M. Cristiani

Direct Observation of Tunneling and Nonlinear Self-Trapping in a Single Bosonic Josephson Junction

We report on the first realization of a single bosonic Josephson junction, implemented by two weakly linked Bose-Einstein condensates in a double-well potential. In order to fully investigate the nonlinear tunneling dynamics we measure the density distribution in situ and deduce the evolution of the relative phase between the two condensates from interference fringes. Our results verify the predicted nonlinear generalization of tunneling oscillations in superconducting and superfluid Josephson junctions. Additionally, we confirm a novel nonlinear effect known as macroscopic quantum self-trapping, which leads to the inhibition of large amplitude tunneling oscillations.

Bloch Oscillations and Mean-Field Effects of Bose-Einstein Condensates in 1D Optical Lattices

We have loaded Bose-Einstein condensates into one-dimensional, off-resonant optical lattices and accelerated them by chirping the frequency difference between the two lattice beams. For small values of the lattice well depth, Bloch oscillations were observed. Reducing the potential depth further, Landau-Zener tunneling out of the lowest lattice band, leading to a breakdown of the oscillations, was also studied and used as a probe for the effective potential resulting from mean-field interactions as predicted by Choi and Niu [Phys. Rev. Lett. 82, 2022 (1999)]. The effective potential was measured for various condensate densities and trap geometries, yielding good qualitative agreement with theoretical calculations.

Experimental properties of Bose-Einstein condensates in one-dimensional optical lattices: Bloch oscillations, Landau-Zener tunneling, and mean-field effects

We report experimental results on the properties of Bose-Einstein condensates in 1D optical lattices. By accelerating the lattice, we observed Bloch oscillations of the condensate in the lowest band, as well as Landau-Zener (L-Z) tunneling into higher bands when the lattice depth was reduced and/or the acceleration of the lattice was increased. The dependence of the L-Z tunneling rate on the condensate density was then related to mean-field effects modifying the effective potential acting on the condensate, yielding good agreement with recent theoretical work. We also present several methods for measuring the lattice depth and discuss the effects of the micromotion in the TOP-trap on our experimental results.

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.

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.

Free expansion of a Bose-Einstein condensate in a one-dimensional optical lattice

We have experimentally investigated the free expansion of a Bose-Einstein condensate in an array of two-dimensional traps created by a one-dimensional optical lattice. If the condensate held in a magnetic trap is loaded adiabatically into the lattice, the increase in chemical potential due to the additional periodic potential is reflected in the expansion of the condensate after switching off the magnetic trap. We have calculated the chemical potential from measurements of the transverse expansion of the condensate as a function of the lattice parameters.

Decay and revival of phase coherence of a Bose-Einstein condensate in a one-dimensional lattice

The dynamics of a Bose-Einstein condensate nonadiabatically loaded into a one-dimensional optical lattice is studied by analyzing the phase coherence between sites along the lattice as well as the radial profile of the condensate after a time-of flight. A simple model is proposed that predicts the short-time dephasing as a function of the condensate parameters. In the radial direction, heavily damped oscillations are observed, as well as an increase in the condensate temperature. These findings are interpreted as a rethermalization due to dissipation of the initial condensate excitations into high-lying modes.

Coherent transport of cold atoms in angle-tuned optical lattices

Optical lattices with a large spacing between the minima of the optical potential can be created using the angle-tuned geometry where the one-dimensional periodic potential is generated by two propagating laser beams intersecting at an angle different from {pi}. The present work analyzes the coherent transport for the case of this geometry. We show that the potential depth can be kept constant during the transport by choosing a magic value for the laser wavelength. This value agrees with that of the counterpropagating laser case, and the magic wavelength does not depend on the optical lattice geometry. Moreover, we find that this scheme can be used to implement controlled collision experiments under special geometric conditions. Finally we study the transport of hyperfine-Zeeman states of {sup 87}Rb.

Fast nondestructive temperature measurement of two-electron atoms in a magneto-optical trap

We extend the technique originally proposed by [Honda et al., Phys. Rev. A 59, R934 (1999)] to measure the temperature of ytterbium and alkaline-earth-metal atoms confined in a magneto-optical trap (MOT). The method is based on the analysis of excitation spectra obtained by probing the