Two-step pulse observation for Raman-Ramsey coherent population trapping   atomic clocks
  We propose a two-step pulse observation method to enhance frequency stability for coherent population trapping (CPT) atomic clocks. The proposed method is a Raman-Ramsey scheme with low light intensity at resonance observation, and provides a Ramsey-CPT resonance with both reduced frequency sensitivity to the light intensity and a high signal-to-noise ratio by reducing the repumping into a steady dark state. The resonance characteristics were calculated based on density matrix analysis of a $\Lambda$-type three level system that was modeled on the $^{133}$Cs-D$_1$ line, and the characteristics were also measured using a vertical-cavity surface-emitting laser and a Cs vapor cell. 
Yuichiro Yano, Shigeyoshi Goka and Masatoshi Kajita
physics.atom-ph