arXiv subjects

M. Naraschewski

Publications and source records attributed to M. Naraschewski.

7 recordsLinked to original sources

arXiv1998

Berry's Phase Induced Bose-Einstein condensation into a Vortex State

The existence of a geometric phase in magnetic traps can be used to Bose condense a magnetically trapped atomic gas into a vortex state. We propose an experimental setup where a magnetic trap together with a blue detuned laser beam form a multiply connected trap geometry. The local variation of the magnetic quantization axis induces a geometric or Berry's phase that allows the atoms to acquire an effective gauge charge interacting with the analog of a magnetic solenoid. It is shown that the ground state of such a system may be given by a vortex state. We also discuss the influence of atomic interactions on the proposed vortex production scheme in the context of present Bose-Einstein condensation experiments with dilute gases.

cond-mat↗

arXiv1998

The Markov approximation for the atomic output coupler

The regions of validity of the Markov approximation for the coupling of atoms out of an atomic trap are determined. We consider radio-frequency output coupling in the presence of gravity and collisional repulsion, and Raman output coupling. The Markov approximation is crucial in most theoretical descriptions of an atom laser that assume a continuous process of output coupling from a trapped Bose-Einstein condensate. In this regime many techniques proved to be useful for modeling the optical laser, such as master equations, can be used to describe the dynamics of the damping of the condensate mode undergoing output coupling.

cond-mat.stat-mech↗

arXiv1998

Spatial coherence and density correlations of trapped Bose gases

We study first and second order coherence of trapped dilute Bose gases using appropriate correlation functions. Special attention is given to the discussion of second order or density correlations. Except for a small region around the surface of a Bose-Einstein condensate the correlations can be accurately described as those of a locally homogeneous gas with a spatially varying chemical potential. The degrees of first and second order coherence are therefore functions of temperature, chemical potential, and position. The second order correlation function is governed both by the tendency of bosonic atoms to cluster and by a strong repulsion at small distances due to atomic interactions. In present experiments both effects are of comparable magnitude. Below the critical temperature the range of the bosonic correlation is affected by the presence of collective quasi-particle excitations. The results of some recent experiments on second and third order coherence are discussed. It is shown that the relation between the measured quantities and the correlation functions is much weaker than previously assumed.

cond-mat↗

arXiv1998

Analytical description of a trapped semi-ideal Bose gas at finite temperature

Present experiments with Bose condensed gases can be largely described by a semi-ideal two-gas model. In this model, the condensate is influenced only by the mean-field repulsion among condensed atoms, while the thermal cloud is considered an ideal gas confined by an effective potential that consists of the external trap and the mean-field repulsion by the condensate. This simple, intuitive model provides explicit analytical expressions for the density distributions of the condensate and the thermal component. It describes the reduction of the condensate fraction relative to that of an ideal gas as a consequence of the positive chemical potential due to interactions in the condensate.

cond-mat↗

arXiv1997

Output of a pulsed atom laser

We study the output properties of a pulsed atom laser consisting of an interacting Bose-Einstein condensate (BEC) in a magnetic trap and an additional rf field transferring atoms to an untrapped Zeeman sublevel. For weak output coupling we calculate the dynamics of the decaying condensate population, of its chemical potential and the velocity of the output atoms analytically.

quant-ph↗

arXiv1996

Interference of Bose condensates

We investigate the prospects of atomic interference using samples of Bose condensed atoms. First we show the ability of two independent Bose condensates to create an interference pattern, even if both condensates are described by Fock states. Thus, the existence of an experimental signature for a broken gauge symmetry, seen in a single run of the experiment, is not necessarily reflected by a broken symmetry on the level of the quantum mechanical state vector. Based on these results, we simulate numerically a recent experiment with two independent Bose condensates, performed by the group of W.Ketterle (MIT). The calculated expansion of the condensates is in good agreement with the experimental data. In addition the existence of interference fringes is predicted based on the nonlinear Schroedinger equation. Finally we study theoretically the influence of finite temperatures on the visibility of the interference in a double pinhole experiment.

atom-ph↗

arXiv1996

Continuous Observation of Interference Fringes from Bose Condensates

We use continuous measurement theory to describe the evolution of two Bose condensates in an interference experiment. It is shown how the system evolves in a single run of the experiment into a state with a fixed relative phase, while the total gauge symmetry remains unbroken. Thus, an interference pattern is exhibited without violating atom number conservation.

atom-ph↗