The $^{229}$Th nucleus has the unique property of an extremely low lying isomeric first excited state (denoted $^{229\text{m}}$Th). 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, $^{229}$Th 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 a (potential and theoretically predicted) time variation of fundamental constants like the fine structure constant compared to electronic transitions in other atoms.
While the isomeric state was recently directly excited with laser light in $^{229}$Th doped CaF$_2$ and other solid-state crystals, the nuclear clock project at LMU focusses on the trapped ion approach with sympathetically laser-cooled $^{229\text{(m)}}$Th$^{3+}$, 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 $^{229(\text{m})}$Th$^{3+}$ ions embedded in mixed-species $^{229(\text{m})}$Th$^{3+}$/$^{88}$Sr$^+$ Coulomb crystals. We will show simultaneous fluorescence imaging of trapped $^{88}$Sr$^+$ ions at 422 nm and $^{229}$Th$^{3+}$ ions at 690 nm using two cameras, and report on the progress towards laser spectroscopy of the hyperfine structure of the electronic transitions in $^{229(\text{m})}$Th$^{3+}$, measuring the lifetime of $^{229\text{m}}$Th$^{3+}$ in vacuum, and quantum state readout of the nuclear transition after VUV excitation.