Quantum and classical correlations in four-wave mixing from cold ensembles of two-level atoms

TL;DR

This paper investigates quantum correlations in four-wave mixing from cold two-level atomic ensembles, revealing their dependence on system parameters and potential for generating robust, narrow-band biphotons.

Contribution

It provides an extended experimental analysis of quantum correlations, highlighting their dependence on atom number, optical depth, and excitation power, with insights into classical and quantum regimes.

Findings

Quantum correlations persist without filtering over background light.

Decay rate of correlations varies with atom number, indicating superradiance.

Distinct timescales reveal different biphoton creation pathways.

Abstract

Quantum correlations in four-wave-mixing from ensembles of cold two-level atoms may prevail without filtering over background light with well-known classical interpretations, such as Rayleigh scattering, as recently experimentally demonstrated in Phys. Rev. Lett. {\bf 128}, 083601 (2022). Here we provide an extended investigation of this effect, in which we detail the experimental procedure and the variation of the quantum correlation with various parameters of the system. Particularly, we show that the decay rate of the quantum correlations changes with the number of atoms in the sample, providing another indication of its superradiance-like nature. The nonclassical aspects of the signal occur for short timescales, but the long timescales carry as well a lot of information on the classical correlations of the system. This slow classical regime presents also two clearly distinct timescales, which we explain by two different pathways for the creation of biphotons. From the global analysis of the data in all its timescales, we are able to derive an empirical expression to fit the data, resulting in information on, among other parameters, the sample's temperature and superradiant-like acceleration. In general, the reported quantum correlations present a dependence on critical parameters of the system, such as optical depth and excitation power, that is quite different from other systems used for biphoton generation, and are more robust to changes in these parameters. This opens the possibility of exploring this process for efficient generation of narrow-band biphotons or of other quantum-correlated photonic states of higher order.