Transitions in Atom-Atom Collisions Produced by a Dipole-Dipole Interaction

Bongjune Kim, Ko-Tang Chen, Kuei-You Chen +4 more

Dark-state polaritons (DSPs) based on the effect of electromagnetically induced transparency are bosonic quasiparticles, representing the superpositions of photons and atomic ground-state coherences. It has been proposed that stationary DSPs are governed by the equation of motion closely similar to the Schrödinger equation and can be employed to achieve Bose-Einstein condensation (BEC) with transition temperature orders of magnitude higher than that of the atomic BEC. The stationary-DSP BEC is a three-dimensional system and has a far longer lifetime than the exciton-polariton BEC. In this Letter, we experimentally demonstrated the stationary DSP dressed by the Rydberg-state dipole-dipole interaction (DDI). The DDI-induced phase shift of the stationary DSP was systematically studied. Notably, the experimental data are consistent with the theoretical predictions. The phase shift can be viewed as a consequence of elastic collisions. In terms of thermalization to achieve BEC, the μm^{2}-size interaction cross section of the DDI can produce a sufficient elastic collision rate for the stationary DSPs. This Letter makes a substantial advancement toward the realization of the stationary-DSP BEC.

E. Duman

R. F. Stebbings

Vladimir A. Sautenkov

In this paper, our experimental studies of dipole-dipole interactions in excited resonance gases are discussed. In weakly excited gas the dipole broadening of optical transitions is proportional to its density N. A gas medium can be identified by its density N as rarefied gas, where dipole broadening of atomic transitions is much less than Doppler width, or as dense gas, where dipole broadening is much more than Doppler width. According to our experiments the dipole broadening in rarefied gas does not depend on the optical excitation. This behavior is in a good agreement with predictions of standard theory for impact atomic collisions. In excited dense gas we observed the strong reduction of the dipole broadening. These experimental results are not described by standard theory for impact collisions. We suggested a simple description of the optical properties for excited dense gas.

Dominikus Hoffmann

In this dissertation I present a variety of experiments on collisions between ultra-cold rubidium atoms. These collisions take place in light-force atom traps, where temperatures below 1 mK are reached. At these low temperatures weak, long-range interactions between the atoms are important, and collision times are long. Consequently, the absorption and spontaneous emission of photons plays an important role in determining the collision dynamics. The collisions studied in this dissertation involve excitation of the atoms to a state where the relatively strong dipole-dipole interaction with another atom results in attraction (or repulsion) after which spontaneous emission may take place. The resulting energy transfer in such a collision can eject the atoms from the trap. We have observed such collisions in both stable isotopes of rubidium. If the excitedstate is involved, the two isotopes behave quite di ff erently. This makes clear the importance of hyperfine interactions in the collision dynamics. In contrast, for the excited-state such di ff erences disappeare, because the hyperfine interaction is strong enough to eliminate hyperfine mixing of the potentials. Another collision process involved collisions with a repulsive dipole-dipole interaction. This experiment gives strong support for a Landau-Zener treatment of the excitation process. These collisions enabled the measurement of the trap depth. Finally, collisions between rubidium atoms that have undergone a two-step excitation by two near-infrared photons produce violet fluorescence as a signature of the collision. We have observed this e ff ect. The data shows unexpected features, which warrant further investigation. P3 2 ⁄

Boris Michailovich Smirnov