  We present evidence for Rydberg-Rydberg interaction in a gas of rubidium
atoms above room temperature. Rabi oscillations on the nanosecond timescale to
different Rydberg states are investigated in a vapor cell experiment. Analyzing
the atomic time evolution and comparing to a dephasing model we find a scaling
with the Rydberg quantum number n that is consistent with van der Waals
interaction. Our results show that the interaction strength can be larger than
the kinetic energy scale (Doppler width) which is the requirement for
realization of thermal quantum devices in the GHz regime.
