Experimental observation of some phenomena in Bose-Einstein condensates

In this chapter we review our experimental research on cold atoms and Bose- Einstein condensates, including the properties of the condensate in a tight confinement and across an optical dipole potential, the parametric excitation of cold atoms in a quadrupole-Ioffe-configuration trap, and the precise manipulation of atoms on a microchip. The behavior of BEC in a trap without free-falling is described in detail, and the sudden shrink of the long axis of the atomic cloud in a cigar-shape trap was observed directly when the condensation occurs. The experimental exploration supports the use of the reduction in the effective sample size as evidence for the onset of the condensate in the case that the trap cannot be switched off completely, just like permanent-magnet traps, or if the time-of-flightmethod is not convenient in a restricted geometry of the traps. The dynamical evolution of a cold atomic cloud and a condensate passing through a far red-detuned Gaussian beam is studied both numerically and experimentally. Several exotic phenomena, such as the focusing and advancement of atomic clouds, were exhibited. It shows that the BEC can be operated in a controllable manner, and it can be used to simulate a wide variety of quantum phenomena which benefit from the wave-like characteristic of the condensate and flexible manipulation of optical potentials. Furthermore, we discuss the parametric excitation of 87Rb atoms in a quadrupole-Ioffe-configuration trap, and measure the dependence of the temperature and the number of the atomic cloud on modulation frequency of the parametric excitation field. It is found that the contribution of atomic collisions to the energy distributions results in a lower temperature of the atomic cloud than the theoretical prediction. Finally, we review our efforts in realizing the first BEC on an atom chip in China and report a systematic study on precisely controlling the behavior of cold atoms on chips, including magnetic trapping, adiabatically compressing, guiding, dynamically splitting and merging. Surface-induced evaporative cooling and parametric resonance induced by the modulation of the current in chip wires are also described.