Generating coherence and entanglement with a finite-size atomic ensemble in a ring cavity

Abstract

We propose a model to study the coherence and entanglement resulting from the interaction of a finite-size atomic ensemble with degenerate counter-propagating field modes of a high-Q ring cavity. Our approach applies to an arbitrary number of atoms N and includes the spatial variation of the field throughout the ensemble. We report several new interesting aspects of coherence and entangled behaviour that emerge when the size of the atomic ensemble is not taken to the thermodynamic limit of N→∞. Under such conditions, it is found that although in the thermodynamic limit the counter-propagating cavity modes are mutually incoherent and exhibit no one-photon interference, the modes can, however, be made mutually coherent and exhibit interference after interacting with a finite-size atomic ensemble. It is also found that the spatial redistribution of the atoms over a finite size results in non-orthogonality of the collective bosonic modes. This non-orthogonality leads to the super-bunching effect that the correlations of photons of the individual cavity modes and of different modes are stronger than those of a thermal field. We also investigate the spectral distributions of the logarithmic negativity and the variances of the output fields. These functions determine the entanglement properties of the output cavity fields and can be measured by a homodyne technique. We find that the entanglement is redistributed over several components of the spectrum and the finite-size effect is to concentrate the entanglement at the central component of the spectrum.