Experimental observation of nonclassical correlations in photon pairs generated from an ensemble of pure two-level atoms

We report the experimental verification of nonclassical correlations for an unfiltered spontaneous four-wave-mixing process in an ensemble of cold two-level rubidium atoms, obtaining $R = (1.98\pm0.03) \nleq 1$ for the violation of a Cauchy-Schwarz inequality in the system, confirming theoretical predictions by Du {\it et al.} in 2007. Quantum correlations are observed in a nano-seconds timescale, in the interference between the sidebands dislocated by the detuning to the atomic resonance. They prevail over the noise background coming from Rayleigh 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.