Purifying a thermal field in a lossless micromaser

We show that a thermal field may be ‘‘purified’’ by its interaction with a flux of atoms having definite dipoles. In these circumstances, a transfer of coherence from the atoms to the field is realized. We show results for the Poissonian factor and for the purity factor (${\xi }_{\mathit{f}}$=1-${\mathit{Tr}}_{\mathit{f}}${${\mathrm{\rho }}_{\mathit{f}}^2$}). We also study the evolution of the phase distribution of the field in the framework of the Pegg-Barnett phase operator.