Cancellation of light-shifts in an N-resonance clock
  We demonstrate that first-order light-shifts can be cancelled for an all-optical, three-photon-absorption resonance ("N-resonance") on the D1 transition of Rb87. This light-shift cancellation enables improved frequency stability for an N-resonance clock. For example, using a table-top apparatus designed for N-resonance spectroscopy, we measured a short-term fractional frequency stability (Allan deviation) 1.5e-11 tau^(-1/2) for observation times 1s< tau < 50s. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated N-resonance clock. 
Irina Novikova, Alexei V. Taichenachev, Valery I. Yudin, David F.   Phillips, Alexander S. Zibrov, Ronald L. Walsworth