  We present the experimental observation of the antiblockade in an ultracold
Rydberg gas recently proposed by Ates et al. [Phys. Rev. Lett. 98, 023002
(2007)]. Our approach allows the control of the pair distribution in the gas
and is based on a strong coupling of one transition in an atomic three-level
system, while introducing specific detunings of the other transition. When the
coupling energy matches the interaction energy of the Rydberg long-range
interactions, the otherwise blocked excitation of close pairs becomes possible.
A time-resolved spectroscopic measurement of the Penning ionization signal is
used to identify slight variations in the Rydberg pair distribution of a random
arrangement of atoms. A model based on a pair interaction Hamiltonian is
presented which well reproduces our experimental observations and allows one to
analyze the distribution of nearest neighbor distances.
