A novel absorption resonance for all-optical atomic clocks
  We report an experimental study of an all-optical three-photon-absorption resonance (known as a "N-resonance") and discuss its potential application as an alternative to atomic clocks based on coherent population trapping (CPT). We present measurements of the N-resonance contrast, width and light-shift for the D1 line of 87Rb with varying buffer gases, and find good agreement with an analytical model of this novel resonance. The results suggest that N-resonances are promising for atomic clock applications. 
Sergei Zibrov, Irina Novikova, David F. Phillips, Aleksei V.   Taichenachev, Valeriy I. Yudin, Ronald L. Walsworth, Alexande...