Berlin 2001 – wissenschaftliches Programm

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AMPD: EPS AMPD

AMPD 6: Sitzung 6

AMPD 6.3: Vortrag

Mittwoch, 4. April 2001, 10:10–10:35, H104

Linear and Nonlinear Atom Optics with Bose–Einstein Condensates — •K. Sengstock — Institut für Quantenoptik der Universität Hannover, Welfengarten 1, 30167 Hannover, Germany

The realization of Bose–Einstein condensation (BEC) of weakly interacting atomic gases stimulates strongly the exploration of nonlinear properties of matter waves. This supports the new field of nonlinear atom optics, e.g., four wave mixing in BEC’s, as well as the study of various types of excitations. Of particular interest are macroscopically excited Bose condensed states, such as vortices and solitons. Soliton–like solutions of the Gross–Pitaevskii equation are closely related to similar solutions in nonlinear optics describing the propagation of light pulses in optical fibres. Here, bright soliton solutions correspond to short pulses where the dispersion of the pulse is compensated by the self–phase modulation, i.e., the shape of the pulse does not change. Similarly, optical dark solitons correspond to intensity minima within a broad light pulse. In the case of nonlinear matter waves, bright solitons are only expected for an attractive interparticle interaction (s–wave scattering length a < 0), whereas dark solitons, also called ’kink–states’, are expected to exist for repulsive interactions (a > 0). Conceptually, solitons as particle–like objects provide a link of BEC physics to fluid mechanics, nonlinear optics and fundamental particle physics. In this presentation we report on the experimental investigation of dark solitons in cigar shaped Bose–Einstein condensates of ⁸⁷Rb. Low lying excited states are produced by imprinting a local phase onto the BEC wavefunction. This is done by applying the dipole potential of a far detuned laser pulse to part of the BEC and thus imprinting the necessary phase structure for the generation of a dark soliton state. As a result of this additional phase the BEC wavefunction generates density minima. By monitoring the evolution of the density profile we study the successive dynamics of the wavefunction. The evolution of density minima travelling at a smaller velocity than the speed of sound in the trapped condensate is observed. By comparison to analytical and numerical solutions of the 3D Gross–Pitaevskii–equation for our experimental conditions we identify these density minima to be moving dark solitons [1]. We have performed series of measurements with different parameter sets for the product of laser intensity and imprinting time. The velocity of the dark soliton could thereby be varied between 50% and 90% of the velocity of sound. The initial stages of the evolution and the radial ballistic expansion of the sample are well described by a T = 0 approach which also shows the absence of dynamical instabilities. A decrease of the soliton contrast is observed and gives a clear signature of thermodynamical dissipation in the soliton dynamics.

We also report on preliminary measurements of the interaction of two moving dark solitons in BEC, as well as on the realization of a 1D–matter–wave–guide [2] and the study of solitons in a 1D–geometry.

[1] S. Burger, K. Bongs, S. Dettmer, W. Ertmer, K. Sengstock, A. Sanpera, G. V. Shlyapnikov, M. Lewenstein, Phys. Rev. Lett. 83, 5198 (1999).

[2] K. Bongs, S. Burger, S. Dettmer, D. Hellweg, J. Arlt, W. Ertmer, K. Sengstock, Phys. Rev. A, Rap. Com. 63, March (2001).