  We present the experimental demonstration of interaction induced enhancement
in Rydberg excitation or Rydberg anti-blockade in thermal atomic vapor. We have
used optical heterodyne detection technique to measure Rydberg population due
to two-photon excitation to the Rydberg state. The anti-blockade peak which
doesn't satisfy the two-photon resonant condition is observed along with the
usual two-photon resonant peak which can't be explained using the model with
non-interacting three-level atomic system. A model involving two interacting
atoms is formulated for thermal atomic vapor using the dressed states of
three-level atomic system to explain the experimental observations. A
non-linear dependence of vapor density is observed for the anti-blockade peak
which also increases with increase in principal quantum number of the Rydberg
state. A good agreement is found between the experimental observations and the
proposed interacting model. Our result implies possible applications towards
quantum logic gates using Rydberg anti-blockade in thermal atomic vapor.
