High resolution laser excitation of np Rydberg states of cesium atoms shows a dipole blockade at Förster resonances corresponding to the resonant dipole-dipole energy transfer of the np+np --> ns+(n+1)s reaction. The dipole-dipole interaction can be tuned on and off by the Stark effect, and such a process, observed for relatively low n(25-41), is promising for quantum gate devices. Both Penning ionization and saturation in the laser excitation can limit the range of observation of the dipole blockade.

Cold inelastic collisions between confined cesium (Cs) atoms and Cs2 molecules are investigated inside a CO2 laser dipole trap. Inelastic atom-molecule collisions can be observed and measured with a rate coefficient of ∼ 2.5 × 10 −11 cm 3 s −1 , mainly independent of the molecular ro-vibrational state populated. Lifetimes of purely atomic and molecular samples are essentially limited by rest gas collisions. The pure molecular trap lifetime ranges 0,3-1 s, four times smaller than the atomic one, as is also observed in a pure magnetic trap. We give an estimation of the inelastic molecule-molecule collision rate to be ∼ 10 −11 cm 3 s −1 .

High resolution laser Stark excitation of np (60 < n < 85) Rydberg states of ultra-cold cesium atoms shows an efficient blockade of the excitation attributed to long-range dipole-dipole interaction. The dipole blockade effect is observed as a quenching of the Rydberg excitation depending on the value of the dipole moment induced by the external electric field. Effects of eventual ions which could match the dipole blockade effect are discussed in detail but are ruled out for our experimental conditions. Analytic and Monte-Carlo simulations of the excitation of an ensemble of interacting Rydberg atoms agree with the experiments indicates a major role of the nearest neighboring Rydberg atom.PACS numbers: 32.80. Rm; 32.80.Pj; 34.20.Cf; 34.60.+z Long-range dipole-dipole interactions often play an important role in the properties of an assembly of cold atoms. One example is the efficiency of photoassociation of cold atoms and the formation of cold molecules [1]. In the case of a Rydberg atomic ensemble, the range of the dipole-dipole interactions can exceed several micrometers, leading to many-body effects [2,3,4]. An interesting application of the dipole-dipole interaction is the dipole blockade (DB) in Rydberg excitation. This effect offers exciting possibilities for quantum information [5] with the fascinating possibilities for manipulating quantum bits stored in a single collective excitation in mesoscopic ensembles, or for realizing scalable quantum logic gates [6]. The DB process for an ensemble of atoms is the result of shifting the Rydberg energy from its isolated atomic value due to the dipole-dipole interaction with the surrounding atoms. In a large volume, a partial or local blockade, corresponding to a limitation of the excitation is expected when the dipole-dipole energy shift exceeds the resolution of the laser excitation. In a zero electric field, Rydberg atoms present no permanent dipole and usually no DB is expected. Nevertheless, a van der Waals blockade, corresponding to a second order dipole-dipole interaction, has been observed through a limitation of the excitation of high Rydberg states np (n ∼ 70 − 80) of rubidium, using a pulsed amplified single mode laser [7]. CW excitations have also been performed [8,9] showing the suppression of the excitation and affecting the atom counting statistics [8]. The DB phenomenon itself has been observed for the first time, in the case of cesium Rydberg atoms, for a so called Förster Resonance Energy Transfer (FRET) reaction, np + np −→ ns + (n + 1)s [10]. The FRET configuration has several advantages: the dipole-dipole interaction can be tuned on and off by the Stark effect, the dipole-dipole interaction having its * Laboratoire Aimé Cotton is associated to Université ParisSud and belongs to Fédération de Recherche Lumière Matière (LUMAT). maximum effect at the resonant field. Its main drawback comes from the fact that the resonance exists only for n ≤ 41 in the cesium case, which limited the observed DB to an efficiency of ∼ 30%.In this letter, we report t...

We present an experimental demonstration of converting a microwave field to an optical field via frequency mixing in a cloud of cold ^{87}Rb atoms, where the microwave field strongly couples to an electric dipole transition between Rydberg states. We show that the conversion allows the phase information of the microwave field to be coherently transferred to the optical field. With the current energy level scheme and experimental geometry, we achieve a photon-conversion efficiency of ∼0.3% at low microwave intensities and a broad conversion bandwidth of more than 4 MHz. Theoretical simulations agree well with the experimental data, and they indicate that near-unit efficiency is possible in future experiments.

Dense samples of cold Rydberg atoms have previously been observed to spontaneously evolve to a plasma, despite the fact that each atom may be bound by as much as 100 cm −1 . Initially, ionization is caused by blackbody photoionization and Rydberg-Rydberg collisions. After the first electrons leave the interaction region, the net positive charge traps subsequent electrons. As a result, rapid ionization starts to occur after 1 s caused by electron-Rydberg collisions. The resulting cold plasma expands slowly and persists for tens of microseconds. While the initial report on this process identified the key issues described above, it failed to resolve one key aspect of the evolution process. Specifically, redistribution of population to Rydberg states other than the one initially populated was not observed, a necessary mechanism to maintain the energy balance in the system. Here we report new and expanded observations showing such redistribution and confirming theoretical predictions concerning the evolution to a plasma. These measurements also indicate that, for high n states of purely cold Rydberg samples, the initial ionization process which leads to electron trapping is one involving the interactions between Rydberg atoms.

We present a method to model the interaction and the dynamics of atoms excited to Rydberg states. We show a way to solve the optical Bloch equations for laser excitation of the frozen gas in good agreement with the experiment. A second method, the Kinetic Monte Carlo method gives an exact solution of rate equations. Using a simple N-body integrator (Verlet), we are able to describe dynamical processes in space and time. Unlike more sophisticated methods, the Kinetic Monte Carlo simulation offers the possibility of numerically following the evolution of tens of thousands of atoms within a reasonable computation time. The Kinetic Monte Carlo simulation gives good agreement with dipole-blockade type of experiment. The role of ions and the individual particle effects are investigated.

We perform spectroscopic measurements of electromagnetically induced transparency (EIT) in a strongly interacting Rydberg gas. We observe a significant spectral shift and attenuation of the transparency resonance due to the presence of interactions between Rydberg atoms. We characterize the attenuation as the result of an effective dephasing, and show that the shift and the dephasing rate increase versus atomic density, probe Rabi frequency, and principal quantum number of Rydberg states. Moreover, we find that the spectral shift is reduced if the size of a Gaussian atomic cloud is increased, and that the dephasing rate increases with the EIT pulse duration at large parameter regime. We simulate our experiment with a semi-analytical model, which yields results in good agreement with our experimental data.

We demonstrate microwave-to-optical conversion using six-wave mixing in 87 Rb atoms where the microwave field couples to two atomic Rydberg states, and propagates collinearly with the converted optical field. We achieve a photon conversion efficiency of ∼ 5% in the linear regime of the converter. In addition, we theoretically investigate all-resonant six-wave mixing and outline a realistic experimental scheme for reaching efficiencies greater than 60%.