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 µs. PACS numbers: 42.50.Gy, 42.50.Ex, 42.50.Va Light beams carrying orbital angular momentum (OAM) have attracted an enormous recent interest owing to the possibility of encoding quantum information in a multidimensional state space [1], to their use to excite vortices in Bose-Einstein condensates [2], as well as to a number of others interesting applications [3]. One important family of these beams, the Laguerre-Gaussian (LG) modes of the electromagnetic field [4], possesses wave-fronts dislocation or vortices specified by a topological charge m, which sets its OAM along the propagation direction as being equal to mh per photon. The coherent and nonlinear interaction of light beams carrying OAM with atomic systems have been reported previously using different experimental schemes [5,6,7,8,9]. From the perspective of quantum information processing, the use of multidimensional state space has a promising prospect to achieve higher quantum efficiency [10]. Indeed, entanglement between photons with OAM and a cold atomic ensemble was already reported in [11] and more recently the generation of twin light beams with OAM was achieved via four-wave mixing in a hot atomic vapor [12].However, further development in this field is strongly conditioned to our capability of reversibly store and manipulate these higher dimensional quantum states of light into long-lived atomic coherences. The light storage (LS) in an electromagnetically induced transparency (EIT) medium [13], which allow us to obtain later information of a previously stored light pulse, is a well understood phenomenon and was originally described in terms of a mixed two component light-matter excitation, called dark state polariton [14]. However, in a simpler alternative picture, LS can be described as being due to the creation of a ground state coherence grating which contains information on the amplitude and phase of an optical field and which survives after the switching off of the incident fields. To date, several experimental observations of this phenomenon were realized in different systems [15,16,17,18,19,20]. Recent theoretical and experimental work have also addressed the storage of spatial structures of light beams (images) in atomic vapors [21,22,23,24]. For instance, a light vortex was stored in a hot vapor for hundreds of microseconds and its robustness against diffusion demonstrated [21]. However, to date the storage and manipulation of superpositions of OAM states into an atomic ensemble, as well as the characterization of the retrieved states, has not yet been achieved.In this work, we report the storage of su...
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.
We investigate the evolution of a Zeeman coherence grating induced in a cold atomic cesium sample in the presence of an external magnetic field. The gratings are created in a three-beam light storage configuration using two quasi-collinear writing laser pulses and reading with a counterpropagating pulse after a variable time delay. The phase conjugated pulse arising from the atomic sample is monitored. Collapses and revivals of the retrieved pulse are observed for different polarizations of the laser beams and for different directions of the applied magnetic field. While magnetic field inhomogeneities are responsible for the decay of the coherent atomic response, a fivefold increase in the coherence decay time, with respect to no applied magnetic field, is obtained for an appropriate choice of the direction of the applied magnetic field. A simplified theoretical model illustrates the role of the magnetic field mean and its inhomogeneity on the collective atomic response.
We report a detailed investigation on the generation of pulse pairs during the readout of a coherence grating stored in a cold atomic ensemble. The pulse shapes and the split of the retrieved energy between the two pulses are studied as a function of the relative intensities of the two reading fields, and a minimum is observed for the total retrieved energy. We introduce a simplified analytical theory for the process, considering a three-level atomic system, which explains all the most striking experimental features.PACS. 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption -32.80.Qk Coherent control of atomic interactions with photons
We report on the simultaneous observation, by delayed Bragg diffraction, of four- and six-wave mixing processes in a coherently prepared atomic ensemble consisting of cold cesium atoms. For each diffracted order, we observe different temporal pulse shapes and dependencies with the intensities of the exciting fields, evidencing the different mechanisms involved in each process. The various observations are well described by a simplified analytical theory, which considers the atomic system as an ensemble of three-level atoms in Λ configuration.
We report a spectroscopic investigation of the reading process of a cold atomic ensemble coherently prepared in a superposition of its degenerate ground states. Specifically we measure the spectra of the generated signal for different frequencies of the reading laser pulse and for intensities below saturation. The spectra present a double-peaked structure, with a decrease of the signal around the atomic resonance. A simple theory using the density matrix formalism and accounting for propagation effects qualitatively describes the experimentally observed results.
Atomic vapors are systems well suited for nonlinear optics studies but very few direct measurements of their nonlinear refractive index have been reported. Here we use the z-scan technique to measure the Kerr coefficient, n 2 , for a Cs vapor. Our results are analyzed through a fourlevel model, and we show that coherence between excited levels as well as cross-population effects contribute to the Kerr-nonlinearity.
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