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Published online by Cambridge University Press: 13 October 2009
After the introduction of the two-level model, we add a third level to the quantum mechanical system to discuss some of the effects that cannot be described with only two states. The main feature is the exchange of order between states and transitions, which may proceed through populations or coherences. We concentrate our attention on the latter possibility.
Phenomenological introduction
Model atoms
In the preceding section, we discussed the interaction between light and two-level atoms – probably one of the most popular physical models. The basis of this popularity is its intuitively simple interpretation combined with the posibility of explaining a wide range of physical phenomena. An interesting aspect of the two-level system is that, although its dynamics are formally equivalent to those of a classical angular momentum, it can explain many aspects of quantum mechanics. Once these aspects are understood, it is tempting to look further into the behaviour of real systems, trying to find patterns inconsistent with the predictions of the two-level model.
This section discusses aspects of the interaction between matter and radiation that are incompatible with the two-level model. We do not consider specific atomic systems or attempt a complete analysis of the dynamics that a three-level system can exhibit. Instead we select a number of phenomena that play an important role in the discussion of those physical systems that form the subject of the subsequent sections.
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