Atomic systems dressed-atom model

Lee et al. [34] have shown that the physical origin of the cancellation of the central line in the spectrum can be explained clearly by the dressed-atom model of the system [35]. In this model we use a fully quantum-mechanical description of the Hamiltonian (65), which in a frame rotating with the laser frequency (0L can be written as... 

An alternative description of the observed phenomena can be obtained by the dressed atom model [80]. In this approach the eigenvalues of the global system atom + field, the dressed states, are sought. The strong pump beam dresses the atom with laser photons which leads to energy levels split by the Rabi frequency. The resonances discussed in Fig. 20 then arise naturally as transitions between the (infinite) ladder of dressed states. The dressed atom model delivers a physically comprehensible description of atomic energy levels in strong fields and the reader is referred to Ref. 80 for a detailed presentation. 

Consider a simple example such as Friedrichs model, which corresponds to an excited atom interacting with a continuous photon field. Two cases are possible. The first one is when the frequency associated to the atom is outside the spectral range of the continuum. This case corresponds to integrable systems. We can then easily define a dressed excited state in terms of the density matrix. 

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