[1] F. Riehle. Frequency Standards: Basics and Applications(2006).

[2] Y. Du, R. Wei, R. Dong, F. Zou, J. Lin, W. Wang, Y. Wang. Chin. Opt. Lett., 13, 091201(2015).

[3] F. Zou, R. Wei, R. Dong, T. Chen, W. Wang, Y. Wang. Chin. Opt. Lett., 14, 081201(2016).

[4] J. Camparo. Phys. Today., 60, 33(2007).

[5] T. Bandi, C. Affolderbach, C. Stefanucci, F. Merli, A. K. Skrivervik, G. Miletil. IEEE Trans. Ultrasonics, Ferroelectr. Freq. Control, 61, 1769(2014).

[6] A. Godone, F. Levi, S. Micalizio, C. Calosso. Phys. Rev. A, 70, 012508(2004).

[7] Z. Du, W. Zhao, J. Liu, S. Zhang. Geomatics Inf. Sci. Wuhan Univ., 36, 1236(2011).

[8] S. Kang, M. Gharavipour, C. Affolderbach, F. Gruet, G. Mileti. J. Appl. Phys., 117, 104510(2015).

[9] A. Godone, S. Micalizio, F. Levi. Phys. Rev. A., 70, 023409(2004).

[10] S. Micalizio, C. E. Calosso, A. Godone, F. Levi. Metrologia, 49, 425(2012).

[11] J. Deng, Z. Hu, H. He, Y. Wang. Chin. Phys. Lett., 23, 1745(2006).

[12] Y. Bo, Y. Ma, Y. Wang. Phys. Rev. A, 79, 063820(2009).

[13] J. Deng, Z. Hu, L. Li, H. He. Proceedings of 7th Symposium on Frequency Standards and Metrology, 348(2008).

[14] Y. Ma, J. Deng, Z. Hu, H. He, Y. Wang. Chin. Opt. Lett., 11, 032701(2013).

[15] J. Lin, J. Deng, Y. Ma, H. He, Y. Wang. Opt. Lett., 37, 5036(2012).

[16] J. Deng, J. Lin, J. Qian, G. Dong, H. He, Y. Wang. Proceedings of 2014 IEEE International Frequency Control Symposium, 285(2014).

[17] J. Lin, J. Deng, J. He, Y. Wang. Appl. Opt., 52, 2169(2013).

[18] S. Micalizio, C. E. Calosso, F. Levi, A. Godone. Phys. Rev. A, 88, 033401(2013).

[19] R. H. Dicke. Phys. Rev., 89, 472(1953).

[20] J. Vanier, C. Audoin. The Quantum Physics of Atomic Frequency Standards(1989).

[21] Y. Du, R. We, R. Dong, F. Zou, Y. Wang. Chin. Phys. B., 24, 070601(2015).