The relation between normally ordered and unordered products of creation and annihilation operators is examined, and it is emphasized that the former correspond to counting correlations and the latter to counting moments. Both can be measured. It is shown that there exists a particularly simple relation between the generating functions for the two kinds of products. This relation can also be obtained by semiclassical considerations, which give more insight into its significance. The result provides further indication of the very close connection between the semiclassical and quantum-mechanical theories of optical coherence.

• Received 26 June 1964

DOI:https://doi.org/10.1103/PhysRev.136.B1221