Impurities in materials are responsible for the colours of gemstones or the performance of modern semiconductors. The same applies to quantum systems, although research has been limited. For the first time, Kaiserslautern researchers were able to implant individual impurities formed by caesium atoms into an ultracold quantum gas of rubidium atoms in a controlled manner. They observed how the impurities exchange quantum mechanical angular momentum (spin) with the gas. They also demonstrated for the first time that caesium atoms can store quantum information. The study was published in the renowned journal Physical Review Letters.