A chain of N identical two-level atoms coupled with the electromagnetic field, prepared via a single-photon Fock state, is investigated. It is found that, if the interaction between atoms is negligible, than the obtained dynamic equations for the probability amplitudes allow, in a certain sense, an interpretation of the dynamics of states in the classical fashion in terms of a superposition of oscillatory modes of the system under study. The derived equations reveal how a space configuration of the system of atoms affects the dynamics of the atomic states, particularly the "decay" rates of separate atoms and the system as the whole. K e y w o r d s: one-photon scattering, atomic system, Fock state, Schrödinger equation.

This paper investigates stimulated emission and absorption near resonance for a driven system of interacting two-level atoms. Microscopic kinetic equations for the density matrix elements of N -atom states including atomic motion are built, taking into account atom-field and atom-atom interactions. Analytical solutions are given for the resulting macroscopic equations in different limits, for a system composed of a strong coherent "pump" field and a weak counter-propagating "probe" field. It was shown that the existence of a dipole-dipole (long-range) interaction between atoms separated by distance less than the pump wave-length can cause the formation of periodic polarization and population structures (gratings in time and space) in the pumped medium without a probe field. The magnitude of pump induced population grating can have a strong dependence on the relation between the pump field detuning and the polarization decay rate. The "interaction" between pump and probe induced polarization/population gratings through a dipole-dipole interaction mechanism causes the absorption line shape asymmetry. Under certain conditions, this asymmetry is revealed in increasing probe gain for the "red"-shifted (relative to pump) probe and suppressing the gain for the "blue"-shifted probe field when pump is "red"-shifted relative to the ensemble averaged resonant frequency. The theoretical results are consistent with experimental data for the probe gain or absorption as the function of frequency and the dependance of the gain on atomic density for sodium vapor when the pump laser is tuned near the D 2 line. Here the dependance of gain on particle density was explained in the terms of the long-range interaction between the atoms.

The quantum optical theory of absorption/reemission properties of a system of interacting atoms is discussed. The calculation method of the absorption coefficient is developed with regard for the quantization of field, thermal atomic motion, Doppler effect, and the model interaction between atoms. It is shown that the formulation of the absorption coefficient in the quantum optical context is based on the commutation relation between the operators of electric field and intensity. The revealed non-linear dependence of the absorption coefficient on the atomic density, even in the case of negligible binary interaction, can be referred to a certain kind of quantum-optic collective effects.

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