  The Lindblad approach to continuous quantum measurements is applied to a
system composed of a two-level atom interacting with a stationary quantized
electromagnetic field through a dispersive coupling fulfilling quantum
nondemolition criteria. Two schemes of measurements are examined. The first one
consists in measuring the atomic electric dipole, which indirectly allows one
to infer the photon distribution inside the cavity. The second one schematizes
a measurement of photon momentum, which permits to describe the atomic level
distribution. Decoherence of the corresponding reduced density matrices is
studied in detail for both cases, and its relationship to recent experiments is
finally discussed.
