High-precision spectroscopy on simple systems such as atomic hydrogen has reached an unprecedented level of accuracy in recent years . Experimentally determined energy levels are used to test bound-state quantum electrodynamics (QED). However, theoretical values for the energy levels were limited by the uncertainty of experimentally determined parameters such as the proton charge radius (rp). In 2010, the CREMA collaboration performed a spectroscopic measurement on muonic hydrogen. From this, rp was extracted with a ten times higher accuracy but also showed a 5σ discrepancy to the CODATA-2010 value . This so-called proton radius puzzle remains unsolved. A possible solution to this problem can be obtained from measurements in other systems.