Abstract
Dissipation, often associated with plasmons, leads to decoherence and is generally considered fatal for quantum nonlinearities and entanglement. Counterintuitively, by introducing a dissipative plasmonic nanoantenna into a typical cavity quantum electrodynamics (QED) system, we unveil the wide existence of the atom‐photon quasi‐bound state (qBS), a kind of exotic eigenstate with anomalously small decay, in the hybrid plasmonic‐photonic cavity. To derive the analytical condition of atom‐photon qBS, we formulate a quantized two‐mode model of the local density of states by connecting the interacting uncoupled cavity modes to the macroscopic QED. With resonant plasmon‐photon coupling, we showcase the single‐atom qBS that improves the efficiency of single‐photon generation over one order of magnitude; and the two‐atom qBS that significantly enhances spontaneous entanglement generation compared with a bare photonic cavity. Notably, such single‐atom and multi‐atom qBS can be simultaneously accessed in realistic plasmonic‐photonic cavities, providing a versatile platform for advanced quantum technologies, such as quantum light sources, quantum computation, and quantum information.