  In recent years, much attention has been paid to the development of
techniques which transfer trapped particles to very low temperatures. Here we
focus our attention on a heating mechanism which contributes to the finite
temperature limit in laser sideband cooling experiments with trapped ions. It
is emphasized that similar heating processes might be present in a variety of
composite quantum systems whose components couple individually to different
environments. For example, quantum optical heating effects might contribute
significantly to the very high temperatures which occur during the collapse
phase in sonoluminescence experiments. It might even be possible to design
composite quantum systems, like atom-cavity systems, such that they
continuously emit photons even in the absence of external driving.
