The major research activities of the Institute of Quantum Physics are connected to theoretical quantum optics with a special emphasis on the interface to other branches of physics. Tests of general relativity with the help of cold atoms and, in particular, Bose Einstein condensates, accelerator physics and matter wave optics giving rise to the quantum free electron laser, the connection between number theory and quantum mechanics as well as fundamental questions about the theory of measurement in quantum mechanics and insights provided by the modern techniques of experimental quantum optics represent only a few of many more examples illustrating interesting border lines between quantum optics and other fields.