Number 100 (Story #2), October 23, 1992 by Phillip F. Schewe and Ben Stein LASER ACTIVITY WITHOUT POPULATION INVERSION has reportedly been achieved in a sodium vapor. Population inversion is the condition in which a majority of atoms in a laser's active medium is maintained in excited states. This prevents ground-state atoms, which readily absorb laser photons, from sabotaging the laser process. In a paper to be published in the journal Optics Communications, Jin-Yue Gao of Jilin University in China reports the use of microwave radiation to prevent ground-state atoms in a sodium vapor from absorbing photons produced in the vapor, thereby obviating the need for population inversion. The microwave radiation brings about an "induced transparency" in the medium by coherently linking two closely spaced ground states. The probability amplitudes for absorption from either of the two superimposed states cancel, and the result is no absorption at all; laser light passes through unimpeded. Marlan Scully of Texas A&M refers to materials in which atomic states are coherently linked as "phase controlled materials," or "phaseonium." The use of such materials has led to reduced quantum noise in some laser systems. Scully believes that it could also lead to highly refracting, non-absorbing, transparent lenses. These lenses would result in higher-resolution optical microscopes and perhaps even setups enabling laser light to boost electrons in accelerators. (Science, 2 October 1992.)