关键词: thorium-229, 148 nm laser, optical clock, four-wave mixing, VUV combs

Abstract: The thorium-229 nucleus possesses a uniquely low-energy nuclear transition ($\sim 8.4$ eV, corresponding to a wavelength of $\sim 148$ nm), which is the first confirmed nuclear excitation that can be coherently manipulated by narrow-linewidth lasers. Consequently, this transition has garnered widespread interest over the past decades. Owing to the small nuclear size and strong resistance to environmental perturbations, a 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 high-performance 148 nm excitation source 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 high-quality 148 nm laser sources represents a frontier for scientists worldwide. In this article, we provide a systematic overview of the current development of 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 recent progress in nuclear transition measurements using such sources. Finally, we discuss potential future directions.

Key words: thorium-229, 148 nm laser, optical clock, four-wave mixing, VUV combs