G. E. Astrakharchik

Superfluidity versus Bose-Einstein condensation in a Bose gas with disorder

We investigate the phenomenon of Bose-Einstein condensation and superfluidity in a Bose gas at zero temperature with disorder. By using the diffusion Monte Carlo method, we calculate the superfluid and the condensate fraction of the system as a function of density and strength of disorder. In the regime of weak disorder we find agreement with the analytical results obtained within the Bogoliubov model. For strong disorder the system enters an unusual regime where the superfluid fraction is smaller than the condensate fraction.

Sharp crossover from composite fermionization to phase separation in microscopic mixtures of ultracold bosons

We show that a two-component mixture of a few repulsively interacting ultracold atoms in a one-dimensional trap possesses very diverse quantum regimes and that the crossover between them can be induced by tuning the interactions in one of the species. Starting from the composite fermionization regime, in which the interactions between both components are large and neither gas is phase coherent, our results show that a phase-separated state can be reached by increasing the interaction in one of the species. In this regime, the weakly interacting component stays at the center of the trap and becomes almost fully phase coherent, while the strongly interacting one is expelled to the edges of the trap. The crossover is sharp, as can be witnessed in the systems energy and in the occupation of the lowest natural orbital of the weakly interacting species. We show that such a transition is a few-atom effect which disappears for a large population imbalance.

Wave functions of the super Tonks-Girardeau gas and the trapped 1D hard sphere Bose gas

Recent theoretical and experimental results demonstrate a close connection between the super-Tonks-Girardeau (STG) gas and a one-dimensional (1D) hard-sphere Bose (HSB) gas with hard-sphere diameter nearly equal to the 1D scattering length a{sub 1D} of the STG gas, a highly excited gaslike state with nodes only at interparticle separations |x{sub jl}|=x{sub node{approx_equal}}a{sub 1D}. It is shown herein that when the coupling constant g{sub B} in the Lieb-Liniger interaction g{sub B{delta}}(x{sub jl}) is negative and |x{sub 12}|{>=}x{sub node}, the STG and HSB wave functions for N=2 particles are not merely similar, but identical; the only difference between the STG and HSB wave functions is that the STG wave function allows a small penetration into the region |x{sub 12}| 2. The STG and HSB wave functions for N=2 are given exactly in terms of a parabolic cylinder function, and for N{>=}2, x{sub node} is given accurately by a simple parabola. The metastability of the STG phase generated by a sudden change of the coupling constant from large positive to largemorexa0» negative values is explained in terms of the very small overlap between the ground state of the Tonks-Girardeau gas and collapsed cluster states.«xa0less

One-dimensional Bose gas in optical lattices of arbitrary strength

One-dimensional Bose gas with contact interaction in optical lattices at zero temperature is investigated by means of the exact diffusion Monte Carlo algorithm. The results obtained from the fundamental continuous model are compared with those obtained from the lattice (discrete) Bose-Hubbard model, using exact diagonalization, and from the quantum sine-Gordon model. We map out the complete phase diagram of the continuous model and determine the regions of applicability of the Bose-Hubbard model. Various physical quantities characterizing the systems are calculated, and it is demonstrated that the sine-Gordon model used for shallow lattices is inaccurate.

Thermal and quantum fluctuations in chains of ultracold polar molecules

Ultracold polar molecules, in highly anisotropic traps and interacting via a repulsive dipolar potential, may form one-dimensional chains at high densities. According to classical theory, at low temperatures there exists a critical value of the density at which a second order phase transition from a linear to a zigzag chain occurs. We study the effect of thermal and quantum fluctuations on these self-organized structures using classical and quantum Monte Carlo methods, by means of which we evaluate the pair correlation function and the static structure factor. Depending on the parameters, these functions exhibit properties typical of a crystalline or of a liquid system. We compare the thermal and the quantum results, identifying analogies and differences. Finally, we discuss experimental parameter regimes where the effects of quantum fluctuations on the linear - zigzag transition can be observed.

Number fluctuations in cold quantum gases

In ultracold gases many experiments use atom imaging as a basic observable. The resulting image is averaged over a number of realizations and mostly only this average is used. Only recently the noise has been measured to extract physical information. In the present paper we investigate the quantum noise arising in these gases at zero temperature. We restrict ourselves to the homogeneous situation and study the fluctuations in particle number found within a given volume in the gas, and more specifically inside a sphere of radius

High-fidelity entanglement via molecular dissociation in integrated atom optics

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Condensate fraction of cold gases in a nonuniform external potential

. We show that zero-temperature fluctuations are not extensive and the leading term scales with sphere radius

Breakdown of scale invariance in the vicinity of the Tonks-Girardeau limit

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QUANTUM MONTE CARLO STUDY OF THE GROUND-STATE PROPERTIES OF A FERMI GAS IN THE BCS-BEC CROSSOVER

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