Sudden transition from naked atom decay to dressed atom decay

The studies on quantum open system play key roles not only in fundamental problems in quantum mechanics but also in quantum computing and information processes. Here we propose a scheme to use a one dimensional coupling cavity array (CCA) as an artificial electromagnetic environment of a two-level atom. For a finite length of CCA, we find that after a turning time the population of excited state deviates suddenly from the exponential decay. We show that physically this phenomena corresponds to a transition from a naked atom decay to a dressed state decay. We hope that our finding will promote the studies on quantum system with a finite size environment.