Interacting Bose-Einstein Condensates: Tunneling, Localization, and Beyond Mean-Field

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Lithium atoms exhibit extraordinarily broad Feshbach resonances. The Feshbach resonance for the lowest hyperfine sublevel of Li-7 extends over a range of approximately 200 Gauss (G), where it goes through a zero crossing with a slope of only 0.1 ao/G, where ao is the Bohr radius. Our work exploits the properties of this Feshbach resonance to study bosons with attractive, weak, or very strong interactions.The creation of bright matter wave solitons, previously demonstrated with weakly attracting Li-7 atoms, enables the exploration of fundamental quantum phenomena. We study single particle tunneling by causing the soliton to execute dipole oscillations in a one-dimensional optical trap in the presence of a central barrier formed from a light sheet. This system may show a remarkable coherent recombination that could form the basis for a bright matter wave soliton interferometer. Longer term goals are to work towards true macroscopic quantum tunneling in order to create a Schrödinger Cat state.Our second project is study of the effect of a disordered potential, created from optical speckle, on the transport properties of a Bose-Einstein condensate. Recent proposals in condensed matter to demonstrate a "superinsulator", in which there is a finite temperature transition between a conducting and insulating state in 1D may be realizable using cold atoms. A closely related topic is to understand the role of interactions in Anderson Localization by measuring the localization length as a function of interparticle interaction.Finally, the broad Feshbach resonance enables access to the regime of strong interactions where the mean-field theory of Bose-Einstein condensation breaks down. Both the perturbative regime, where the energy is corrected in powers of many time the cube of the scattering length and the regime of strong condensate depletion are being explored.Wave/particle duality is at the heart of quantum physics. At very low temperatures, we find that particles that ordinarily act as compact solid objects behave as if they are waves, that is, they reflect, diffract, and interfere. Bose-Einstein condensates (BECs) of atoms are iconic examples of such behavior. A BEC is a collection of atoms that become a single quantum mechanical wave at temperatures as low as one millionth of a degree above absolute zero. In this program, we create BECs of interacting lithium atoms and use them to explore and test some of the most fundamental ideas of quantum mechanics, including the tunneling of particles through otherwise impenetrable barriers. These experiments are enabled by the ability to tune the strength of the interatomic interactions and to even change whether they are repulsive or attractive. By making them weakly attractive, the BECs form a soliton, which is a packet of waves that can travel over a distance without dissipating. Under some conditions a BEC soliton may behave as a single "super-atom", thus stretching the realm of quantum mechanics to ever larger objects, much like the famous Schrodinger Cat. We also tune the interactions to the opposite extreme, where the interactions are repulsive and very strong. These BECs are being used to test our theories of strongly interacting matter. These experiments will give us a greater understanding of the quantum realm, which hopefully, will allow us to exploit quantum phenomena, such as superconductivity, for practical applications.