• Cold atoms
• many-body systems
• Rydberg atoms

Atoms in highly excited states, known as Rydberg atoms, have been proposed as a possi-
ble candidate to simulate many-body problems with cold atoms. While ground state atoms
interact weakly, in Rydberg atoms the high dipole moments enable strong interactions. To
take advantage of this peculiarity, we need a simple representation of Rydberg atoms, that
helps to separate single-particle dynamics from many-body behavior. Ideally, we would like to
associate Rydberg atoms with a two-level system considering only ground and excited state.
However not only many-body effects, which we are mainly interested in, modify the two level
picture by shifting the energy of the excited state, but also multi-level atomic structure mod-
ifies the model with the introduction of additional levels. In this thesis we investigate the
deviations from the simple two-level model arising in a sample of cold Rb atoms excited to
Rydberg state, due to many-body and multi-level effects.
Manybody effects discussed in this thesis focus mainly on van der Waals interaction that
may shift, broaden or split the Rydberg state. Such variations contain information on the
sample geometry and the atom motion. To probe them we use a de-excitation technique that
transfers atoms with a specific energy to a fast decaying level. The same technique also allows
us to observe the emergence of multi-level effects in Rydberg decay. Multilevel effects arise
from the interaction of a Rydberg state with the black body radiation of a room temperature
environment. That radiation couples the orginal Rydberg state to other Rydberg states that
are closely spaced in energy and spreads the initial population over a number of different
Rydberg states, each with its own properties.
By using de-excitation to selectively remove atoms from a given state we are able to separate
its population from that of other Rydberg levels and to observe its evolution over time. De-
excitation technique used to probe both effects is characterized within the thesis. Multilevel
effects arise also during excitation, the use of two-photon excitation introduces a virtual level
close to a real atomic level. The proximity of the virtual and the real level makes it probable
to populate the latter. We discuss the possibility to neglect population of the real level to
recover the two-level model. Our measurements show the limits of the two-level model in
excitation, decay and de-excitation of Rydberg atoms, and demonstrate the usefulness of a
new technique to probe strong interactions in Rydberg atoms.