text: letin**54**, 348 (2009). 
  
  [16] D. Yu and J. Chen, Phys. Rev. A**78**, 013846 (2008). 
  [17] J. Chen, In*Frequency Standards and Metrology: Proceedings*
  *of the 7th Symposium*, edited by Maleki Lute (World Scientiﬁc 
  Publishing Company, 2009). 
  
  We acknowledge Yiqiu Wang and Deshui Yu for fruitful 
  discussions. This work is supported by MOST of China 
  (grant 2005CB724500, National Natural Science Foundation 
  of China (grant 60837004, 10874009), National Hi-Tech Re- 
  search and Development (863) Program. 
  
  Lett.**102**, 163601 (2009) 
  
  [20] F. Strumia, Metrologia**8**, 85 (1972). 
  [21] G. Kramer, J. Opt. Soc. Am.**68**, 1634 (1978). 
  [22] V. S. Letokhov and B. D. Pavlik, Opt. Spectrosc. USSR**32**, 455 
  (1972). 
  [23] Ye. V. Baklanov, B. Ya, Dubetsky, V. P. Chebotayev, Appl. 