Quantum correlations of an atomic ensemble via an incoherent bath

A rich variety of quantum features can be found in a collection of atoms driven only by an incoherent bath. To demonstrate this, we discuss a sample of three-level atoms in ladder configuration interacting via the surrounding bath, and show that the fluorescence light emitted by this system exhibits nonclassical properties. Realizations could be thermal baths for microwave transitions, or incoherent broadband fields for optical transitions. In a small sample of atoms, the emitted light can be switched from sub- to super-Poissonian and from antibunching to superbunching controlled by the mean number of atoms in the sample. Larger samples allow us to generate superbunched light over a wide range of bath parameters and thus fluorescence light intensities. We also identify parameter ranges where the fields emitted on the two transitions are strongly correlated or anticorrelated, such that the Cauchy-Schwarz inequality is violated. As in a moderately strong bath this violation occurs also for larger numbers of atoms, such samples exhibit macroscopic quantum effects