Abstract

The field of quantum optics had been greatly enriched with the discovery of dark states and electromagnetically induced transparency (EIT). Although these two processes give similarly effects, they are distinctly different. Dark states¹ are quantum states created in a three-level lambda system that is decoupled from the excited state. Some of the main features are zero linear absorption, no population in the excited state and no spontaneous emission. One limiting factor of a dark state system is that self-phase modulation disappears together with absorption. EIT is a nonlinear effect that can provide transparency to a probe field in a variety of systems. Much work has been done on two-level,² cascaded-three-level³ and even four-level systems.⁴ It has also been shown that a large third-order nonlinear susceptibility is present in the absence of absorption. However, spontaneous emission cannot be eliminated in such system, thereby limiting their applications, particularly, to squeezed light generation.

© 2001 Optical Society of America