Classical versus quantum coherence.
Quantum dots are structures engineered to have desirable quantum-mechanical properties. Much of their potential usefulness stems from the ability to design such structures so that their passive properties can be exploited. However, there are many plans to use dots as the basis of a more active quantum engineering, in which the detailed quantum states of the dot's components are manipulated by externally imposed fields. This leads to a new requirement: our ability to model the time development of individual driven quantum systems. I shall discuss some of the problems which arise when single systems, rather than ensembles, are considered and give examples which illustrate the effects of classical and quantum incoherences. I propose a new classification of incoherent processes, based on their effect on individual wave functions.