Decoherence and coherent population transfer between two coupled systems -art. no. 013413

We show that an arbitrary system described by two dipole moments exhibits c oherent superpositions of internal states that can be completely decoupled fi om the dissipative interactions (responsible for decoherence) and an ext ernal driving laser field. These superpositions, known as dark or trapping states, can he completely stable or can coherently interact with the remain ing states. We examine the master equation describing the dissipative evolu tion of the system and identify conditions for population trapping and also classify processes that can transfer the population to these undriven and nondecaying states. It is shown that coherent transfers are possible only i f the two systems are nonidentical, that is the transitions have different frequencies and/or decay rates. in particular, we find that the trapping co nditions can involve both coherent and dissipative interactions, and depend ing on the energy level structure of the system, the population can be trap ped in a linear superposition of two or more bare states, a dressed state c orresponding to an eigenstate of the system plus external fields or, in som e cases. in one of the excited states of the system. A comprehensive analys is is presented of the different processes that are responsible for populat ion trapping, and we illustrate these ideas with three examples of two coup led systems: single V- and Lambda-type three-level atoms and two nonidentic al tao-level atoms, which are known to exhibit dark states. We show that th e effect of population trapping does not necessarily require decoupling of the antisymmetric superposition from the dissipative interactions. We also find that the vacuum-induced coherent coupling between the systems could be easily observed in Lambda-type atoms. Our analysis of the population trapp ing in two nonidentical atoms shows that the atoms can be driven into a max imally entangled state which is completely decoupled from the dissipative i nteraction.