The thorium nuclear optical clocks proposed were based on the special 8.4 eV nuclear transition exist in thorium-229 (²²⁹Th) nucleus. Because of the isolation of the thorium-229 nucleus and the narrow linewidth of the nuclear transition, the thorium nuclear optical clocks are anticipated to surpass the accuracy of existing atomic clocks, which are the existing most accurate clocks.

Our pathway to realize the nuclear optical clocks is based on thorium ions. The trivalent thorium-229 ions are confined by an ion trap. They are cooled by laser cooling or sympathetic cooling into several mK. At this temperature, the thorium-229 ions form into an ordered structure called a Coulomb crystal. These ions in a Coulomb crystal are isolated from the environment and can be confined for a long time, therefore are favorable for the nuclear transition excitation. Electronic bridge transitions, which are transitions coupled with both electronic transitions and the nuclear transition, can be used to excite the thorium-229 nuclear transition. Some electronic transitions have larger transition rates than the direct nuclear transition.

There are several challenges still needs to overcome, such as the 148 nm laser sources and the closed-loop manipulation. The high-precision nuclear optical clocks based on the ²²⁹Th ions are expected to achieve a frequency uncertainty at the 10^-19 level.

 

Fig. 1: The electronic levels and electronic transitions and the electronic bridge excitation of the trivalent thorium-229 ions