Enhancing non-classical correlations for light scattered by an ensemble of cold two-level atoms

TL;DR

This paper demonstrates that filtering the spectrum of biphotons emitted from cold two-level atoms significantly enhances quantum correlations, surpassing classical limits and improving the violation of the Cauchy-Schwarz inequality.

Contribution

It introduces a spectral filtering technique to enhance non-classical correlations in biphotons from cold atomic ensembles, achieving a fourfold increase in quantum correlation strength.

Findings

Filtering the Rayleigh component enhances quantum correlations.

Violation of Cauchy-Schwarz inequality increased to 4.8.

Spectral filtering improves quantum correlation measurements.

Abstract

We report the enhancement of quantum correlations for biphotons generated via spontaneous four-wave mixing in an ensemble of cold two-level atoms. This enhancement is based on the filtering of the Rayleigh linear component of the spectrum of the two emitted photons, favoring the quantum-correlated sidebands reaching the detectors. We provide direct measurements of the unfiltered spectrum presenting its usual triplet structure, with Rayleigh central components accompanied by two peaks symmetrically located at the detuning of the excitation laser with respect to the atomic resonance. The filtering of the central component results in a violation of the Cauchy-Schwarz inequality to $4.8 \pm 1.0 \leq 1$ for a detuning of 60 times the atomic linewidth, representing an enhancement by a factor of four compared with the unfiltered quantum correlations observed at the same conditions.