Progresses on 148 nm Light Sources for Precision Measurement of Nuclear Transition of 229mTh

Abstract The thorium-229 nucleus possesses a uniquely low-energy nuclear transition (~8.4 eV, corresponding to a wavelength of ~148 nm), which is the first confirmed nuclear excitation that can be coherently manipulated by narrow linewidth lasers. Consequently, this transition has garnered widespread interest in the past decades. Owing to the small nuclear size and its strong resistance to environmental perturbations, the thorium-based nuclear clock is theoretically capable of achieving an unprecedented fractional frequency uncertainty at the 10 -20 level, offering great promise as a next-generation frequency standard. Among the key ingredients of such a thorium-based nuclear clock, a 148 nm excitation source with high-performance is of critical importance. Since the feasibility of directly exciting the transition, as well as the overall clock performance, depends heavily on the availability and quality of such a source, the development of 148 nm laser sources with high-quality represents a frontier for scientists globally. In this article, we provide a systematic overview of the development of the current 148 nm laser sources. First, we briefly introduce the scientific motivation for high-precision spectroscopy of the thorium nuclear transition and the corresponding technical requirements for 148 nm laser sources. Then, we summarize four main types of existing 148 nm source generation schemes and their working principles, along with the recent progress in nuclear transition measurements using such sources. Finally, we discuss potential future directions.