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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Coherent back scattering and Anderson Localization of ultra cold atoms

Alain Aspect

Published May 1, 2013• 0 citations
Publisher

Abstract

Summary form only given. An ensemble of ultra cold atoms released in a well-chosen disordered optical potential is a system that offers a privileged way to directly observe Anderson Localization. Localization has been observed in 1D [1,2] and 3D [3,4], while 2D experiments are promising [5]. Theory supports the conclusion that what we observe is AL [6,7], but a smoking gun of the role of coherence is still missing. Recently, we have observed CBS [8] (see Figure), an indisputable coherent effect in quantum transport, related to the first order manifestation of localization (weak localization). Anderson Localization is a research field where many theoretical questions remain open. In particular, when there is interaction between particles, the corresponding quantum many-body problems are exceedingly difficult to solve. We thus plan to implement controlled interactions between atoms in our system, using Feschbach resonances, in order to provide answers to such problems. The system will then deserve the name of "quantum simulator" in the sense used by Feynman in his pioneering paper [9].

DOI
10.1109/cleoe-iqec.2013.6802008

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Summary form only given. An ensemble of ultra cold atoms released in a well-chosen disordered optical potential is a system that offers a privileged way to directly observe Anderson Localization. Localization has been observed in 1D [1,2] and 3D [3,4], while 2D experiments are promising [5]. Theory supports the conclusion that what we observe is AL [6,7], but a smoking gun of the role of coherence is still missing. Recently, we have observed CBS [8] (see Figure), an indisputable coherent effect in quantum transport, related to the first order manifestation of localization (weak localization). Anderson Localization is a research field where many theoretical questions remain open. In particular, when there is interaction between particles, the corresponding quantum many-body problems are exceedingly difficult to solve. We thus plan to implement controlled interactions between atoms in our system, using Feschbach resonances, in order to provide answers to such problems. The system will then deserve the name of "quantum simulator" in the sense used by Feynman in his pioneering paper [9].
We use ultra cold atoms transport in a disordered potential created with a laser speckle, to study Anderson Localization (AL) and Coherent Back Scattering (CBS). Localization has been observed in 1D and 3D, and 2D experiments are promising. Theory supports the conclusion that what we observe is AL, but a smoking gun of the role of coherence is still missing. Recently, we have observed CBS (see Figure), an indisputable coherent effect in quantum transport, related to the first order manifestation of localization (weak localization).
Anderson localization – the halt of quantum transport in a strongly disordered potential – has been observed for quantum gases in optical potentials. An important stage of experiments is the creation and characterization of the optical (speckle) potential that typically features nontrivial correlations complicating the interpretation of measurements. We realize an alternative route towards Anderson localization of cold atoms using a mixture of two strongly mass-imbalanced atomic species without any externally imposed disordered potential. On short time scales, the heavy component can be considered immobile and creates a quasi-static scattering potential for the light component that experiences Anderson localization under appropriate conditions. Experimental results allow for a quantitative description based on the model of randomly distributed resonant point scatterers. This demonstrates an interesting and fruitful link between the phenomenon of (single-particle) Anderson localization and the so-called many-body localization that takes place in interacting quantum systems composed of many particles, which is the case of our experimental system.
Cold atoms in optical lattices offer an exciting new laboratory where quantum many-body phenomena can be realized in a highly controlled way. They can even serve as quantum simulators for notoriously difficult problems like high-temperature superconductivity. This review is focussed on the recent developments and new results in multi-component systems. Fermionic atoms with SU(N) symmetry have exotic superfluid and flavor-ordered ground states. We discuss symmetry breaking, collective modes, and detection issues, e.g. in Bragg scattering. On the other hand, bosonic multiflavor ensembles allow for engineering of spin Hamiltonians which are interesting from a quantum computation point of view. Finally, we address the role of disorder in optical lattices. Fermionic atoms experience Anderson localization at sufficiently strong disorder. Interactions among the atoms induce a competing tendency towards delocalization. We present a complete phase diagram obtained within dynamical mean-field theory and discuss experimental observability of the Mott and Anderson phases.
We report on the direct observation of coherent backscattering (CBS) of ultracold atoms in a quasi-two-dimensional configuration. Launching atoms with a well-defined momentum in a laser speckle disordered potential, we follow the progressive build up of the momentum scattering pattern, consisting of a ring associated with multiple elastic scattering, and the CBS peak in the backward direction. Monitoring the depletion of the initial momentum component and the formation of the angular ring profile allows us to determine microscopic transport quantities. We also study the time evolution of the CBS peak and find it in fair agreement with predictions, at long times as well as at short times. The observation of CBS can be considered a direct signature of coherence in quantum transport of particles in disordered media. It is responsible for the so called weak localization phenomenon, which is the precursor of Anderson localization.
Large ensembles of cold, immobile atoms might serve as ideal model systems to study Anderson localization of light. In contrast to dielectric systems, atomic clouds allow for a fine experimental control and admit a precise theoretical description. In this talk, we will review the recent theoretical developments aimed at paving a path towards an experimental realization of Anderson localization of light in three-dimensional (3D) atomic systems. Because atoms are the simplest point scatterers, considering light scattering on them is a good opportunity to illustrate several basic concepts and methods of the localization theory (Thouless conductance and Ioffe-Regel criterion, the mobility edge and the critical behavior, the scaling and self-consistent theories, the finite-size scaling procedure, etc.) on a concrete example. We will show that, surprisingly, Anderson localization of light by atoms is impossible in 3D if no special efforts are made to suppress near-field interactions between atoms. The necessary suppression may be achieved by subjecting the atoms to a constant external magnetic field or by arranging them in a crystalline structure in which a small amount of disorder is introduced. However, even when spatially localized states appear in a system of immobile atoms, their experimental observation remains challenging because of the residual thermal motion of atoms.
The understanding of disordered quantum systems is still far from being complete, despite many decades of research on a variety of physical systems. In this review we discuss how Bose–Einstein condensates of ultracold atoms in disordered potentials have opened a new window for studying fundamental phenomena related to disorder. In particular, we direct our attention to recent experimental studies on Anderson localization and on the interplay of disorder and weak interactions. These realize a very promising starting point for a deeper understanding of the complex behaviour of interacting, disordered systems.