Thibault Vogt

Atom-molecule collisions in an optically trapped gas.

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 approximately 2.6 x 10(-11) cm3 s(-1), mainly independent of the molecular rovibrational 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, 4 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 approximately 10(-11) cm3 s(-1).

Electric-field induced dipole blockade with Rydberg atoms.

High resolution laser Stark excitation of np (60<n<85) Rydberg states of ultracold 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 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 and indicate a major role of the nearest neighboring Rydberg atom.

Back and forth transfer and coherent coupling in a cold Rydberg dipole gas

Coupling by the resonant dipole-dipole energy transfer between cold cesium Rydberg atoms is investigated using time-resolved narrow-band deexcitation spectroscopy. This technique combines the advantage of efficient Rydberg excitation with high-resolution spectroscopy at variable interaction times. Dipole-dipole interaction is observed spectroscopically as avoided level crossing. The coherent character of the process is linked to back and forth transfer in the np + np <--> ns + (n + 1)s reaction. Decoherence in the ensemble has two different origins: the atom motion induced by dipole-dipole interaction and the migration of the s-Rydberg excitation in the environment of p-Rydberg atoms.

Kinetic Monte Carlo modeling of dipole blockade in Rydberg excitation experiment

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 (KMC) method, gives an exact solution of rate equations. Using a simple N-body integrator (Verlet), we are able to describe dynamic processes in space and time. Unlike more sophisticated methods, the KMC simulation offers the possibility of numerically following the evolution of tens of thousands of atoms within a reasonable computation time. The KMC simulation gives good agreement with dipole-blockade type of experiment. The role of ions and the individual particle effects are investigated.

Star cluster dynamics in a laboratory : electrons in an ultracold plasma

Electrons in a spherical ultracold, quasi-neutral plasma at temperature in the Kelvin range can be created by laser excitation of an ultracold, laser-cooled atomic cloud. The dynamical behaviour of the electrons is similar to the one described by conventional models of star cluster dynamics. The single mass component, the spherical symmetry and no star evolution are here accurate assumptions. The analogue of binary star formations in the cluster case is a three-body recombination in Rydberg atoms in the plasma case with the same Heggie’s law: soft binaries get softer and hard binaries get harder. We demonstrate that the evolution of such an ultracold plasma is dominated by Fokker‐Planck kinetics equations formally identical to the ones controlling the evolution of a star cluster. The Virial theorem leads to a link between the plasma temperature and the ions and electron numbers. The Fokker‐Planck equation is approximate using gaseous and fluid models. We found that the electrons are in a Kramers‐Michie‐King type quasi-equilibrium distribution as stars in clusters. We suggest that the evaporation rate can be used to determine the temperature. As an example, knowing the electron distribution and using forced fast electron extraction, in a ‘violent extraction’ way, we are able to determine the plasma temperature knowing the trapping potential depth.

Spectral shift and dephasing of electromagnetically induced transparency in an interacting Rydberg gas

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 regimes. We simulate our experiment with a semianalytical model, which yields results in good agreement with our experimental data.

Rapid nonadiabatic loading in an optical lattice

We present a scheme for nonadiabatically loading a Bose-Einstein condensate into the ground state of a one-dimensional optical lattice within a few tens of microseconds, i.e., typically in less than half the Talbot period. This technique of coherent control is based on sequences of pulsed perturbations, and the experimental results demonstrate its feasibility and effectiveness. As the loading process is much shorter than the traditional adiabatic loading time scale, this method may find many applications.

Lensing effect of electromagnetically induced transparency involving a Rydberg state

We study the lensing effect experienced by a weak probe field under conditions of electromagnetically induced transparency (EIT) involving a Rydberg state. A Gaussian coupling beam tightly focused on a laser-cooled atomic cloud produces an inhomogeneity in the coupling Rabi frequency along the transverse direction and makes the EIT area acting like a gradient-index medium. We image the probe beam at the position where it exits the atomic cloud, and observe that a red-detuned probe light is strongly focused with a greatly enhanced intensity whereas a blue-detuned one is de-focused with a reduced intensity. Our experimental results agree very well with the numerical solutions of Maxwell-Bloch equations.

Controllable Interactions between Rydberg Atoms and Ultracold Plasmas

We discuss the control of dipole-dipole interactions in a frozen assembly of Rydberg atoms. We report the evidence of dipole blockade of the Rydberg excitation for two configurations: dipole blockade induced by electric field and dipole blockade in Forster resonance. We demonstrate that two individual atoms separated by ~ 4 μm can act as a collective dipole if their interaction is strong enough to be in the dipole blockade regime. This observation is crucial for the quantum entanglement of two or more atoms using dipole-dipole interaction. The dipole-dipole interactions between Rydberg atoms are also responsible for Penning ionization leading to the formation of an ultracold plasma. We have demonstrated that Penning ionization of np Rydberg cesium atoms can be prevented by considering repulsive dipole-dipole interactions.

Cooperative scattering measurement of coherence in a spatially modulated Bose gas

Correlations of a Bose gas released from an optical lattice are measured using superradiant scattering. Conditions are chosen so that, after initial incident light pumping at the Bragg angle for diffraction, superradiant scattering into the Bragg diffracted mode is preponderant due to matter-wave amplification and mode competition. A temporal analysis of the superradiant scattering gain reveals periodical oscillations and damping due to the initial lack of coherence between lattice sites. Such damping is used for characterizing first-order spatial correlations in our system with a precision of one lattice period.