Nuclear clocks: What now?

In 1976 Kroger and Reich established the existence of a low-lying nuclear excited state in 229 Th through the spectroscopy of rays emitted following the decay of 233 U. The prospects of a laser-accessible nuclear transition touched off a flurry of proposals to utilize this apparently unique nuclear transition as a sensitive probe of both nuclear structure and chemical environment, to constrain physics beyond the Standard Model, and to construct a clock with unprecedented performance. Unfortunately, Kroger and Reich could only tell us that the transition energy was less than about 100 eV and therefore, scientists have spent the intervening 48 years searching for the thorium nuclear transition.

Last year, there was a breakthrough in this search and the nuclear transition was finally laser excited in Th:CaF 2 [1,2] at PTB and JILA, and in Th:LiSrAlF 6 [3], ThF 4 [4], and ThO 2 [5] at UCLA. With this new data, a clearer theoretical picture of the nuclear-electronic couplings in the solid-state hosts is emerging [6-8]. In this talk, we willsummarize the current understanding of these systems and highlight the new opportunities they suggest for optimizing nuclear clock performance. This work was funded by the NSF, DARPA, and ARO.