Nonlinear Interference effect for Optical Clocks in Yb Atoms
  In this paper, two-photon absorption resonance has been investigated . We studied this phenomenon in three-level system with common upper level, two lower long-life levels and two quasi-resonant laser fields with different frequency spectrum. It has been shown that, the absorption coefficient in the far distance from absorption line at the frequency of two-photon resonance has considerable amplitude and narrow width. Frequency difference between two lower levels is transition frequency for optical clocks. This resonance is much less sensitive to the effects of noise components and probing fields compared with electromagnetically induced transparency. Stark shift of the two-photon resonance can be offset and reduced by the changes in the probing field amplitude. We recommended to use an additional laser with narrow line to stabilize the frequency at optical clock to get a line in sub-Hertz range. 
Tara Ahmadi, Sergei.A Pulkin
physics.atom-ph quant-ph