The hyperfine structure of hydrogen-like ions are a unique probe to access nuclear magnetic
moments and nuclear structure. Thus, while eliminating the ignorance of essential links in
metrology due to insufficiently known magnetic moments, at the same time these ions
provide complementary insight into the inner nucleus. The very recently started 3He-
experiment exploits these characteristics to provide a new standard for absolute precision-
magnetometry and determine the nuclear charge and current distribution of 3He.
To this end, a novel four Penning-trap experiment was designed and built. Using novel
techniques, this system enables non-demolition measurements of the nuclear quantum state
and allows sympathetic laser cooling of single, spatially separated ions to sub-thermal
In the first measurement campaign, 3He was investigated by exciting microwave transitions
between the ground-state hyperfine states. This enabled us to determine the nuclear g-factor,
the electronic g-factor and the zero-field ground-state hyperfine-splitting of 3He+ with a
precision of 5*10-10, 3*10-10 and 2*10-11, respectively .
Our measurement constitutes the first direct and most precise determination of the 3He+
nuclear magnetic moment. The result is of utmost relevance for absolute precision
magnetometry, as it allows the use of He NMR probes as an independent new standard with
much higher accuracy. In addition, the comparison to advanced theoretical calculations
enables us to determine the size of the 3He nucleus with a precision of 2,4*10-17m.
In future, we aim at a direct determination of the bare nuclear magnetic moment of 3He to be
compared to the bound-state result. For this measurement, it is essential to implement new
methods and technology such as sympathetic laser cooling and a high-precision voltage
source based on Josephson junctions . The latest results, status and the future prospect of
the experiment will be presented.
 A Mooser et al., J. Phys.: Conf. Ser. 1138, 012004 (2018)
 A. Schneider et al., Nature 606, 878 (2022)
 A. Schneider et al., Ann. Phys. 531, 1800485 (2019)