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arXiv:0901.0939 · Physical Review A

Collapses and revivals of stored orbital angular momentum of light in a cold-atom ensemble

D. Moretti, D. Felinto, J. W. R. Tabosa

Published Feb 20, 2009• 112 citations
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Abstract

We report on the storage of orbital angular momentum of light in a cold ensemble of cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The stored information can be manipulated by an applied magnetic field and we were able to observe collapses and revivals due to the rotation of the stored atomic Zeeman coherence for times longer than $15\text{ }\ensuremath{\mu}\text{s}$.

DOI
10.1103/physreva.79.023825

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We report on the storage of orbital angular momentum of light in a cold ensemble of cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The stored information can be manipulated by an applied magnetic field and we were able to observe collapses and revivals due to the rotation of the stored atomic Zeeman coherence for times longer than 15 \ensuremathμs15\text{ }\ensuremath{\mu}\text{s}15 \ensuremathμs.
The paper reports on the storage of superposition of orbital angular momentum (OAM) states, as well as its manipulation through an applied transverse magnetic field. In the experiment , Zeeman sublevels of the degenerate two-level system is used. In another series of experiments,an external dc magnetic field is applied, nearly orthogonal to the plane defined by the incident beams. In this case,a series of collapses and revivals of the stored Zeeman coherence is observed. These observations demonstrate that superposition of OAM states can be stored into the atomic ensemble, and also the reversible transfer of OAM between light and atoms. Moreover, it also shows that the stored OAM can be manipulated with an external magnetic field.
We report on the generation of light carrying orbital angular momentum through Bragg diffraction into an electromagnetically induced coherence grating in a degenerate two-level system of cold cesium atoms. The induced Zeeman coherence grating is shown to contain the spatial phase structure of the incident beams. The exchange of phase information between a light beam with orbital angular momentum and a long-lived atomic coherence opens up the way to process quantum information encoded in a multidimensional state space.
Summary form only given. Light beams carrying orbital angular momentum (OAM) have attracted a great interest owing to their several applications ranging from the mechanical manipulation of macroscopic particles to the encoding of quantum information [1]. A well-known family of these beams is constituted by Laguerre-Gaussian modes specified by a topological charge ℓ, which gives to the mode a hélicoïdal phase structure and a corresponding OAM per photon equal to ℓħ. The nonlinear interaction of these beams with atomic systems has been investigated via four-wave mixing processes and the corresponding conservation of OAM demonstrated both in cold and thermal atoms [2]. The storage of OAM in cold and thermal atomic ensembles was also previously demonstrated [3].Differently from the previous observations, in this work we demonstrate that the stored OAM of a light beam can be retrieved along a nearly non-collinear direction with a reasonable fidelity. The experiment is performed in cold cesium atoms obtained from a MOT, using a delayed FWM configuration. Light beams with topological charges ℓ=0,1,2,3, produced by a spatial light modulator (SLM) are incident into the medium along the z direction, as shown in the simplified experimental scheme depicted in Fig. 1 (A). The frequency of the beams is resonant with the transition 6S1/2, F=3 - 6P3/2, F=2. The phase structure of the beam carrying OAM is stored into the Zeeman coherence grating induced by the incident writing beams W and W' that form a small angle (~ή between them and have opposite circular polarization. The writing beams are kept on for a period long enough to create a stationary Zeeman coherence grating and then are switched off. After the storage time, a reading beam R, counter-propagating and with opposite circular polarization to the writing beam W, is turned on and the retrieved beam (C), whose propagation direction is determined by the phase matching condition is monitored by a CCD camera. In Fig. 1(B) (a, b) we show images of the incident beams (W) intensity for the cases ℓ=1,2. In Fig. 1B (c, d) the corresponding images of the retrieved C beams. In order to determine the topological charges of the beams we create interference with a reference Gaussian beam. Fig 1(B) (e,f) and (g,h), show the interferograms for the incident (W) and the retrieved (C) beams respectively. Considering the detection geometry we conclude that the incident and retrieved beams carry the same OAM. These results clearly demonstrate that for small angles between W and W' beams, the OAM stored along the z direction can ben retrieved along the non-collinear direction z'. Additional results concerning the conservationof OAM as well as the manipulation of the stored OAM by a magnetic field will also be presented.
We report on the generation of light carrying orbital angular momentum through Bragg diffraction into an electromagnetically induced coherence grating in a degenerate two-level system of cold cesium atoms, demonstrating the c onservation the c onservation of orbital angular momentum of light in a coherent four-wave mixing process. The induced Zeeman coherence grating is shown to contain the spatial phase structure of the incident beams. The e xchange of phase information between a light beam with orbital angular momentum and a long-lived atomic coherence opens up the way to process quantum process information encoded in a multidimensional state space.
Abstract We report on the amplification and the storage of a light vortex beam carrying orbital angular momentum (OAM) in an ensemble of cold cesium atoms in the presence of an external transverse magnetic field. This magnetically assisted optical gain was previously reported in a recent publication and originates from the manipulation of the induced dark state by the external magnetic field. Single pass optical gain exceeding 100 % was measured for beams with different topological charges. Furthermore, we also demonstrate the storage and retrieval of the vortex beams, showing that the amplification and the storage processes preserve the coherent nature of the optical vortex. A simple theoretical model based on density matrix numeric calculation and which accounts for the presence of the external magnetic field was developed and present qualitative agreement with the observed results. Owing to the recent proposal of using light carrying OAM for efficient data transmission systems, we believe our results could find application in communications protocols involving distant nodes.
We report on a detailed investigation of the dynamics and the saturation of a light grating stored in a sample of cold cesium atoms. We employ Bragg diffraction to retrieve the stored optical information impressed into the atomic coherence by the incident light fields. The diffracted efficiency is studied as a function of the intensities of both writing and reading laser beams. A theoretical model is developed to predict the temporal pulse shape of the retrieved signal and compares reasonably well with the observed results.