Over the last decades quantum effects have become more and more controllable, leading to the implementations of various quantum information protocols. These protocols are all based on utilizing quantum correlation. In this thesis we consider how states of an atomic ensemble with such correlations can be created and characterized.

First we consider a spin-squeezed state. This state is generated by performing quantum non-demolition measurements of the atomic population difference. We show a spectroscopically relevant noise reduction of -1.7dB, the ensemble is in a many-body entangled state. Furthermore, the nonclassical properties of the created state is inferred through the use of atomic quadrature quasi-probability distributions.

The second generated state is a collective-single-excitation state — the atomic equivalent of a single photon. This state is created by the detection of a heralding photon and characterized using atomic homodyne tomography. Using this hybrid continues-discrete method we show a significant increase in the variance of the measurements conditioned on a click. A clear signature of the collective-single-excitation state.

Last we consider a new experimental venture — a nanofiber based light-atom interface. Using a dual-frequency probing method we measure and prepare an ensemble with a sub-Poissonian atom number distribution. This is a first step towards the implementation of more exotic quantum states.