Collective excitations of a dilute Bose gas were probed above and below the Bose-Einstein condensation temperature. The temperature dependencies of the frequency and damping rates of condensate oscillations indicate significant interactions between the condensate and the thermal cloud. Hydrodynamic oscillations of the thermal cloud analogous to first sound were observed. An outof-phase dipolar ...

The recent achievement of Bose-Einstein condensation (BEC) in trapped atoms of dilute gases [1–3] has been quickly followed by the quantitative measurements of the condensate’s properties. In particular the dependence of the fraction of atoms in the condensate on the temperature has been found to be in agreement with the theory of the noninteracting Bose gas in a harmonic trap [4]. From the poi...

A gas of magnons in magnetic films differs from all other known systems demonstrating Bose-Einstein condensation (BEC), since it possesses two energetically degenerate lowest-energy quantum states with non-zero wave vectors ±k(BEC). Therefore, BEC in this system results in a spontaneously formed two-component Bose-Einstein condensate described by a linear combination of two spatially non-unifor...

The interference observed between two Bose-Einstein condensates demonstrated clearly the long-range coherence of a Bose condensate [1]. The fringe contrast is directly related to first-order coherence. Our paper [2] showed that previous measurements of the interaction energy of a condensate can be reinterpreted as a measure for second-order coherence and therefore provide direct evidence for th...

A Bose-Einstein condensate illuminated by a single off-resonant laser beam ("dressed condensate") shows a high gain for matter waves and light. We have characterized the optical and atom-optical properties of the dressed condensate by injecting light or atoms, illuminating the key role of long-lived matter wave gratings produced by the condensate at rest and recoiling atoms. The narrow bandwidt...

By shining a tightly focused laser light on a Bose–Einstein condensate (BEC) and moving the centre of the beam along a spiral path one may stir the BEC and create vortices. It is shown that one can induce rotation of the BEC in the direction opposite to the direction of stirring. When a spoon, stick or another object of a similar shape is used to stir a liquid, the liquid rotates in the directi...

The quantized vortex state is investigated in a Bose-Einstein condensate, confined in a multiply connected geometry formed by a Laguerre-Gaussian optical trap. Solving the Gross-Pitaevskii equation variationally, we show that the criterion for vortex stability is that the interatomic interaction strength must exceed a critical interaction strength. The time evolution of a freely expanding Lague...

We derive an exact recursion formula for the calculation of thermodynamic functions of finite systems obeying Bose-Einstein statistics. The formula is applicable for canonical systems where the particles can be treated as noninteracting in some approximation, e.g. like Bose-Einstein condensates in magnetic traps. The numerical effort of our computation scheme grows only linear with the number o...

In this paper we study the relation between long cycles and Bose-Condensation in the Infinite range Bose-Hubbard Model with a hard core interaction. We calculate the density of particles on long cycles in the thermodynamic limit and find that the existence of a non-zero long cycle density coincides with the occurrence of Bose-Einstein condensation but this density is not equal to that of the Bo...

The coherent and collective nature of a Bose–Einstein condensate can enhance or suppress physical processes. Bosonic stimulation enhances scattering in already occupied states which leads to matter wave amplification, and the suppression of dissipation leads to superfluidity. In this article we present several experiments where enhancement and suppression have been observed and discuss the comm...