Investigation on spontaneously generated coherence

Spontaneously Generated Coherence(SGC) refers to a kind of quantum coherence induced by the process of spontaneous emission. It can greatly affect the dynamics of a quantum system, and accounts for a variety of important phenomena. Many efforts have been devoted to this topic, aiming to investigate the essence of quantum coherence and advanced technologies. However, the existence of SGC needs rigorous requirements which can hardly been fulfilled in atoms placed in a free space. Therefore we must give particular considerations to investigate this coherence experimentally. In this paper, a few interesting phenomena related to SGC are summarized, such as gain without inversion, coherent population trapping, phase sensitive spectra, and modifications of absorption, emission, and refraction. We also review the investigations on the realization of SGC, such as modifying the vacuum, coupling levels with static fields, simulating SGC with coherence induced by coherent fields, and studying SGC in special materials.