“…At that time it was stimulated by the search for new laser materials and optical storage devices. A new wave of activity has emerged in connection with the recent experimental observations of new phenomena in these materials such as electromagnetically induced transparency [1][2][3][4], slow light and stored light [5,6], stimulated Raman adiabatic passage [7][8][9], Ramsey fringes [10], coherent reversible inhomogeneous broadening [11] and atomic frequency combs [12]. These phenomena lead to many promising applications such as controllable optical delay lines, low intensity optical switches and frequency converters, quantum information storage, etc.…”