Modulation induced frequency shifts in a CPT-based atomic clock
  We investigate systematic errors associated with a common modulation technique used for phase sensitive detection of a coherent population trapping (CPT) resonance. In particular, we show that modification of the CPT resonance lineshape due to the presence of off-resonant fields leads to frequency shifts which may limit the stability of CPT-based atomic clocks. We also demonstrate that an alternative demodulation technique greatly reduces these effects. 
D. F. Phillips, I. Novikova, C. Y.-T. Wang, M. Crescimanno, and R. L.   Walsworth