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arXiv:1803.07860 · Physical Review Letters

Spin–Orbital-Angular-Momentum Coupled Bose-Einstein Condensates

Haiqing Chen, Kuan-Yu Lin, P.-K. Chen, N.-C. Chiu, J.-B. Wang, C.-A. Chen, Panpan Huang, Sungkit Yip, Yuki Kawaguchi, Y.-J. Lin

Published Sep 13, 2018• 93 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevlett.121.113204

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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.
We examine the use of Laguerre-Gaussian (LG) laser beams in the tripod scheme to create the coupling between spin and orbital angular momentum in a Bose–Einstein condensate (BEC) confined by a harmonic trap. We derive and solve the Gross-Pitaeskii equation numerically to obtain the density distribution of the BEC. Orbital angular momentum from the LG beam is transferred to the BEC, inducing the formation of a giant vortex at the BEC’s centre. The macroscopic flow of spinor BEC around the vortex leads to a persistent spin current in the BEC.
The exchange of orbital angular momentum (OAM) between paraxial optical vortex and a Bose-Einstein condensate (BEC) of atomic gases is well known. In this paper, we develop a theory for the microscopic interaction between matter and an optical vortex beyond paraxial approximation. We show how superposition of vortex states of BEC can be created with a focused optical vortex. Since the polarization or spin angular momentum (SAM) of the optical field is coupled with OAM of the field, in this case, these angular momenta can be transferred to the internal electronic and external center-of-mass motion of atoms provided both the motions are coupled. We propose a scheme for producing the superposition of matter-wave vortices using Gaussian and a focused Laguerre-Gaussian beam. We study how two-photon Rabi frequencies of stimulated Raman transitions vary with focusing angles for different combinations of OAM and SAM of optical states. We demonstrate the formation of vortex-antivortex structure and discuss interference of three vortex states in a BEC.
By inducing a Raman transition using a pair of Gaussian and Laguerre-Gaussian laser beams, we realize a ^{87}Rb condensate whose orbital angular momentum (OAM) and its internal spin states are coupled. By varying the detuning and the coupling strength of the Raman transition, we experimentally map out the ground-state phase diagram of the system for the first time. The transitions between different phases feature a discontinuous jump of the OAM and the spin polarization, and hence are of first order. We demonstrate the hysteresis loop associated with such first-order phase transitions. The role of interatomic interaction is also elucidated. Our work paves the way to explore exotic quantum phases in the spin-orbital-angular-momentum coupled quantum gases.
The experimental realization of emergent spin-orbit coupling through laser-induced Raman transitions in ultracold atoms paves the way for exploring novel superfluid physics and simulating exotic many-body phenomena. A recent proposal with the use of Laguerre-Gaussian lasers enables another fundamental type of coupling between spin and orbital angular momentum (SOAM) in ultracold atoms. We hereby study quantum phases of a realistic Bose-Einstein condensate (BEC) with this synthetic SOAM coupling in a disk-shaped geometry, respecting radial inhomogeneity of the Raman coupling. We find that the experimental system naturally resides in a strongly interacting regime in which the phase diagram significantly deviates from the single-particle picture. The interplay between SOAM coupling and interaction leads to rich structures in spin-resolved position and momentum distributions, including a stripe phase and various types of immiscible states. Our results would provide a guide for an experimental investigation of SOAM-coupled BECs.
Spin-orbit coupling (SOC) plays a crucial role in many branches of physics. In this context, the recent experimental realization of the coupling between spin and linear momentum of ultracold atoms opens a completely new avenue for exploring new spin-related superfluid physics. Here we propose that another important and fundamental SOC, the coupling between spin and orbital angular momentum (SOAM), can be implemented for ultracold atoms using higher-order Laguerre-Gaussian laser beams to induce Raman coupling between two hyperfine spin states of atoms. We study the ground-state phase diagrams of SOAM-coupled Bose-Einstein condensates on a ring trap and explore their applications in gravitational force detection. Our results may provide the basis for further investigation of intriguing superfluid physics induced by SOAM coupling, such as collective excitations.