Dark-state polaritons in electromagnetically induced transparency

We identify form-stable coupled excitations of light and matter ("dark-stat e polaritons") associated with the propagation of quantum fields in electro magnetically induced transparency. The properties of dark-state polaritons such as the group velocity are determined by the mixing angle between light and matter components and can be controlled by an external coherent field as the pulse propagates. In particular, light pulses can be decelerated and "trapped" in which case their shape and quantum state are mapped onto meta stable collective states of matter. Possible applications of this reversibl e coherent-control technique are discussed.