Content area
Full Text
Rayleigh scattering off a Bose-Einstein condensate was studied. Exposing an elongated condensate to a single off-resonant Laser beam resulted in the observation of highly directional scattering of light and atoms. This collective light scattering is caused by the coherent center-of-mass motion of the atoms in the condensate. A directional beam of recoiling atoms was built up by matter wave amplification.
Rayleigh scattering, the scattering of a photon off an atom, is the most elementary process involving atoms and light. It is responsible for the index of refraction of gases, the blue sky, and resonance fluorescence. This process can be divided into absorption of a photon and subsequent spontaneous emission. Photon scattering imparts a recoil momentum to the atom. Because of the random nature of spontaneous emission, the direction of the recoil is random, leading to momentum diffusion and heating of the atomic motion.
With the realization of Bose-Einstein condensation (BEC) (1), it is now possible to study the interactions of coherent light with an ensemble of atoms in a single quantum state. The high degree of spatial and temporal coherence of a condensate was confirmed in several experiments (2, 3). This raises the important question of whether the coherent external motion of the atoms can alter the interactions between atoms and light. Here we show that the long coherence time of a Bose-Einstein condensate introduces strong correlations between successive Rayleigh scattering events. The scattering of photons leaves an imprint in the condensate in the form of long-lived excitations that provide a positive feedback and lead to directional Rayleigh scattering.
This phenomenon is analogous to the superradiance discussed by Dicke (6). He showed that the optical emission of incoherently excited atoms can be highly directional. The key feature of superradiance (or super-- fluorescence) (11) is that spontaneous emission is not a single-atom process but a collective process of all atoms, leaving the atoms in a coherent superposition of ground and excited states (12). The condensate at rest "pumped" by the off-resonant laser corresponds to the electronically excited state in the Dicke case. It can decay by a spontaneous Raman process to a state with photon recoil (corresponding to the ground state). The rate of superradiant emission in Dicke's treatment is proportional to the square of...