[en] One of the driving forces behind the development of quantum mechanics at the start of the last century was the need to understand why atoms only emit light at certain wavelengths. Shortly afterwards quantum mechanics was applied to molecules and then to solids. Moving in the other direction, it was also applied to predict the properties of fundamental particles, notably the electron. In the December issue of Physics World, Gerhard Rempe of the Max Planck Institute for Quantum Optics, Germany, explains how, some 100 years after the birth of quantum mechanics, physicists are still learning more about the interactions between light and matter. (U.K.)