  We report the experimental verification of nonclassical correlations for a
four-wave-mixing process in an ensemble of cold two-level atoms, confirming
theoretical predictions by Du et al. in 2007 for the violation of a
Cauchy-Schwarz inequality in the system, and obtaining $R = (1.98\pm0.03) \nleq
1$. Quantum correlations are observed in a nano-seconds timescale, in the
interference between the central exciting frequency and sidebands dislocated by
the detuning to the atomic resonance. They prevail without filters over the
noise background coming from linear scattering from the same optical
transition. These correlations are fragile with respect to processes that
disturb the phase of the atomic excitation, but are robust to variations in
number of atoms and to increasing light intensities.
