Garchinger Maier-Leibnitz-Kolloquium: Laser spectroscopy of electronic transitions of the nuclear clock isomer 229Th3+ in a cryogenic Paul trap

by Dr Markus Wiesinger (LMU Munich (LS Parodi))

Thursday 11 Dec 2025, 16:15 → 17:15 Europe/Berlin

Lecture Hall, ground floor (west) (LMU building, Am Coulombwall 1, campus Garching)

The 229Th nucleus has the unique property of an extremely low lying isomeric first excited state (229mTh). With an excitation energy of 8.356 eV — corresponding to a wavelength of 148.4 nm — and an expected lifetime of the ionic thorium isomer in vacuum of about 2000 s, the isomeric state can be directly excited with laser light from state-of-the-art VUV laser systems. Consequently, 229Th is the ideal candidate for a nuclear optical clock.

Furthermore, due to a near-cancellation between the large Coulomb energies of ground and isomeric state as part of the nuclear binding energy, the nuclear clock transition has an about 10^3 larger sensitivity to potential and theoretically predicted time variation of the fine structure constant compared to electronic transitions in other atoms.

While the isomeric state was recently directly excited with laser light in 229Th doped CaF2 and other solid-state crystals, the nuclear clock project at LMU focusses on the trapped ion approach with sympathetically laser-cooled 229(m)Th3+, which features suitable electronic transitions for fast nuclear state readout and extremely low systematic uncertainties.

This talk will present the status of the experiment at LMU with an emphasis on trapping and sympathetic laser-cooling of 229(m)Th3+ ions embedded in mixed-species 229(m)Th3+ /88Sr+ Coulomb crystals. We will show simultaneous  fluorescence imaging of trapped 88Sr+ ions at 422 nm and 229Th3+ ions at 690 nm using two cameras, laying the foundations for laser spectroscopy of the hyperfine structure of the electronic transitions in 229(m)Th3+, for measuring the lifetime of 229mTh3+ in vacuum, and for quantum state readout of the nuclear transition after VUV excitation.

Hybrid access via ZOOM:
https://lmu-munich.zoom.us/j/98457332925?pwd=TWc3V1JkSHpyOTBPQVlMelhuNnZ1dz09
Meeting ID: 984 5733 2925
Passcode: 979953

kolloquium-11-12-25.pdf