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Starting from a three-level atom coupled to two modes of radiation field, we derive a Raman-coupled Hamiltonian by a unitary transformation, evaluated perturbatively in coupling constants. The Rabi oscillation frequency and the collapse and revival times of the atomic coherence are found to have strikingly different photon-intensity dependence than those found previously.PACS number(s): 42.50.Hz, 42.50.Ar

We calculate the quantum mechanical, temporal second-order coherence function for a singlemode, degenerate parametric amplifier for a system in the Gaussian state, viz., a displaced-squeezed thermal state. The calculation involves first the dynamical generation at time t of the Gaussian state from an initial thermal state and subsequent measurements of two photons a time τ ≥ 0 apart. The generation of the Gaussian state by the parametric amplifier ensures that the temporal second-order coherence function depends only on τ , via τ /t, for given Gaussian state parameters, Gaussian state preparation time t, and average numbern of thermal photons. It is interesting that the time evolution for displaced thermal states shows a power decay in τ /t rather than an exponential one as is the case for general, displaced-squeezed thermal states.

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