Abstract
The possibility to store charged particles in Penning traps for long times basically at rest
has enabled a rich variety of intriguing measurement opportunities. The purely static
magnetic and electric fields, in combination with the extremely good vacuum in cryogenic
Penning traps, largely decouples the ion from the complicated environment and thus gives
a clear and unobscured view onto the fundamental properties of the ion. Moreover, the
theoretical calculations in atomic physics, using the quantum field theories of the Standard
Model, have reached an impressive precision. Consequently, by measuring observables with
similar precision we get the opportunity to test our most fundamental theories in physics.
Here, highly charged ions (HCI) play a special role. The bound electrons in such HCI are
exposed to the extremely strong electromagnetic fields of the nucleus - the strongest fields
we have available in the laboratory in stable systems. Moreover, in HCI typically only
single or a few electrons are left and calculations of the atomic structure are relatively
simple and accurate. Consequently, HCI provide close to ideal conditions for stringent tests
of the Standard Model, specifically quantum electrodynamics (QED). In the last years, my
group at the Max Planck Institute for Nuclear Physics (MPIK), part of the “stored and
cooled ions” division led by Klaus Blaum, has developed two experiments, Alphatrap
and Liontrap. Liontrap is dedicated to the determination of the atomic masses of the
lightest ions. We have provided world-leading values for the proton, the deuteron and
the HD+ molecular ion. This way, we have shed light on the puzzle of light ion masses,
a long-standing discrepancy in the literature values of these fundamentally important
constants. At Alphatrap and its predecessor experiment at Mainz we have performed a
series of measurements on the g-factor of the bound electron(s), among others the to date
most stringent test of QED in strong fields and the determination of the electron atomic
mass from the g-factor of hydrogenlike carbon to 11 digits. Recently, we have connected
Alphatrap to the Heidelberg HD-EBIT high energy ion source and are now working on
progressing our experiments towards the heavy-HCI regime, where electric fields up to
1016 V cm−1
can be found. With this programme, Alphatrap is part of the Collaborative
Research Centre 1225 ISOQUANT. Furthermore, new techniques enable performing precise
laser spectroscopy in HCI and other systems that are notoriously difficult to address,
such as the molecular hydrogen ion. The development of sympathetic laser cooling of
ions in separate traps will open up a precision regime that was previously beyond reach.
Overall, we have a unique toolbox available, which will give us the opportunity to perform
intriguing measurements that will help to advance our understanding of fundamental
physics.
