The mechanism of using the anisotropic Purcell factor to control the spontaneous emission linewidths in a four-level atom is theoretically demonstrated; if the polarization angle bisector of the two dipole moments lies along the axis of large/small Purcell factor, destructive/constructive interference narrows/widens the fluorescence center spectral lines. Large anisotropy of the Purcell factor, confined in the subwavelength optical mode volume, leads to rapid spectral line narrowing of atom approaching a metallic nanowire, nanoscale line width pulsing following periodically varying decay rates near a periodic metallic nanostructure, and dramatic modification on the spontaneous emission spectrum near a custom-designed resonant plasmon nanostructure. The combined system opens a good perspective for applications in ultracompact active quantum devices.

In the coherently trapped populations of a four-level atom, we demonstrated the quantum beats with different mechanism, which originate from the interference between transition channels with different dipole moments. The beat frequency is determined by the intrinsic atomic parameters, i.e., the spacing of upper levels and ratio of dipole moments. The resonant plasmonic nanoantenna, as a candidate for the creation of anisotropic vacuum, was proposed to achieve the nanoscale realization of the quantum beats, spontaneous emission cancellation, and Rabi oscillation in two-photon correlations through the enhanced near-field and modified decay rates.

We demonstrate that the Gaussian entanglement cascading can be realized unconditionally for any nonzero squeezing in all of the entangled sources. We obtain the upper bound of the Gaussian cascaded entanglement, which cannot be exceeded by any entanglement cascading protocol based on Gaussian local operations and classical communications. We then propose an unconditional entanglement cascading protocol that can produce the cascaded entanglement reaching the upper bound. The protocol, as a generalization of the standard continuous variable teleportation, includes two steps: (i) each repeater site locally implements the Bell measurement and classically sends the results to Bob; (ii) Bob optimally displaces his own mode based on these results.