Realization of cavity linewidth narrowing via interacting dark   resonances in a tripod-type electromagnetically induced transparency system
  Cavity linewidth narrowing via double-dark resonances has been experimentally observed using the 87Rb Zeeman splitting sublevels. With the steep dispersion led by the interacting dark resonances in the tripodtype electromagnetically induced transparency system, we narrow the cavity linewidth to 250 KHz at room temperature. Furthermore, the position of this ultranarrow cavity linewidth could be tuned in a 60 MHz coupling field detuning range. 
Kang Ying, Yueping Niu, Dijun Chen, Haiwen Cai, Ronghui Qu
physics.optics physics.atom-ph