• 4-body process
• Cold gases
• Collective excitation
• Cooperative effects
• Dicke collective states
• Few-body
• Interactions
• Many-body
• Quantum correlation
• Rydberg atoms
• Sub-poissonian Statistics

During this thesis, the Physics of interacting systems has been investigated experimentally using Cold Rydberg gases. Rydberg atoms are very excited atoms and have the particularity to interact strongly together through long-range electrostatic interaction. A first highlight of the thesis is a direct experimental observation of a 4-body process. This process consists in internal exchanges of excitations between 4 Rydberg atoms due to their mutual interactions. In addition to its experimental observation, the process has been succefully described theoretically at a quantum level. A second highlight of this thesis has concerns the study of the laser excitation of strongly interacting Rydberg gases. In this regime, many-body quantum correlations play a central role leading to fascinating many-body behavior. In addition to fundamental interests, such systems could be used to implement quantum simulator or non-classical light sources. The system has been investigated during this thesis through the counting statistics of the Rydberg excitation. A highly sub-poisonian, i.e correlated, statistics has been observed in the regim of strong interactions proving the many-body nature of the system. A third highlight of this thesis is the development of an original theoretical model to describe the laser excitation of strongly interacting Rydberg gases. Using the so-called Dicke collective states, it has been possible to catch several interesting features of this many-body problem.