Observing the onset of effective mass.

The response of a particle in a periodic potential to an applied force is commonly described by an effective mass, which accounts for the detailed interaction between the particle and the surrounding potential. Using a Bose-Einstein condensate of (87)Rb atoms initially in the ground band of an optical lattice, we experimentally show that the initial response of a particle to an applied force is in fact characterized by the bare mass. Subsequently, the particle response undergoes rapid oscillations and only over time scales that are long compared to those of the interband dynamics is the effective mass observed to be an appropriate description. Our results elucidate the role of the effective mass on short time scales, which is relevant for example in the interaction of few-cycle laser pulses with dielectric and semiconductor materials.

Enhanced coherence between condensates formed resonantly at different times.

We demonstrate coherence between exciton-polariton condensates created resonantly at different times. The coherence persists much longer than the individual particle dephasing time, and this persistence increases as the particle density increases. The observed coherence time exceeds that of the injecting laser pulse by more than an order of magnitude. We show that this significant coherence enhancement relies critically on the many-body particle interactions, as verified by its dependence on particle density, interaction strength, and bath temperature, whereas the mass of the particles plays no role in the condensation of resonantly injected polaritons. Furthermore, we observe a large nonlinear phase shift resulting from intra-condensate interaction energy. Our results provide a new approach for probing ultrafast dynamics of resonantly-created condensates and open new directions in the study of coherence in matter.