F. Minardi

Josephson junction arrays with Bose-Einstein condensates.

We report on the direct observation of an oscillating atomic current in a one-dimensional array of Josephson junctions realized with an atomic Bose-Einstein condensate. The array is created by a laser standing wave, with the condensates trapped in the valleys of the periodic potential and weakly coupled by the interwell barriers. The coherence of multiple tunneling between adjacent wells is continuously probed by atomic interference. The square of the small-amplitude oscillation frequency is proportional to the microscopic tunneling rate of each condensate through the barriers and provides a direct measurement of the Josephson critical current as a function of the intermediate barrier heights. Our superfluid array may allow investigation of phenomena so far inaccessible to superconducting Josephson junctions and lays a bridge between the condensate dynamics and the physics of discrete nonlinear media.

Superfluid and Dissipative Dynamics of a Bose-Einstein Condensate in a Periodic Optical Potential

We create Bose-Einstein condensates of 87Rb in a static magnetic trap with a superimposed blue-detuned 1D optical lattice. By displacing the magnetic trap center we are able to control the condensate evolution. We observe a change in the frequency of the center-of-mass oscillation in the harmonic trapping potential, in analogy with an increase in effective mass. For fluid velocities greater than a local speed of sound, we observe the onset of dissipative processes up to full removal of the superfluid component. A parallel simulation study visualizes the dynamics of the Bose-Einstein condensate and accounts for the main features of the observed behavior.

Double Species Bose-Einstein Condensate with Tunable Interspecies Interactions

We produce Bose-Einstein condensates of two different species, 87Rb and 41K, in an optical dipole trap in proximity of interspecies Feshbach resonances. We discover and characterize two Feshbach resonances, located around 35 and 79 G, by observing the three-body losses and the elastic cross section. The narrower resonance is exploited to create a double species condensate with tunable interactions. Our system opens the way to the exploration of double species Mott insulators and, more in general, of the quantum phase diagram of the two-species Bose-Hubbard model.

Observation of Heteronuclear Atomic Efimov Resonances

Building on the recent experimental observation with ultracold atoms, we report the first experimental evidence of Efimov physics in a heteronuclear system. A mixture of ;{41}K and ;{87}Rb atoms was cooled to few hundred nanokelvins and stored in an optical dipole trap. Exploiting a broad interspecies Feshbach resonance, the losses due to three-body collisions were studied as a function of the interspecies scattering length. We observe an enhancement of the three-body collisions for three distinct values of the interspecies scattering lengths, both positive and negative, where no Feshbach resonances are expected. We attribute the two features at negative scattering length to the existence of two kinds of Efimov trimers, KKRb and KRbRb.

Expansion of a Coherent Array of Bose-Einstein Condensates

We investigate the properties of a coherent array containing about 200 Bose-Einstein condensates produced in a far detuned 1D optical lattice. The density profile of the gas, imaged after releasing the trap, provides information about the coherence of the ground-state wave function. The measured atomic distribution is characterized by interference peaks. The time evolution of the peaks, their relative population, as well as the radial size of the expanding cloud are in good agreement with the predictions of theory. The 2D nature of the trapped condensates and the conditions required to observe the effects of coherence are also discussed.

Degenerate Bose-Bose mixture in a three-dimensional optical lattice

Quantum degenerate gases are formidable systems to shine light on fundamental quantum phenomena occurring at extremely low temperatures, such as superconductivity and superfluidity. In combination with optical lattices and scattering resonances, degenerate gases give rise to strongly correlated systems, enriching even further the breadth of phenomena that can be directly probed. Indeed, the pioneering experiment on the superfluid to Mott-insulator transition [1] has shown how physical models long studied in the field of condensed matter can be realized almost ideally. With two different atomic species, the wealth of quantum phases grows to a daunting complexity [2], only marginally explored by experiments. Actually, experiments with heteronuclear mixtures in three-dimensional (3D) optical lattice have been performed very recently only for Fermi-Bose systems [3, 4], while Fermi-Fermi and Bose-Bose mixtures are yet uncharted territory. The importance of mixtures in optical lattices is hardly overstated: association of dipolar molecules [5], mapping of spin arrays [6], schemes for quantum calculation [7], and implementation of disorder [8] represent only a few major research lines that potentially will greatly benefit from such systems. In particular, Bose-Bose mixtures seem well suited for all these purposes, provided that collisional losses are adequately suppressed. This work reports the first realization of a degenerate Bose-Bose heteronuclear mixture in a 3D optical lattice. Exploiting the large mass difference, we investigate the regime where one species lies well in the superfluid domain, while the other exhibits the disappearence of the interference pattern usually associated with the transition from a superfluid to a Mott insulator. We focus only

Quasi-2D Bose-Einstein condensation in an optical lattice

We study the phase transition of a gas of 87Rb atoms to quantum degeneracy in the combined potential of a harmonically confining magnetic trap and the periodic potential of an optical lattice. For high optical lattice potentials we observe a significant change in the temperature dependency of the population of the ground state of the system. The experimental results are in good agreement with a model assuming the subsequent formation of quasi-2D condensates in the single lattice sites.

Entropy Exchange in a Mixture of Ultracold Atoms

We investigate experimentally the entropy transfer between two distinguishable atomic quantum gases at ultralow temperatures. Exploiting a species-selective trapping potential, we are able to control the entropy of one target gas in presence of a second auxiliary gas. With this method, we drive the target gas into the degenerate regime in conditions of controlled temperature by transferring entropy to the auxiliary gas. We envision that our method could be useful both to achieve the low entropies required to realize new quantum phases and to measure the temperature of atoms in deep optical lattices. We verified the thermalization of the two species in a 1D lattice.

Collective Oscillations of Two Colliding Bose-Einstein Condensates

Two 87Rb condensates ( F = 2, m(f) = 2, and m(f) = 1) are produced in highly displaced harmonic traps and the collective dynamical behavior is investigated. The mutual interaction between the two condensates is evidenced in the center-of-mass oscillations as a frequency shift of 6.4(3)%. Calculations based on a mean-field theory well describe the observed effects of periodical collisions both on the center-of-mass motion and on the shape oscillations.

Scattering in mixed dimensions with ultracold gases.

We experimentally investigate the mix-dimensional scattering occurring when the collisional partners live in different dimensions. We employ a binary mixture of ultracold atoms and exploit a species-selective 1D optical lattice to confine only one atomic species in 2D. By applying an external magnetic field in proximity of a Feshbach resonance, we adjust the free-space scattering length to observe a series of resonances in mixed dimensions. By monitoring 3-body inelastic losses, we measure the magnetic field values corresponding to the mix-dimensional scattering resonances and find a good agreement with the theoretical predictions based on simple energy considerations.