We study effects of collective states (CS) in both superradiant and electromagnetically induced transparent (EIT) systems. For superradiance, we incorporate the cooperative effects into a two-atom master equation, which results in a superradiant time close to the experimental data. Additionally, we identify the role of collectivity in Raman superradiance in a Bose–Einstein Condensate (BEC). As an application of CS to quantum information, we study the feasibility of EIT in semiconductors and a flexible multiple beam splitter for generating entangled single photons. Specifically, we find that diffusion on stored coherence makes the decoherence rate of a vortex state is surprisingly larger than that of a Gaussian beam. Generally, the less phase gradient, the better for stored coherence against diffusion. Additionally, we study of the effect of laser noise on EIT systems and good agreement between the theory and the experiment.

Finally, I will briefly mention about our other research related to condensed matter systems.