Surface-plasmon-enhanced quantum field entanglement through anisotropic Purcell factors

We theoretically propose a mechanism for the enhancement of quantum field entanglement generated by four-wave mixing through anisotropic Purcell factors in a three-level atomic system. With anisotropic Purcell factors, the dependence of the entanglement on the relative polarization direction between two dipole moments is investigated. It is found that, for the two-mode quantum field entanglement, if the bisector of the two orthogonal dipole moments lies in the large (small) Purcell factor axis, the entanglement would be enhanced (decreased) with changing the crossing damping accordingly. Moreover, larger anisotropism of Purcell factors leads to further enhancement of the entanglement. We also reveal a mechanism for increasing the duration of four-mode quantum field entanglement through the anisotropic Purcell factors. With the help of the custom-designed plasmon nanostructure which creates the anisotropic Purcell factors, we demonstrate that the quantum field entanglement can be effectively modulated at subwavelength scale. Our paper provides a promising way towards the entangled source in nanophotonic structures and may have potential applications in on-chip devices for quantum information.