The nucleus of thorium-229 possesses a property that is unique among all known nuclides: It should be possible to excite it with ultraviolet light. To date, little has been known about the low-energy state of the Th-229 nucleus that is responsible for this property. Together with their colleagues from Munich and Mainz, researchers at the Physikalisch-Technische Bundesanstalt (PTB) have now performed the first-ever measurements – using optical methods – of some important properties of this nuclear state such as the shape of its charge distribution. In this way, a laser excitation of the atomic nucleus can be monitored, thus allowing an optical nuclear clock to be realized that “ticks” more precisely than present-day atomic clocks. The scientists have reported their results in the current issue of Nature.