M. Anderlini

Asymmetric Landau-Zener Tunneling in a Periodic Potential

Using a simple model for nonlinear Landau-Zener tunneling between two energy bands of a Bose-Einstein condensate in a periodic potential, we find that the tunneling rates for the two directions of tunneling are not the same. Tunneling from the ground state to the excited state is enhanced by the nonlinearity, whereas in the opposite direction it is suppressed. These findings are confirmed by numerical simulations of the condensate dynamics. Measuring the tunneling rates for a condensate of rubidium atoms in an optical lattice, we have found experimental evidence for this asymmetry.

Photoionization of ultracold and Bose-Einstein-condensed Rb atoms

Photoionization of a cold atomic sample offers intriguing possibilities for observing collective effects at extremely low temperatures. Irradiation of a rubidium condensate and of cold rubidium atoms within a magneto-optical trap (MOT) with laser pulses ionizing through one-photon and two-photon absorption processes was performed. Losses and modifications in the density profile of the remaining trapped cold cloud or the remaining condensate sample were examined as functions of the ionizing laser parameters. Ionization cross sections were measured for atoms in a MOT, while in magnetic traps losses larger than those expected for ionization process were measured.

Sympathetic cooling and collisional properties of a Rb-Cs mixture

We report on measurements of the collisional properties of a mixture of {sup 133}Cs and {sup 87}Rb atoms in a magnetic trap at {mu}K temperatures. By selectively evaporating the Rb atoms using a radio-frequency field, we achieved sympathetic cooling of Cs down to a few {mu}K. The interspecies collisional cross section was determined through rethermalization measurements, leading to good agreement with a theoretical prediction of 595a{sub 0} for the triplet s-wave scattering length for Rb in the vertical bar F=2,m{sub F}=2> and Cs in the vertical bar F=4,m{sub F}=4> magnetic states. We briefly speculate on the prospects for reaching the Bose-Einstein condensation of Cs inside a magnetic trap through sympathetic cooling.

Instabilities of a Bose-Einstein condensate in a periodic potential: an experimental investigation

By accelerating a Bose-Einstein condensate in a controlled way across the edge of the Brillouin zone of a 1D optical lattice, we investigate the stability of the condensate in the vicinity of the zone edge. Through an analysis of the visibility of the interference pattern after a time-of-flight and the widths of the interference peaks, we characterize the onset of instability as the acceleration of the lattice is decreased. We briefly discuss the significance of our results with respect to recent theoretical work.

Atomic micromotion and geometric forces in a triaxial magnetic trap

Nonadiabatic motion of Bose-Einstein condensates of rubidium atoms arising from the dynamical nature of a time-orbiting-potential (TOP) trap was observed experimentally. The orbital micromotion of the condensate in velocity space at the frequency of the rotating bias field of the TOP was detected by a time-of-flight method. A dependence of the equilibrium position of the atoms on the sense of rotation of the bias field was observed. We have compared our experimental findings with numerical simulations. The nonadiabatic following of the atomic spin in the trap rotating magnetic field produces geometric forces acting on the trapped atoms.

Two-photon ionization of cold rubidium atoms with a near resonant intermediate state

By means of a simple theoretical model and experimental results, we analyse the two-photon ionization of cold rubidium atoms. We consider a two-step process in which ground state atoms are ionized by two blue (421 nm) photons, chosen to be quasi-resonant with the 5S → 6P transition. We show that for a good description of the process we have to take into account not only the three states coupled by the laser radiation, but also all the atomic states involved in the spontaneous emission cascade from the 6P to the 5S state, since optical pumping from 5S1/2 (F = 2) to the 5S1/2 (F = 1) states modifies the ionization efficiency when ionizing near resonance. The experimental and theoretical determination of ionization cross sections for excitations near resonance with the 6P1/2 and the 6P3/2 intermediate levels is presented.

Two-photon ionization of cold rubidium atoms

Two-photon ionization cross sections from the rubidium ground state have been measured with both a cw 421-nm laser and a combination of cw 421- and 1002-nm lasers. The measurements were performed within a high-vacuum magneto-optical trap while the trapping lasers were switched off, exploiting the long trap lifetime and also the trap laser confinement. The two-photon cross sections were determined for the blue laser near resonance with the 6P1/2 and 6P3/2 states and compared with the estimates of a theoretical model. In near resonance with the 6P states, large two-photon photoionization cross sections were measured.

One-dimensional bichromatic standing-wave cooling of cesium atoms

We report the results of an experimental study on the interaction of cooled cesium atoms with the optical field of two standing waves having different wavelengths (852 and 894 nm) and opposite circular polarizations. The spatial modulation of the superposition of the two optical potentials and the polarization properties of this configuration are expected to produce cooling of the atoms and a spatial modulation of their density with the periodicity of the beat of the two wavelengths. We performed temperature measurements of the cesium sample and observed the density distribution of the atoms for several configurations of the standing wave by means of time-of-flight absorption imaging and fluorescence imaging techniques. Experimentally we could not observe a pronounced density modulation on the length scale of the superperiod. Reasons for this are revealed by a one-dimensional numerical simulation including the complexity of the full Zeeman structure of the cesium atoms. That simulation reproduces the experimental results for the temperatures and spatial confinement.

Photoionization of Bose-Einstein condensates

Photoionization of Bose-Einstein condensates is an interesting topic at the interface between plasma physics and quantum statistics. We irradiated a BEC of rubidium atoms with ionizing laser pulses and studied losses and density profile modifications in the remaining trapped cloud of atoms.

Beyond the usual approximations: Time-dependent magnetic traps revisited

Abstract The motion of atoms in time-orbiting potential (TOP) traps is usually described by applying adiabatic and time-averaging approximations. In this work we show that careful experimental analysis reveals corrections to both of these approximations in the shape of a micromotion of the atoms at the frequency of the time-dependent trapping field and of a dependence of the equilibrium position of the atoms on the sense of rotation of the bias field. Whereas the former can be easily explained theoretically, the latter requires a more sophisticated analytical treatment involving geometric forces. We present experimental results and numerical simulations which agree well with the data. Furthermore, implications for Bose–Einstein condensation experiments are discussed.