A simple model of spontaneous emission
  We present a very simple model of a spontaneous emission from a two-level atom, interacting with a field of a finite number of states. Such a process is often said to occur because of the large number of equally-probable states of environment. We show that in our model increasing the number of field states may and may not cause a practically permanent emission, depending on the details of the model. We also describe how irreversibility emerges with growing number of states. Mathematical tools are reduced to a necessary minimum and hopefully can be well understood by undergraduate students. 
Krzysztof Piotr W\'ojcik
quant-ph