QUANTUM ISLAND STATES IN THE MICROMASER

We study the photon statistics of a micromaser with a nonzero thermal background for a large range of the pumping parameter theta. The photo n statistics are examined in a two-dimensional space, parametrized by theta and k, where k is the photon number. For values of theta which a re not too large, there are prominent peaks in the photon-number distr ibution. The locations of these peaks are determined by well-defined s tructures in theta-k space. These structures lie along curves in this space. As theta is increased, these structures disappear and the numbe r distribution becomes diffuse. When theta becomes sufficiently large, however, new structures which are well localized in theta and k appea r. We call these states ''island states.'' These states exhibit a stro ng squeezing of the photon number and are fairly insensitive to the th ermal background. Their noise increases slowly due to fluctuations of the atomic beam as long as the spread in values of the pump parameter theta is smaller than the distance between the islands. Bistable behav ior can be induced by fluctuations that overlap adjacent island states . We also present the photon statistics of a micromaser pumped by atom s in a mixture (incoherent superposition) of their upper and lower lev els. It is shown that the noise in island states can be drastically re duced by an optimum amount of injected absorption. These features rend er these states experimentally feasible, so that they could be used fo r generating squeezed quantum states of the radiation field that are w ell localized in theta and k.