Manipulating Bose-Einstein condensates with laser light

Abstract

A dilute gas Bose-Einstein condensate was probed and manipulated by off-resonant laser beams. Spontaneous and stimulated off-resonant light scatterings were studied experimentally. Stimulated, two-photon Bragg scattering was used for spectroscopic measurement of the mean-field energy and of the intrinsic momentum uncertainty of the condensate. The high momentum and energy resolution of this method allowed the determination of the coherence length of the condensate, which was shown to be equal to its size. Spontaneous, off-resonant Rayleigh scattering was studied by exposing an elongated condensate to a single off-resonant laser beam. Highly directional scattering of light and atoms was observed. This collective light scattering is shown to be directly analogous to Dicke superradiance, where the electronic coherence is replaced by the coherent center-of-mass motion of the atoms in the condensate. Superradiant Rayleigh scattering was used to amplify atomic matter waves. The active medium was a Bose-Einstein condensate, pumped by off-resonant laser light ("Dressed condensate"). An atomic wave packet was amplified with a gain of 10 to 100. Phase-coherence of the amplifier was verified by observing the interference of the output wave with a reference wave packet. Optical properties of the dressed condensate were also characterized, focusing on the key role of long-lived matter wave gratings produced by interference between the condensate at rest and the recoiling atoms. The narrow bandwidth for the optical gain gave rise to an extremely slow group velocity of an amplified light pulse ([approx.] 1 m/s).

(cont.) The role of quantum statistics in these enhanced scatterings was studied. It was shown that the macroscopic occupation of a single quantum state is not necessary. These processes are in principle possible for fermionic or non-degenerate samples, provided the atomic ensemble has a sufficiently long coherence time. By moving a focused, far off-resonant laser beam through a condensate, vortex excitations were created in a Bose-Einstein condensate. They were observed as dislocations in the interference fringes formed by the stirred condensate and a second unperturbed condensate. The technique was shown to be a powerful tool to study turbulent superfluid flow.