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Journal de Physique IV (Proceedings)

Strong correlation effects and quantum information theory of low dimensional atomic gases

B. Paredes, Juan José García‐Ripoll, P. Zoller, J. I. Cirac

Published Oct 1, 2004• 2 citations
Publisher

Abstract

These lecture notes present an introduction to the strongly correlated regime of low dimensional atomic gases. The discussion is concentrated on situations in which the strongly correlated limit is achieved by creating degeneracies in the one-particle motional states. Three different schemes of experimental relevance are analyzed: bosonic atoms in a two dimensional rapidly rotating trap, bosonic atoms in a one dimensional optical lattice, and bosonic atoms with frozen motional degrees of freedom and two internal states. The corresponding entangled multiparticle states (Laughlin liquids, Mott phases, squeezed states), and the different strongly correlated phenomena that appear (fermionization, fractional statistics) are studied. Emphasis is given to the possibility of observing novel strongly correlated phenomena as well as to the possible implementations for quantum computation and quantum information.

DOI
10.1051/jp4:2004116005

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These lecture notes present an introduction to the strongly correlated regime of low dimensional atomic gases. The discussion is concentrated on situations in which the strongly correlated limit is achieved by creating degeneracies in the one-particle motional states. Three different schemes of experimental relevance are analyzed: bosonic atoms in a two dimensional rapidly rotating trap, bosonic atoms in a one dimensional optical lattice, and bosonic atoms with frozen motional degrees of freedom and two internal states. The corresponding entangled multiparticle states (Laughlin liquids, Mott phases, squeezed states), and the different strongly correlated phenomena that appear (fermionization, fractional statistics) are studied. Emphasis is given to the possibility of observing novel strongly correlated phenomena as well as to the possible implementations for quantum computation and quantum information.
In this thesis we theoretically study low-dimensional, strongly corrleated systems of cold atoms, which are not in an equilibrium situation. This is motivated by recent experimental progress, which has made it possible to study quantum many-body physics in a controllable and clean setting; and parameters can be changed during the experiment. In Chapter 2 and 3 we study phases and quantum phase transitions of ‘tilted ’ Mott insulator of bosons. We analyze a variety of lattices and tilt directions in two dimensions: square, decorated square, triangular, and kagome. We show that there are rich possibilities for correlated phases with non-trivial entanglement of pseudospin degrees of freedom encoded in the boson density. For certain configurations three-body interactions are necessary to ensure that the energy of the effective resonant subspace is bounded from below. We find quantum phases with Ising density wave order, with superfluidity transverse to the tilt direction, a quantum liquid state with no broken symmetry. We also find cases for which the resonant subspace is described by effective quantum dimer models. In Chapter 4 we study spin 1/2 chains with a Heisenberg interaction which are
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In this thesis we investigate strongly correlated phenomena in the field of ultracold atomic gases. Chapter 2 addresses a question of the insulating phases of cold spin-one bosonic particles with antiferromagnetic interactions, such as 23 Na, in optical lattices. Magnetic properties of the ground state in the insulating regime are studied using various techniques. Chapter 3 considers a one dimensional interacting Bose-Fermi mixture with equal masses of bosons and fermions, and with equal repulsive interactions between Bose-Fermi and Bose-Bose particles. Properties of such mixture are studied using exact Bethe-ansatz techniques. Chapter 4 deals with certain phenomena which appear in the experiments with imbalanced fermionic mixtures in strongly anisotropic traps. Chapter 5 gives a comprehensive review of interference phenomena, analyzing effects which contribute to the reduction of the interference fringe contrast in matter interferometers. iii Contents
Ultracold bosonic gases in optical lattices are strongly correlated quantum systems similar to solids. The strong correlation between the electrons in a solid on the one hand, and the bosonic atoms in optical lattice on the other, exhibit various quantum phenomena like insulation, conductivity, localization of electrons and atoms, respectively. Controlled by the intensity of the lattice laser, the ultracold bosonic gas can be transferred from a regime with superfluid character for shallow lattices into a regime of strong correlations, the Mott insulator. As an additional external parameter besides the lattice depth, one can generate spatial inhomogeneities by superimposing an additional standing wave (so-called two-color superlattices), which gives rise to localization effects or the formation of a Bose-glass phase. In the present work, numerical simulations are employed in order to investigate characteristic signatures of the quantum phases in the low-energy excitation spectrum of one-dimensional systems. We simulate temporal small amplitude modulations of the optical lattice in analogy to experiments, and evaluate the response of the system from the time-evolved initial state. The lattice systems are described in the framework of the Bose-Hubbard model. For the evaluation of the time-evolved state, we employ several numerical methods. We analyze systems of small size (6 particles on 6 sites) using an exact time-evolution by integration of the time-dependent Schrödinger equation. The formulation of an importance truncation scheme enables us to retain only the relevant components of the model space in the strongly correlated regime and, thus, allows for the investigation of systems with 10 particles on 10 sites using exact time-evolution. Based on this method, we present results of the Mott-insulating regime as well as for the Bose-glass phase. Furthermore, we employ particle-hole methods, which allow for the treatment of systems with experimental lattice sizes and particle numbers. Starting from the equation of motion method we adapt the Tamm-Dancoff approximation as well as the random-phase approximation for the occupation number representation of the Bose-Hubbard model. We present results of simulations of up to 50 particles on 50 sites and discuss the impact of the lattice depth on the low-energy excitations (U-resonance). Moreover, the impact of a two-color superlattice and the variation of its amplitude is investigated.
I describe in these notes the physical properties of one dimensional interacting quantum particles. In one dimension the combined effects of interactions and quantum fluctuations lead to a radically new physics quite different from the one existing in the higher dimensional world. Although the general physics and concepts are presented, I focuss in these notes on the properties of interacting bosons, with a special emphasis on cold atomic physics in optical lattices. The method of bosonization used to tackle such problems is presented. It is then used to solve two fundamental problems. The first one is the action of a periodic potential, leading to a superfluid to (Mott)-Insulator transition. The second is the action of a random potential that transforms the superfluid in phase localized by disorder, the Bose glass. Some discussion of other interesting extensions of these studies is given.
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