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Journal of the Optical Society of America B

Quantum-fluid q -plates in spinor Bose–Einstein condensates

Netzer Moriya

Published Mar 25, 2026• 0 citations
Publisher

Abstract

We propose a reconfigurable way to convert the polarization of light into orbital angular momentum using a spinor Bose–Einstein condensate (BEC). In conventional optics, this conversion is performed by a q -plate, a patterned birefringent element that couples circular polarization (spin angular momentum) to a helical wavefront (orbital angular momentum). Here, we show that an in-plane spin (or nematic) texture in a spinor BEC can play the same role: it imprints a geometric phase that converts incident circularly polarized light into a new orbital-angular-momentum channel set by the texture winding number q . In the low-loss, adiabatic regime, we derive a closed-form mapping for the transmitted field, which yields a quantized selection rule Δ ℓ =2 s q ( s =±1) and a simple back-action law for the reaction torque on the condensate. Notably, the predicted torque vanishes at q =1 in the ideal limit, providing a clear experimental signature of angular-momentum exchange. We analyze robustness to smooth azimuthal distortions and identify breakdown when defects form or loss dominates. Monte Carlo sampling and two-component beam-propagation simulations confirm the predicted conversion efficiency, OAM spectrum, and torque response.

DOI
10.1364/josab.589072

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We propose a reconfigurable way to convert the polarization of light into orbital angular momentum using a spinor Bose–Einstein condensate (BEC). In conventional optics, this conversion is performed by a q -plate, a patterned birefringent element that couples circular polarization (spin angular momentum) to a helical wavefront (orbital angular momentum). Here, we show that an in-plane spin (or nematic) texture in a spinor BEC can play the same role: it imprints a geometric phase that converts incident circularly polarized light into a new orbital-angular-momentum channel set by the texture winding number q . In the low-loss, adiabatic regime, we derive a closed-form mapping for the transmitted field, which yields a quantized selection rule Δ ℓ =2 s q ( s =±1) and a simple back-action law for the reaction torque on the condensate. Notably, the predicted torque vanishes at q =1 in the ideal limit, providing a clear experimental signature of angular-momentum exchange. We analyze robustness to smooth azimuthal distortions and identify breakdown when defects form or loss dominates. Monte Carlo sampling and two-component beam-propagation simulations confirm the predicted conversion efficiency, OAM spectrum, and torque response.
Abstract A coherent two-photon optical Raman interaction in a pseudo-spin-1/2 Bose–Einstein condensate (BEC) serves as a q -plate for atoms, converting spin to orbital angular momentum. This Raman q -plate has a singular pattern in its polarization distribution in analogy to the singular birefringent q -plates used in singular optics. The vortex winding direction and magnitude as well as the final spin state of the BEC depend on the initial spin state and the topology of the optical Raman q -plate beams. Drawing on the mathematical and geometric foundations of singular optics, we derive the equivalent Jones matrix for this Raman q -plate and use it to create and characterize atomic spin singularities in the BEC that are analogous to optical C -point singularities in polarization. By tuning the optical Raman parameters, we can generate a coreless vortex spin texture which contains every possible superposition in a two-state system. We identify this spin texture as a full-Bloch BEC since every point on the Bloch sphere is represented at some point in the cross section of the atomic cloud. This spin–orbit interaction and the spin textures it generates may allow for the observation of interesting geometric phases in matter waves and lead to schemes for topological quantum computation with spinor BECs.
We demonstrate coupling between the atomic spin- and orbital-angular momentum (OAM) of the atom's center-of-mass motion in a Bose-Einstein condensate (BEC). The coupling is induced by Raman-dressing lasers with a Laguerre-Gaussian beam and creates coreless vortices in an F=1 ^{87}Rb spinor BEC. We observe correlations between spin and OAM in the dressed state and characterize the spin texture; the result is in good agreement with the theory. In the presence of the Raman field, our dressed state is stable for 0.1 s or longer, and it decays due to collision-induced relaxation. As we turn off the Raman beams, the vortex cores in the bare spin |m_{F}=1⟩ and |-1⟩ split. These spin-OAM coupled systems with the Raman-dressing approach have great potential for exploring new topological textures and quantum states.
We present experimental results on the coherent transfer of orbital angular momentum (OAM) from optical fields to the center-of-mass motion of a Bose-Einstein condensate (BEC), using an approach which results in negligible linear momentum change. Two collinear optical beams of differing OAM are used to couple discrete sublevels of the atomic ground state via a stimulated Raman process. We demonstrate several of the possible optical field configurations which place the BEC into various spinor and vortex states. We also show how this approach can be used to create coherent superpositions of these states and use the interference patterns of such superpositions to confirm the presence of quantized vortices.
Light orbital angular momentum (OAM) has been recognized as a new promising resource for classical and quantum information applications. In contrast to the spin angular momentum, the OAM is an inherently multidimensional. Thus, the information can be encoded in the higher-dimensional OAM alphabets. Recently, Marrucci et al. have invented a new device named q-plate (QP), made of liquid crystal cell patterned in such a way to introduce a topological charge q at the transverse plane, which is able to generate a well-defined values of photon OAM. My research has been aimed at investigating the physics of the QP, of the optical fields that it generates and of its possible applications for optical communication and quantum information. We studied both theoretically and experimentally a novel set of non-orthogonal but over-complete paraxial modes, named Hypergeometric-Gaussian modes, that is typically associated with OAM-carrying optical fields. We have also found the light propagation kernel inside the QP and we have shown analytically that if small losses due to reflection, absorption, and scattering are neglected, the QP can convert the photon spin into OAM with up to 100 \% efficiency. We implemented a technique to control the QP optical retardation by tuning the QP temperature. At the optimal temperature, the QP can generate a beam with up to 97\% efficiency. Moreover, the OAM state generated by QP can be rotated easily in the 2D OAM Hilbert space by proper manipulation of the input polarization state, a fact which opens a new way of beam shape controlling in MHz scale. We also performed a novel way to encode and read two bits information on the 4D OAM space by using only one QP. Finally, we experimentally demonstrated the transfer of quantum information from spin to OAM and vice versa, including the case of bi-photon states having quantum correlations. Furthermore, by exploiting these quantum information transfer devices, we demonstrated the Hong-Ou-Mandel effect and the optimal quantum cloning with OAM-carrying photons.
The Einstein-de Haas effect is a phenomenon in which angular momentum is transferred from microscopic spins to mechanical rotation of a macroscopic rigid body. We report an observation of the Einstein-de Haas effect in a spinor-dipolar Bose-Einstein condensate, in which the intrinsic magnetic dipole-dipole interaction mediates coherent transfer of angular momentum from atomic spins to collective circulation of a quantum fluid. The depolarized spinor components displayed ring-shaped density distributions that were confirmed as quantized vortices through matter-wave interferometry, revealing a coherent conversion between spin and orbital angular momentum. This observation opens a pathway to exploring ground-state phases with broken chiral symmetry, spin textures, and mass circulation, as well as the Barnett effect in dipolar quantum gases.
Theoretical study is presented for a spinor Bose-Einstein condensate, whose two components are coupled by copropagating Raman beams with different orbital angular momenta. The investigation is focused on the behavior of the ground state of this condensate, depending on the atom-light coupling strength. By analyzing the ground state, we have identified a number of quantum phases, which reflect the symmetries of the effective Hamiltonian and are characterized by the specific structure of the wave function. In addition to the well-known stripe, polarized, and zero-momentum phases, our results show that the system can support phases whose wave functions contain a complex vortex molecule. Such a molecule plays an important role in the continuous phase transitions of the system. The predicted behavior of vortex-molecule phases can be examined in cold-atom experiments using currently existing techniques.