The core of the apparatus is a so-called Penning trap. A Penning trap is a precisely manufactured measuring instrument that enables the storage and non-destructive observation of ions. For this purpose, the Penning trap is used to create a precisely defined superposition of static electric and static magnetic fields, which allows individual ions to be stored in their interior.

Subsequently, the ion together with the trap forms an artificially produced atom, which can be manipulated in a variety of ways and examined with a wide variety of methods.

Figure 1 shows the energy levels of a single helium-3 ion stored in the Penning trap. In addition to the properties of the Penning trap that we have defined, the energy levels depend on the specific properties of the helium ion. The result is that the properties of the ion can be derived from the precise measurement of these energy levels. This makes it possible to measure the mass, the magnetic moment and the size of the ion.

As shown in Figure 2, the measurements are carried out in a tower of several Penning traps consisting of cylindrical electrodes [3]. The ions can be produced within the trap using different methods. On the one hand, a glass sphere can be heated so that gaseous atoms such as helium penetrate the glass and are then ionized with the help of an electron beam. On the other hand, an intense laser beam can be directed onto a solid to ignite a plasma.

The trap tower is placed in the cold bore of a 5 T superconducting magnet and cooled to 4 K by thermal contact with liquid helium. In order to be able to excite spin-flips of electrons bound to nuclei, microwaves of frequency 150 GHz are coupled into the setup by a waveguide. In addition, optical access allows ultraviolet laser radiation to be irradiated in order to cool the ions to a few mK.

In a first study, see [1], we investigated the magnetic properties of the helium-3 isotope, compare Figure 3. Here for the first time, it was possible to directly measure the electron and nuclear magnetic moments of the helium-3 ion with a relative accuracy of 10^-10. With an accuracy improved by two orders of magnitude, the magnetic interaction between the electron and the nucleus (zero-field hyperfine splitting) was determined. In addition, the size of a helium-3 nucleus could be determined. The results represent the first direct calibration for helium-3 nuclear magnetic resonance (NMR) probes and are thus groundbreaking in the metrology of magnetic fields.