Principal Investigators:

• D. Budker (Helmholtz Institute Mainz, PRISMA+ Cluster of Excellence, and the JGU QUANTUM group),
• Ch. E. Düllmann (Helmholtz Institute Mainz, PRISMA+ Cluster of Excellence and Institute of Nuclear Chemistry, JGU Mainz, GSI Helmholtzzentrum für Schwerionenforschung Darmstadt)
• F. Schmidt-Kaler (Helmholtz Institute Mainz, JGU, Institute for Physics, PRISMA+ Cluster of Excellence)

Research:

TACTICa uses atomic physics methods to investigate thorium isotopes including the metastable low energy nuclear state in ^229mTh. The optical ground state transition of isomeric ²²⁹Th is the only nuclear transition that potentially can be interrogated using lasers. Controlling this transition opens an interesting window into high-precision tests of the standard model and on possible variation of fundamental constants.
Thus TACTICa aims to deploy ion trapping techniques like quantum logic spectroscopy to gain access to the nuclear structure of thorium.

TACTICa aims for combining unique expertise of nuclear chemistry and experimental quantum science. This way, we will first vastly improve the accuracy of spectroscopic investigations of the nuclear Th isotopes including the isomer in ²²⁹Th, and eventually realize quantum logic spectroscopy and quantum-enhanced measurements on thorium for fundamental research.

Results:

As an important first demonstration, in 2018, single ²³²Th⁺ ions were trapped together with Ca⁺ ions forming a so-called “Wigner crystal”, sympathetically cooled and characterized.

The ions were successfully trapped in a linear Paul trap, and their mass was identified via a time-of-flight (TOF) measurement as well as from the voids in the laser-induced Ca⁺ fluorescence pattern emitted by the Wigner crystal. This paves the way to future high-precision studies of various Th isotopes including ^229mTh in the context of fundamental physics or quantum optics.

The picture shows a linear crystal of Ca⁺ ions when the confinement in the radial direction is tighter than that in the axial direction. When the number of ions is increased and the radial potential can not keep the ions in a linear chain, the ion crystal undergoes a “phase transition” and assumes a zigzag shape, as shown in the lower image. The distance between adjacent ions strongly depends on the axial trap potential; inner ions are closer to their neighbors compared to outer ions since the ions at the inner positions are more confined. Here the average distance between adjacent ions is 10 to 20 µm. A thorium ion trapped in such a crystal would appear as a “crystal defect.”