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 superpos ition state prepared in a definite mode of a high-e cavity can be used to determine the eigenvalues of the reduced density matrix of the fie ld, provided the assumed dynamical conditions are actually fulfilled t o experimental accuracy. These conditions involve (i) a purely dispers ive coupling of the field to the Rydberg atoms used to manipulate and to monitor the cavity field, and (ii) the effective absence of correla tions in the ground state of the system consisting of the cavity coupl ed 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. [S1050-2947(98)08309-7].