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.
@article{arxiv.quant-ph/0503085,
title = {Spectral Narrowing via Quantum Coherence},
author = {Eugeniy E. Mikhailov and Vladimir A. Sautenkov and Yuri V. Rostovtsev and Aihua Zhang and M. Suhail Zubairy and Marlan O. Scully and George R. Welch},
journal= {arXiv preprint arXiv:quant-ph/0503085},
year = {2007}
}