Enhancement of tripartite quantum correlation by coherent feedback control

Measurement-free coherent feedback control is promising to manipulate various quantum optical systems for preparing nonclassical states of light. In this paper, three different coherent feedback control systems are constructed based on cascaded four-wave mixing processes. By utilizing a beam splitter as the feedback controller, we theoretically investigate the enhancement of tripartite quantum correlation and its pairwise correlation. The absorption effect of Rb vapor cells and the loss of optical propagation in the coherent feedback loop are also taken into account. The quantum correlation in the three structures can be characterized by the degree of relative intensity squeezing of the output fields. We find that the tripartite quantum correlation can be maximally enhanced by an optimal feedback ratio. In addition, two of the three pairwise correlations in the quantum regime can also be improved by tuning the strength of feedback, while the other pairwise correlation remains in the classical regime. Our results pave the way for the experimental implementation and may find potential applications in quantum communication and quantum metrology.