Dark Resonances in Quantum Optics

Abstract

Quantum control and quantum computation, representing two important directions of today research, rely on the precise modification of quantum mechanical systems not perturbed by an environment presence. This precise control was reached in different areas of physics using electromagnetic fields interacting with atoms or molecules in different matter phases. The simplest quantum mechanical system to be explored is a two-level atom, with lower and upper levels excited by a resonant or quasi-resonant electromagnetic field, as it occurs in nuclear or electronic magnetic resonance or in two-level laser spectroscopy. There the populations of the two levels or the coherence between the two levels are precisely modified by a control on the frequency, amplitude and interaction time of the electromagnetic field. The weak perturbation by the environment is realized, for instance, operating with low pressure or with cold/ultracold gases as produced by the combination of laser cooling and evaporative cooling techniques. The next step in level complexity is the three-level systems, with three levels, to be denoted as |0〉, |p〉, |c〉 excited by two separate lasers, the first probe laser at frequency ωp driving the transition |p〉 → |0〉 and the coupling laser at frequency ωc driving the transition |c〉 → |0〉. The atom–laser interaction of a three-level system is based on the evolution of the population of the three levels, of the optical coherences between levels |0〉, |p〉 and |0〉, |c〉. In addition, the three-level system may be prepared by the applied lasers in a coherent superposition state, whence it supports one high order coherence between levels |p〉 and |c〉. The build up of that coherence by the applied lasers produces new processes and new phenomena, peculiar of the three-level configuration. Those processes and phenomena have stimulated a large attention by the laser spectroscopy and quantum optics