Sub-Poissonian photon statistics in resonance fluorescence

Expressions are derived for the probability p(n) that n photons are emitted in a given time in the steady state by a two-level atom, when it is placed in a resonant, coherent, exciting field. The distribution p(n) is shown to be narrower than Poissonian. The ratio - is negative and has an absolute maximum value of 3/4. The possibility of observing the sub-Poissonian statistics is discussed briefly.