Testing QED and the Proton Radius Puzzle with precision spectroscopy on H2 and He+
Frequency comb lasers have made it possible to perform ultra-precise measurements of atomic and molecular structure. This in turn enables table-top experiments to test quantum electrodynamics (QED, the best tested theory in physics), and perform searches for physics beyond the standard model. Spectroscopy of simple atomic systems is, however, currently limited by finite nuclear size effects. If one assumes QED is correct, a comparison between experiment and theory can be used instead to e.g. determine the size of the proton from spectroscopic measurements in atomic hydrogen. Surprisingly, in 2010 spectroscopy on muonic hydrogen (where the electron is replaced with a muon) lead to a proton radius 4% lower than the accepted value based on electronic hydrogen spectroscopy (a 5-sigma deviation known as the Proton Radius Puzzle [1,2]). In September 2017 spectroscopy on the 2S-4P line in atomic hydrogen  resulted in a proton radius and Rydberg constant that actually does agree again with muonic hydrogen.
To bring light in this confusing situation and explore new QED tests, we are developing methods to enable precision spectroscopy at a level of 1 part per trillion or better in molecular hydrogen and He+ ions. Both experiments require short wavelengths (deep-UV  and extreme ultraviolet, respectively) that can only be generated using nonlinear upconversion with high power laser pulses. We developed a method called Ramsey-comb spectroscopy [5,6] that enables this based on excitation with two amplified (phase-coherent) pulses from a frequency comb laser. Recently we demonstrated this type of excitation on the X-EF transition in H2 at 202 nm, and improved its precision by two orders of magnitude. In the talk I will discuss the current status of the Proton Radius Puzzle, our recent Ramsey-comb measurements of H2, and progress towards realisation of the first 1S-2S excitation of He+ ions for QED tests beyond those of atomic hydrogen.
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