Rainer Kaltenbaek's research is aimed towards exploring the frontiers of quantum theory using the novel techniques of quantum optomechanics, a new and exciting field of physics that has been drawing an increasing amount of attention over the last few years. In particular, he will investigate the question whether there are principal limits in size and complexity of physical systems beyond which strange quantum phenomena like entanglement vanish and the laws of quantum theory give way to the concepts we are used to from everyday experience.