Production of number states of the electomagnetic field

It has been demonstrated recently that it is possible to generate a pure number state, or Fock state, of the electromagnetic field in a resonant cavity when a ``micromaser'' is operated under the appropriate conditions. This prospect is examined here by a direct analysis of the equation for the density-matrix-governing operation of the lossless micromaser, without having to solve the equation or perform numerical analyses. This model micromaser affords a unique example of an open quantum system whose von Neumann entropy may increase at first, but must subsequently vanish.