High-precision Ramsey-comb spectroscopy in the DUV and XUV spectral region

Quantum-electrodynamics (QED) theory has been tested with high accuracy. However, QED tests in atomic hydrogen are currently limited by the experimentally determined value of the proton charge radius. Therefore spectroscopic measurements were conducted with muonic-atoms (μH and μD) to determine the proton and the deuteron charge radii with an order of magnitude higher accuracy. However, these values also showed a > 7σ discrepancy with the CODATA-2010 values. In order to contribute to resolving this so called proton radius puzzle we have determined the EF←X transition frequency in molecular hydrogen (H2) at ~ 2 × 202 nm with almost two orders of magnitude higher accuracy than the previous result. We are also building a setup to measure the 1S-2S transition in singly ionized helium at ~ 30 nm.