Laser excitation of the 1S-2S transition in He+
Authors:   , Gründeman Elmer Louis 
Published: October 2024

Our understanding of the physical laws of nature is constantly evolving. When the predictions made by the current theory do not match observations, the theory has to be adapted. Over the last century, examples include the discovery of new particles like antimatter or muons, the formulation of quantum mechanics, and the theory of relativity. The current model, the standard model, is extremely successful in some areas. However, it falls short in describing the nature of dark matter or dark energy, which are estimated to form 95% of the universe. In addition, a grand unified theory, which would combine quantum mechanics and relativity, is lacking. It therefore seems evident that our current understanding of nature is incomplete and should be adapted to include “beyond standard model” physics, but the question remains how.

We aim to contribute to testing fundamental physics and the determination of fundamental constants by measuring the 1S-2S transition in singly-ionized helium (He+). We are specifically interested in testing higher-order quantum electrodynamics (QED) corrections and nuclear size corrections, as they are strongly enhanced compared to H. A precision measurement in He+ could also be compared to already performed measurements in muonic He+. While He+ 1S-2S spectroscopy is a valuable test of QED theory, the inconvenient extreme ultraviolet (XUV) wavelength requirement makes such a precision measurement extremely challenging. In fact, it has been such an obstacle that no measurement of this transition has been presented to date.

Our approach to performing the first precision measurement of this transition relies on the combination of three concepts: the frequency comb (FC), high harmonic generation (HHG), and Ramsey spectroscopy. The FC is a special type of laser that emits ultrashort, extremely intense pulses. The energy spectrum of such a laser is analogous to a ruler, but then in the optical regime. This allows us to determine the laser energy very precisely. By amplifying the FC pulses and focusing them tightly the intensity becomes so high that we can drive HHG in a noble gas. In this process, higher harmonics of the input frequency are created up to a certain cutoff. When the parameters are tuned correctly, these higher harmonics extend all the way to the wavelength regime required for 1S-2S excitation. Lastly, we aim to use a modified version of Ramsey’s method of separated oscillatory fields for precision spectroscopy. We use pairs of XUV FC pulses with a variable time delay in a way that strongly reduces the systematic error of the measurements. This method was developed at the VU and dubbed Ramsey comb spectroscopy (RCS).

This thesis describes two important steps toward reaching the goal of performing the first precision measurement of the 1S-2S transition in He+. The first is laser excitation of the transition, and the second is the development of an ion trap to trap and sympathetically cool a single He+ ion with a laser-cooled Be+ ion. With these demonstrations, the first precision measurement of the 1S-2S transition is now within reach.