1. P.E. Toschek, Ecole d’Eté des Houches 1982, in: New Trends in Atomic Physics, eds G. Grynberg and R. Stora (North-Holland, Amsterdam, (1984) p. 381. See also the lectures of H. Metcalf and S. Chu in this volume.
2. H.J. Kimble, M. Dagenais and L. Mandel, Phys. Rev. Lett. 39, 691 (1977).
3. We only consider in this lecture stationary random processes, so that all correlation functions such as g2 only depend on T.
4. C. Cohen-Tannoudji, Ecole d’Eté des Houches 1975, in: Frontiers in Laser Spectroscopy, eds R. Balian, S. Haroche and S. Liberman (North-Holland, Amsterdam, 1977) p. 1.
5. H.J. Carmichael and D.F. Walls, J. Phys. B8, L-77 (1975); J. Phys. B9, L-43 and 1199 (1976).