Observation of nonclassical correlations in biphotons generated from an ensemble of pure two-level atoms

TL;DR

This paper experimentally demonstrates nonclassical correlations in biphotons generated from cold two-level atoms, confirming theoretical predictions and showing quantum correlations that violate classical inequalities.

Contribution

First experimental verification of nonclassical biphoton correlations from a two-level atomic ensemble, confirming theoretical predictions and analyzing their robustness.

Findings

Violation of Cauchy-Schwarz inequality with R = 1.98

Quantum correlations observed on nanosecond timescales

Correlations are robust to atom number and light intensity variations

Abstract

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.