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We have experimentally investigated laser diffraction on electromagnetically induced gratings in a degenerate open two-level system. The experiment employs the standard backward four-wave mixing configuration and the open 6S 1/2 (Fϭ4)→6 P 3/2 (FЈϭ4) transition of cold cesium atoms. Population and coherence gratings are accessed for different relative polarizations of the incident laser beams. Natural and subnatural linewidths are observed depending on which frequency of the incident beams is scanned and reveal the effect of the atomic motion on the diffraction spectra. A simple theoretical model using a three-level ⌳ system is in reasonable agreement with the observed results and allowed us to make a close connection with the phenomenon of electromagnetically induced transparency.

The interaction of an ensemble of atoms with common vacuum modes may lead to an enhanced emission into these modes. This phenomenon, known as superradiance, highlights the coherent nature of spontaneous emission, resulting in macroscopic entangled states in mundane situations. The complexity of the typical observations of superradiance, however, masks its quantum nature, allowing alternative classical interpretations. Here we stress how this picture changed with the implementation ten years ago of a new process for single-photon generation from atomic ensembles. We present then the last piece of evidence for the superradiant nature of such a process, reporting the observation of an accelerated emission of the photon with a rate that may be tuned by controllably changing the number of atoms in the ensemble. We hope such an investigation will help open up a new bottom-up approach to the study of superradiance.

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 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.

The phenomenon called Electromagnetically Induced Transparency (EIT) may induce different types of correlation between two optical fields interacting with an ensemble of atoms. It is presently well known, for example, that in the vicinity of an EIT resonance the dominant correlations at low powers turn into anti-correlations as power increases. Such correlation spectra present striking power-broadening-independent features, with the best condition for measuring the characteristic linewidth occurring at the highest powers. In the present work we investigate the physical mechanisms responsible for this set of observations. Our approach is first to reproduce these effects in a better controlled experimental setup: a cold atomic ensemble, obtained from a magneto-optical trap. The results from this conceptually simpler system were then compared to a correspondingly simpler theory, which clearly relates the observed features to the interplay between two key aspects of EIT: the transparency itself and the steep normal dispersion near two-photon resonance.

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