ua nt - p h / 05 03 15 6 v 1 1 7 M ar 2 00 5 Probing the light induced dipole - dipole interaction in mo - mentum space

– We theoretically investigate the mechanical effect of the light-induced dipole-dipole interaction potential on the atoms in a Bose-Einstein condensate. We present numerical calculations on the magnitude and shape of the induced potentials for different experimentally accessible geometries. It is shown that the mechanical effect can be distinguished from the effect of incoherent scattering for an experimentally feasible setting. Introduction. – The interaction of a radiation field with an ensemble of atoms has been investigated in many distinct contexts since the early works of Lorentz and Lorenz [1] of light in dense media. Within one century, researchers came up with an innumerable variety of light-matter interaction types [2] and reached a recent summit with the development of laser cooling [3]. The dipole character of an atom, driven by an electromagnetic wave, can be used to apply forces on atoms by intensity gradients of light fields [4] but it can also produce forces between atoms [5]. The implication of interacting atoms on the refractive index was treated theoretically for dilute Bose gases [6] with an extended Lotentz-Lorenz model. The effect of light-induced or excited state collisions shows up in laser cooling by losses in magneto-optical traps [7]. The coherent part of this interaction is recently discussed in basically two directions. The one focuses on the properties of the light absorbed and emitted in dense media, namely radiation trapping [8], level shifts [9], dipole blockade effects as a tool for quantum information processing [10] and collective effects such as superradiance [11]. The other main focus tends towards new forms of interaction in Bose-Einstein condensates as effective 1/r potentials [12] or as a generator for rotons [13]. Recently, the effect of the resonant dipole-dipole interaction in a cold cloud of highly excited Rydberg states was observed by broadening of spectroscopic lines [14] and by resonant energy transfer between different Rydberg states [15]. In this letter, we investigate new physical aspects that arise in dense cold atomic samples irradiated by a near resonant laser beam. Atoms exposed to an electromagnetic wave responds as damped harmonic oscillators and exhibit an alternating electric dipole moment. The interaction energy of such dipoles can exceed the one of magnetic dipoles in atomic ground states