  We experimentally and theoretically investigate the dephasing rates of the
coherent evolution of a resonantly driven pseudo spin emersed in a reservoir of
pseudo spins. The pseudo spin is realized by optically exciting 87 Rb atoms to
a Rydberg state. Hence, the upper spin states are coupled via the strong van
der Waals interaction. Two different experimental techniques to measure the
dephasing rates are shown: the 'rotary echo' technique known from nuclear
magnetic resonance physics and electromagnetically induced transparency. The
experiments are performed in a dense frozen Rydberg gas, either confined in a
magnetic trap or in an optical dipole trap. Additionally, a numerical
simulation is used to analyse the dephasing in the rotary echo experiments.
