1. A. Kastler: J. Physique Radium 11, 255 (1950)
2. J.H. Holloway, R.F. Lacey: Proc. International Congress of Chronometry, Lausanne (1964) p. 317
3. J. Vanier, C. Audoin: The Quantum Physics of Atomic Frequency Standards (Adam Hilger, Bristol 1989) pp. 603?947
4. M. Arditi: Metrology and Fundamental Constants (Societa Italiana di Fisica, Bologna 1980) pp. 289?334
5. L.L. Lewis, M. Feldman: Proc. 35th Annual Frequency Control Symposium, Philadelphia, USA (1981), p. 612, available from Electronic Industries Association, Washington DC