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.

The octupole correlations of the $K^\pi=5/2^+$ ground state and the rotational spectrum built on it in $^{229}$Th are studied using the microscopic relativistic density functional theory on a three-dimensional lattice space and the reflection-asymmetric triaxial particle rotor model. It is found that $^{229}$Th has a ground state with static axial octupole and quadrupole deformations. The occurrence of octupole correlations, driven by the octupole deformation, is analyzed through the evolution of single-particle levels around the Fermi surface. The experimental energy spectrum and the electromagnetic transition probabilities, including $B(E2)$ and $B(M1)$, are reasonably well reproduced.

Owing to the presence of a low-energy, long-lived nuclear isomeric state, $^{229}$Th is an ideal candidate for developing the next generation clock - the nuclear clock - holding great promise for both applied and fundamental physics. The $^{229}$Th ionic nuclear optical clock has garnered considerable attention, attributed to its high precision with a relative uncertainty of $\le 1.5 \times 10^{-19}$ and the potential for common-mode noise cancellation via self-comparison between the nuclear transition and the electronic transition of thorium ions. In this article, we focus on Th$^{n+}$ ions ($n = 1$, 2, 3) and present a comprehensive review of the current progress in the development of ionic nuclear clocks, covering essential steps such as ion generation, trapping, and cooling. Furthermore, we discuss the realization of a closed-loop clock cycle, addressing key aspects including stable isomer excitation and efficient isomer deexcitation.

The $^{229}$Th nucleus has attracted considerable attention due to the existence of its low-energy isomeric state; however, direct laser excitation in ionic systems poses significant challenges for current laser technologies. In the $^{229}$Th$^{3+}$ ion, the electronic bridge (EB) process enables the conversion of direct laser excitation into an effective two-photon process ($I_{\rm g},6{\rm d}_{3/2}\rightarrow I_{\rm g},7{\rm p}_{1/2}\rightarrow I_{\rm m},7{\rm s}_{1/2}$), thereby circumventing the requirement for laser radiation at 148 nm. In this work, we employ many-body perturbation theory (MBPT) to calculate the hyperfine structure constants and field shift factors for several low-lying excited states of the $^{229}$Th$^{3+}$ ion. By combining these theoretical results with previously reported experimental data, we predict three transition frequencies associated with the EB process in the $^{229}$Th$^{3+}$ ion and identify the most suitable transition pathway for EB-assisted nuclear excitation.

Recent advances in atomic optical clocks based on electronic transitions have achieved frequency uncertainties at the $10^{-19}$ level, enabling wide applications in testing variations of physical constants, exploring dark matter signatures, and enhancing precision metrology for position, navigation, and timing systems. To pursue higher-precision optical clocks, the development of nuclear optical clocks has emerged, with the $^{229}$Th system distinguished by its unique low-lying isomeric state at $\sim8.4$ eV and a natural linewidth of approximately 100 μHz, promising uncertainties below $10^{-19}$. The intrinsic insensitivity of nuclear transitions to external perturbations and their subatomic-scale spatial confinement provide significant advantages over electronic transitions in mitigating environmental shifts. Recent experimental breakthroughs include the excitation of the nuclear clock transition in solid-state $^{229}$Th-doped crystals with spectral resolution at the kHz level. However, critical challenges persist, particularly in implementing effective laser excitation schemes (e.g., via the electronic bridge mechanism) and closed-loop quantum control in trapped ion systems. Addressing these requires comprehensive understanding of complex many-body interactions in $^{229}$Th, encompassing electronic structure, nuclear deformation, hyperfine and field shift, and solid-state environmental coupling. This review synthesizes recent advancements in (i) the characterization of nuclear and atomic structures of the $^{229}$Th nuclear clock, and (ii) precise evaluation and mitigation of external perturbations affecting the clock transitions. The analysis provides a solid theoretical and experimental foundation for optimizing $^{229}$Th-based nuclear clock performance.

The isomeric transition of thorium-229 (${}^{229}$Th), as the only known laser-accessible nuclear transition, offers the possibility for the development of a new generation of optical clocks. Solid-state nuclear optical clock based on ${}^{229}$Th-doped crystals or thin films has attracted much attention due to its potential advantages in high stability, miniaturization, and robustness. This paper reviews the research progress of solid-state nuclear optical clock materials, analyzes the preparation, defects, and properties of the candidate solid material systems for ${}^{229}$Th, explores the influence of the local crystal environment on the nuclear transition, focuses on introducing the latest research results of crystal materials such as Th-doped CaF$_{2}$ and LiSrAlF$_{6}$, and looks forward to the future development direction of this field. It could provide a reference for the material selection and optimization of solid-state nuclear optical clocks.