  We show that two-atom correlation measurements of the type involved in a
recent experimental study of the evolution of a mesoscopic superposition state
prepared in a definite mode of a high-Q cavity can be used to determine the
eigenvalues of the reduced density matrix of the field, provided the assumed
dynamical conditions are actually fulfilled to experimental accuracy. These
conditions involve i) a purely dispersive coupling of the field to the Rydberg
atoms used to manipulate and to monitor the cavity field, and ii) the effective
absence of correlations in the ground state of the system consisting of the
cavity coupled to the ``reservoir'' which accounts for the decoherence and
damping processes. A microscopic calculation at zero temperature is performed
and compared to master equation results.
