Electromagnetic Induced Transparency

Three-level atomic and molecular systems coupled to two laser fields exhibit transparency effects that result from the cancellation of absorption at a resonance transition frequency. These effects are deliberately induced by one of the two laser fields upon modifying the medium optical response to the other field. The large degrees of transparency in a medium where strong absorption and hence opacity would normally be expected, has then been termed electromagnetically induced transparency (EIT). It has been been exploited to either control or eliminate the effect that certain materials have on a propagating light beam and, in particular, to control the group velocity of light pulses. Most celebrated is the achievement of two extreme regimes for ultrafast and ultraslow light propagation where a light pulse may travel faster than c, the speed of light in vacuum, or may rather slow down at the a bike's pace of a few meters per second. It has also been used to bring light to a full stop inside a medium-laying basis for light storage techniques in quantum memories. From the outset, electromagnetically induced transparency has also stimulated a considerable amount of work on fundamental issues. Nonlinear optics at low-light levels, photon entanglement and entanglement of atomic ensembles, quantum information processing, quantum information storage, and enhanced acousto-optical effects, just to mention a few. Some of these have been recently observed. Electromagnetically induced transparency is a rather subtle effect. It relies on atomic states coherence and quantum interference, and the necessary background tools to understand the basic underlying physics will be presented. We will start with a preliminary brief discussion of coherent population trapping, a closely related effect whose observation first raised interest in induced transparency phenomena, and we will conclude by overviewing some of its most relevant prospects.