Decoherence of mesoscopic states of cavity fields
  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. 
K. Fonseca Romero (1), M. C. Nemes (2), J. G. Peixoto de Faria (2), A.   N. Salgueiro (1), A. F. R. de Toledo Piza (1) ((1) ...