With lasers it is possible to investigate at which wavelengths light is absorbed by atoms and molecules, when the energy of the photon matches an internal energy interval of the electrons. For the simplest systems, such as atomic hydrogen, it can then be compared to theoretical predictions with extreme precision to test our current understanding of physics, called the Standard Model. In particular, Quantum Electrodynamics (QED) is tested in this way, which is in fact the best tested and most successful part of the Standard Model. Precision spectroscopy also enables to determine some of the fundamental constants of Nature that are required to do accurate calculations with the theory. Any deviation found experimentally from theory could hint at new physics, which we know must be there, e.g. given the existence of 'dark matter' and 'dark energy'. Previous measurements based on e.g. atomic hydrogen, and exotic varieties of it such as muonic-hydrogen, have shown contradicting results.
The aim of this project is to enable precision measurements for the first time on singly-ionised helium on the 1S-2S transition, to contribute to solving the aforementioned issues. Singly-ionized helium is like hydrogen, but has much bigger QED effects. A major challenge is that its internal energy structure requires light at extreme wavelengths for which no lasers exist. We are developing a new laser spectroscopy method, called Ramsey-Comb spectroscopy (RCS), that combines the precision of low-power frequency comb lasers (which get their accuracy from an atomic clock) with high-power laser pulses that can be converted to the extreme ultraviolet by high-harmonic generation (HHG). It required development of a complex laser system, a state-of-the-art ion trap (realized through a collaboration with T. Mehlstäubler from PTB in Germany), laser cooling via a different ion to the lowest quantum-mechanical energy state, and methods to detect that the transition was made.
While the He+ trapping was not completely ready yet during the project, we realized nearly all of the components to do precision spectroscopy of the 1S-2S transition in He+ in the near future. Experiments were conducted on other atoms and molecules to demonstrate the principles and potential of our developed methods. This led to e.g. a more than two orders of magnitude improved molecular spectroscopy of H2 and D2, and in a collaboration with F. Merkt from the ETH Zurich, in improved dissociation energies, which is the benchmark for tests of molecular quantum theory. We now reached an accuracy 3 times better than the influence of the finite proton size in H2. Another important demonstration for the project was the first demonstration of RCS combined with HHG. This enabled us to do the most precise spectroscopic measurement ever performed with light from high-harmonic generation, using our Ramsey-comb technique, which we applied on a transition in xenon at 110 nm (improving the accuracy by 10 000 times).
In the final period of the project, again a major advance was made when we demonstrated the first excitation of the 1S-2S transition in He+, with properties as predicted (and confirmed by theoretical simulations). Although this was not yet a precision measurement, it does prove the excitation principle with 32 nm + 790 nm works very well. Therefore we have demonstrated all the ingredients to do an actual precision determination of the 1S-2S transition, which seems tantalizingly close now. We will pursue this in a follow-up project.
