Quantum coherence and population trapping in three-photon processes

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The spectroscopic properties of a single, tightly trapped atom are studied, when the electronic levels are coupled by three laser fields in an N-shaped configuration of levels, whereby a Λ-type level system is weakly coupled to a metastable state. We show that depending on the laser frequencies the response can be tuned from coherent population trapping at two-photon resonance to novel behaviour at three photon resonance, where the metastable state can get almost unit occupation in a wide range of parameters. For certain parameter regimes the system switches spontaneously between dissipative and coherent dynamics over long time scales.