Spectral Narrowing via Quantum Coherence
  We have studied the transmission of an optically thick Rb vapor that is illuminated by monochromatic and noise broaden laser fields in Lambda configuration. The spectral width of the beat signal between the two fields after transmission through the atomic medium is more than 1000 times narrower than the spectral width of this signal before the medium. 
Eugeniy E. Mikhailov, Vladimir A. Sautenkov, Yuri V. Rostovtsev, Aihua   Zhang, M. Suhail Zubairy, Marlan O. Scully, George ...