Violations of classical inequalities in quantum optics

A review is given of correlation experiments in optics which explicitly distinguish between the predictions of classical and quantum theory. In particular, the Cauchy-Schwarz and Bell's inequalities and the phenomena of photon antibunching and squeezing are discussed. The violation of classical inequalities is shown to be associated with the nonexistence of a positive Glauber-Sudarshan P function. An example is given of a dissipative system where a violation of Bell's inequality occurs.