1. C. Balzer, A. Braun, T. Hannemann, C. Paape, M. Ettler, W. Neuhauser, C. Wunderlich, Phys. Rev. A 73, 041407(R) (2006)
2. F. Strumia, in Proc. 32nd Annu. Symp. Frequency Control (Electronic Industries Association, Washington, 1987)
3. R. Blatt, H. Schnatz, G. Werth, Phys. Rev. Lett. 48, 1601 (1982)
4. G. Werth, Metrologia 22, 190 (1986)
5. R. Blatt, R. Casdorff, V. Enders, W. Neuhauser, P.E. Toschek, in Proc. 4th Symp. Frequency Standards and Metrology (Springer, Berlin, 1989), p. 306