Observation of "double-dark" resonances

Summary form only given. The phenomenon of "dark resonances" or coherent population trapping is a well known concept in optics and laser spectroscopy. It can be considered as the basis for such effects as electromagnetically induced transparency and lasing without inversion. We report the observation of the novel spectral features which arise in systems where multiple quantum superposition states are coupled and interact coherently. These novel resonances correspond to the recently predicted "double-dark states". In the experiment, an optically thin ensemble of rubidium-37 atoms in zero magnetic field is excited in a /spl Lambda/ configuration with circularly polarized light from a pair of 789 nm diode lasers.