The study of quantum light-matter interaction with atomic ensembles is an active research area. This kind of system allows fundamental studies on measurement in the context of continuous variables, in collective entanglement and in quantum simulations. This field of research is also interesting in the context of quantum metrology, quantum networking and quantum computation. In this thesis two complementary aspects of light matter interaction with atomic ensembles have been studied with trapped ions and cold neutral atoms. The trapped ion experiment is intended to evaluate the possibility to use large ion clouds for realizing a quantum memory with long coherence times. Laser cooled trapped ions can reach a crystalline phase due to the strong Coulomb repulsion between ions. In this phase the relative positions between the ions is fixed avoiding several sources of coherence loss. The light matter interaction in a large Coulomb crystal have been measured, and the possible limitations of such system are discussed. The cold atom experiment focused on the use of quantum non demolition measurements to evaluate non-Gaussian states in the context of an experiment similar to quantum networking experiment currently planned.. Non Gaussian atomic states are a resource for quantum computation and quantum communication, in the context of atomic physics experiments, their detection can be difficult. The work presented in this thesis focuses on the detection of non Gaussian states in atomic ensembles using cumulants, and in particular their noise properties.
