atomic transverse coherence length $σ$, which ranges from nanometers to sub-micrometer scales. Larger offsets result in a shifted mean OAM and a finite variance, both controlled by the ratio $b/σ$. The wave-packet nature of light enables electronic transitions that violate standard selection rules, albeit with a clear hierarchy where the dipole transition dominates. For femtosecond pulses, the finite spatial coherence of the photon leads to measurable shaping of the resonant absorption lines. We demonstrate a transverse recoil of the atom in a vicinity of the photonic vortex, dubbed "the superkick", and its dual effect - "the selfkick" - when an initially twisted atomic packet experiences recoil upon absorbing a gaussian photon. These phenomena are within reach of experimental capabilities using structured light in combination with cold atomic beams and ions in Penning traps, providing a route to the controlled generation and manipulation of non-gaussian atomic packets.

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We investigate absorption and scattering of structured light by atoms, treating the photon and the atomic center of mass as spatially localized wave packets. We show that vortex photons can transfer orbital angular momentum (OAM) to the atomic center of mass with near-perfect efficiency in head-on collisions when the impact parameter bbb is smaller than the atomic transverse coherence length σσσ, which ranges from nanometers to sub-micrometer scales. Larger offsets result in a shifted mean OAM and a finite variance, both controlled by the ratio b/σb/σb/σ. The wave-packet nature of light enables electronic transitions that violate standard selection rules, albeit with a clear hierarchy where the dipole transition dominates. For femtosecond pulses, the finite spatial coherence of the photon leads to measurable shaping of the resonant absorption lines. We demonstrate a transverse recoil of the atom in a vicinity of the photonic vortex, dubbed "the superkick", and its dual effect - "the selfkick" - when an initially twisted atomic packet experiences recoil upon absorbing a gaussian photon. These phenomena are within reach of experimental capabilities using structured light in combination with cold atomic beams and ions in Penning traps, providing a route to the controlled generation and manipulation of non-gaussian atomic packets.
We investigate absorption and scattering of structured light by atoms, treating the photon and the atomic center of mass as spatially localized wave packets. We show that vortex photons can transfer orbital angular momentum (OAM) to the atomic center of mass with near-perfect efficiency in head-on collisions when the impact parameter b is smaller than the atomic transverse coherence length σ , which ranges from nanometers to sub-micrometer scales. Larger offsets result in a shifted mean OAM and a finite variance, both controlled by the ratio b / σ . The wave-packet nature of light enables electronic transitions that violate standard selection rules, albeit with a clear hierarchy where the dipole transition dominates. For femtosecond pulses, the finite spatial extent of the photon leads to measurable shaping of the resonant absorption lines. We demonstrate a transverse recoil of the atom in a vicinity of the photonic vortex, dubbed “the superkick,” and its dual effect—“the selfkick”—when an initially twisted atomic packet experiences recoil upon absorbing a Gaussian photon. These phenomena are within reach of experimental capabilities using structured light in combination with cold atomic beams and ions in Penning traps, providing a route to the controlled generation and manipulation of non-Gaussian atomic packets.
Photons can carry angular momentum, not only due to their spin, but also due to their spatial structure. This extra twist has been used, for example, to drive circular motion of microscopic particles in optical tweezers as well as to create vortices in quantum gases. Here we excite an atomic transition with a vortex laser beam and demonstrate the transfer of optical orbital angular momentum to the valence electron of a single trapped ion. We observe strongly modified selection rules showing that an atom can absorb two quanta of angular momentum from a single photon: one from the spin and another from the spatial structure of the beam. Furthermore, we show that parasitic ac-Stark shifts from off-resonant transitions are suppressed in the dark centre of vortex beams. These results show how light's spatial structure can determine the characteristics of light-matter interaction and pave the way for its application and observation in other systems.
Photons carry one unit of angular momentum associated with their spin~\cite{Beth1936}. Structured vortex beams carry additional orbital angular momentum which can also be transferred to matter~\cite{Allen1992}. This extra twist has been used for example to drive motion of microscopic particles in optical tweezers as well as to create vortices in degenerate quantum gases~\cite{He1995,Andersen2006}. Here we demonstrate the transfer of optical orbital angular momentum from the transverse spatial structure of the beam to the internal (electronic) degrees of freedom of an atom. Probing a quadrupole transition of a single trapped 40^{40}40Ca+^++ ion localized at the center of the vortex, we observe strongly modified selection rules, accounting for both the photon spin and the vorticity of the field. In particular, we show that an atom can absorb two quanta of angular momentum from a single photon even when rotational symmetry is conserved. In contrast to previous findings~\cite{Araoka2005,Loeffler2011a,Mathevet2013}, our experiment allows for conditions where the vorticity of the laser beam determines the optical excitation, contributing to the long-standing discussion on whether the orbital angular momentum of photons can be transferred to atomic internal degrees of freedom~\cite{VanEnk1994,Babiker2002,Jauregui2004, Schmiegelow2012, Mondal2014, Scholz-Marggraf2014} and paves the way for its use to tailor light-matter interactions.
An atom placed in an optical vortex close to the axis may, upon absorbing a photon, acquire a transverse momentum much larger than the transverse momentum of any plane-wave component of the vortex lightfield. Although this surprising phenomenon dubbed superkick has been clarified in terms of the atom wave packet evolution in the field of optical vortex, a fully quantum treatment is still lacking. Here, we study the effect within the quantum field theoretical framework. We consider collision of a Bessel twisted wave with a compact Gaussian beam focused to a small focal spot σ\sigmaσ located at distance b from the twisted beam axis. Through a qualitative discussion supported by exact analytical and numerical calculations, we recover the superkick phenomenon for σ≪b\sigma \ll bσ≪b and explore its limits when σ\sigmaσ becomes comparable to b. We also comment on the recent suggestions to observe this effect in nuclear or high-energy physics.
We present a series of four experiments unveiling intriguing characteristics of structured beams when interacting with individual, well-localized atoms. The initial investigation explores the placement of a single atom within the dark center of a vortex/doughnut beam, revealing unexpected excitations. Furthermore, the study demonstrates the transfer of the beam’s chirality to the internal and external motion of the atom, as evidenced by alterations in allowed atomic transitions. The research showcases the ability of both intrinsic (polarization) and extrinsic (orbital/structural) angular momentum of the beam to influence the atom, resulting in the transfer of two units of angular momentum. Notably, structured beams induce motion transversal to their propagation direction. The paper concludes with a position-resolved measurement of the azimuthal Doppler shift of a structured beam, uncovering its characteristic divergence at the center and its scale-invariant nature. These findings offer valuable insights into the understanding of structured beam-atom interactions, potentially constituting the first indirect observation of the elusive super-kicks predicted by Barnett and Berry in 2013.
Near a vortex in a monochromatic light beam, the length of the local wavevector (phase gradient) can exceed the wavenumber in any of the plane waves in the superposition representing the beam. One way to detect these ‘superweak’ momenta could be by ‘superkicks’ imparted to a small particle located near the vortex, by absorbing individual large-momentum photons from the beam. A model for this process is a two-level atom with a transition resonant with the light beam. A semiclassical analysis shows that the momentum distribution of the atom is shifted by interaction with the vortex beam, by amounts that can almost reach the target superkicks and are usually greater than the momenta in the plane waves comprising the beam.