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arXiv:2510.20680 · Nanophotonics

Atomic state interferometry for complex vector light

Kuntal Samanta, Sphinx J. Svensson, Sonja Franke‐Arnold, Niclas Westerberg

Published Nov 6, 2025• 0 citations
arXivHTMLPDF

Abstract

Features of complex vector light become important in any interference effects, including scattering, diffraction, and nonlinear processes. Here, we are investigating the role of polarization-structured light in atomic state interferometers. Unlike optical or atomic path interferometers, these facilitate local interference between atomic transition amplitudes and hence the orthogonal optical polarization components driving these transitions. We develop a fully analytical description for the interaction of generalized structured light with an atomic four state system, that is, multiply connected via optical as well as magnetic transitions. Our model allows us to identify spatially dependent dark states, associated with spatially structured absorption coefficients, which are defined by the geometry of the polarization state and the magnetic field direction. We illustrate this for a range of optical beams including polarization vortices, optical skyrmions, and polarization lattices. This results in a new interpretation and an enhanced understanding of atomic state interferometry, and a versatile mechanism to modify and control optical absorption as a function of polarization and magnetic field alignment.

DOI
10.1515/nanoph-2025-0491

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Features of complex vector light become important in any interference effects, including scattering, diffraction, and nonlinear processes. Here, we are investigating the role of polarization-structured light in atomic state interferometers. Unlike optical or atomic path interferometers, these facilitate local interference between atomic transition amplitudes and hence the orthogonal optical polarization components driving these transitions. We develop a fully analytical description for the interaction of generalized structured light with an atomic four state system, that is, multiply connected via optical as well as magnetic transitions. Our model allows us to identify spatially dependent dark states, associated with spatially structured absorption coefficients, which are defined by the geometry of the polarization state and the magnetic field direction. We illustrate this for a range of optical beams including polarization vortices, optical skyrmions, and polarization lattices. This results in a new interpretation and an enhanced understanding of atomic state interferometry, and a versatile mechanism to modify and control optical absorption as a function of polarization and magnetic field alignment.
We investigate the transmission of vector beams, correlated in their polarization and spatial degrees of freedom, through cold atoms in the presence of a transverse magnetic coupling field. The resulting phase-dependent dynamics allow us to imprint the spatially varying polarization of a vector beam onto atomic spin polarizations, thereby establishing a direct link between optical space-polarization correlations and atomic-state interference. We find that the resulting absorption profiles show interference fringes whose modulation strength is given by the squared concurrence of the vector beam, letting us identify optical concurrence from a single absorption image. We expect impact across a diverse range of applications, including spintronics, quantum memories, metrology, and clocks.
We describe and demonstrate how 3D magnetic field alignment can be inferred from single absorption images of an atomic cloud. While optically pumped magnetometers conventionally rely on temporal measurement of the Larmor precession of atomic dipoles, here a cold atomic vapor provides a spatial interface between vector light and external magnetic fields. Using a vector vortex beam, we inscribe structured atomic spin polarization in a cloud of cold rubidium atoms and record images of the resulting absorption patterns. The polar angle of an external magnetic field can then be deduced with spatial Fourier analysis. This effect presents an alternative concept for detecting magnetic vector fields and demonstrates, more generally, how introducing spatial phases between atomic energy levels can translate transient effects to the spatial domain.
At Glasgow we routinely use a variety of techniques to imprint structure in the spatial amplitude, phase and polarization of a light beam. This allows us to generate vector vortex beams at high fidelity, with applications in polarimetry and strong focusing. Vector vortex beams can also induce magnetic dipole moments in an atomic medium, in our case rubidium atoms at 100 micro Kelvin, with an orientation that depends on the local polarization direction. This affects, in a nontrivial manner, the propagation of the structured light field through the atomic medium, dependent on an external magnetic field. We show that we can retrieve the orientation of the magnetic field from the absorption of the structured light - an atomic compass.
Spatially-structured laser beams, eventually carrying orbital angular momentum, affect electronic transitions of atoms and their motional states in a complex way. We present a general framework, based on the spherical tensor decomposition of the interaction Hamiltonian, for computing atomic transition matrix elements for light fields of arbitrary spatial mode and polarization structures. We study both the bare electronic matrix elements, corresponding to transitions with no coupling to the atomic center-of-mass motion, as well as the matrix elements describing the coupling to the quantized atomic motion in the resolved side-band regime. We calculate the spatial dependence of electronic and motional matrix elements for tightly focused Hermite-Gaussian, Laguerre-Gaussian and for radially and azimuthally polarized beams. We show that near the diffraction limit, all these beams exhibit longitudinal fields and field gradients, which strongly affect the selection rules and could be used to tailor the light-matter interaction. The presented framework is useful for describing trapped atoms or ions in spatially-structured light fields and therefore for designing new protocols and setups in quantum optics, -sensing and -information processing. We provide open code to reproduce our results or to evaluate interaction matrix elements for different transition types, beam structures and interaction geometries.
In our previous studies, we reported polarization-dependent profile variations in modulation transfer spectroscopy, including enhanced error signals and modified absorption profiles, which could not be fully explained by conventional models. In this work, we propose a unified physical framework to interpret these phenomena by introducing the concept of atomic spatial polarization-dependent transmission. Using hybrid polarized vector beams as both pump and probe in a counter-propagating configuration, we demonstrate that polarization-induced anisotropic absorption arises from optical pumping dynamics mediated by the vector beam structure. We further show that, when coherent population trapping (CPT) is introduced via dual-frequency laser excitation, the formation of an atomic dark-state population markedly enhances transmission contrast and gives rise to pronounced spectral features. We observed that narrowing the full width at half maximum of the saturated absorption spectrum nearly doubled the MTS signal amplitude. These effects manifest as spatially modulated intensity patterns in the probe beam, thereby revealing CPT-enhanced, polarization-selective transmission in atomic media. Our results not only explain previously observed polarization-induced signal enhancements but also offer new insights into spatially coherent manipulation of atomic populations, with implications for precision spectroscopy and laser stabilization.
Structured light, when strongly focused, generates highly confined vectorial electromagnetic field distributions which may feature a polarization component along the optical axis. Manipulating and detecting such 3D light fields is challenging, as conventional optical elements and detectors do not interact with the axial polarization component. Vector light can, however, be mapped onto atomic polarizations, making electric dipole transitions an ideal candidate to sense such 3D light configurations. Working in the hyperfine Paschen-Back regime, where the electric dipole transitions are spectrally resolved, we demonstrate direct evidence of the axial polarization component of strongly focused radial light. We investigate the influence of various input polarization states, including radial, azimuthal, and higher-order optical vortices, on atomic absorption profiles. Our results confirm a clear mapping between the 3D vector light and the atomic transition strength. This work provides new insights into vectorial light-matter interaction, and opens avenues for novel quantum sensing applications.