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1. Proposal and theoretical investigation of 229Th-doped nonlinear optical crystals for compact solid-state clocks

   https://doi.org/10.1063/5.0247867  

   Morgan, H_W T; Elwell, R; Terhune, J_E S; Tran_Tan, H B; Perera, U C; Derevianko, A; Alexandrova, A N; Hudson, E R (March 2025, Applied Physics Letters)

   The recent laser excitation of the 229Th isomeric transition in a solid-state host opens the door for a portable solid-state nuclear optical clock. However, at present, the vacuum-ultraviolet laser systems required for clock operation are not conducive to a fieldable form factor. Here, we propose a possible solution to this problem by using 229Th-doped nonlinear optical crystals, which would allow clock operation without a vacuum-ultraviolet laser system and without the need of maintaining the crystal under vacuum. We investigate electronic properties and thorium doping in BaMgF4 and BaZnF4 with density functional theory, predicting BaMgF4 to be the superior material, and evaluate the performance of a Th:BaMgF4 clock. 

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2. Photoinduced quenching of the ${}^{229}\mathrm{Th}$ isomer in a solid-state host

   https://doi.org/10.1103/glzr-thyw  

   Terhune, J_E S; Elwell, R; Tan, H_B Tran; Perera, U C; Morgan, H_W T; Alexandrova, A N; Derevianko, Andrei; Hudson, Eric R (June 2025, Physical Review Research)

   The population dynamics of the ${}^{229}\mathrm{Th}$isomeric state is studied in a solid-state host under laser illumination. A photoquenching process is observed, where off-resonant vacuum-ultraviolet (VUV) radiation leads to relaxation of the isomeric state. The cross-section for this photoquenching process is measured, and a model for the decay process, where photoexcitation of electronic states within the material band gap opens an internal conversion decay channel, is presented and appears to reproduce the measured cross-section. By engineering defects into ${}^{229}\mathrm{Th}$-doped solid-state hosts, this previously unrecognized photoquenching process may be used to reduce the clock transition readout time and thereby increase the stability of the nuclear clock. Published by the American Physical Society2025 

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3. Radiative decay of the ${}^{229m}\mathrm{Th}$ nuclear clock isomer in different host materials

   https://doi.org/10.1103/PhysRevResearch.7.013052  

   Pineda, S V; Chhetri, P; Bara, S; Elskens, Y; Casci, S; Alexandrova, A N; Au, M; Athanasakis-Kaklamanakis, M; Bartokos, M; Beeks, K; et al (January 2025, Physical Review Research)

   A comparative vacuum ultraviolet spectroscopy study conducted at ISOLDE-CERN of the radiative decay of the ${}^{229m}\mathrm{Th}$nuclear clock isomer embedded in different host materials is reported. The ratio of the number of radiative decay photons and the number of ${}^{229m}\mathrm{Th}$embedded are determined for single crystalline ${\mathrm{CaF}}_2, {\mathrm{MgF}}_2, {\mathrm{LiSrAlF}}_6$, AlN, and amorphous ${\mathrm{SiO}}_2$. For the latter two materials, no radiative decay signal was observed and an upper limit of the ratio is reported. The radiative decay wavelength was determined in ${\mathrm{LiSrAlF}}_6$and ${\mathrm{CaF}}_2$, reducing its uncertainty by a factor of 2.5 relative to our previous measurement. This value is in agreement with the recently reported improved values from laser excitation. Published by the American Physical Society2025 

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4. Stable 2 W continuous-wave 261.5 nm laser for cooling and trapping aluminum monochloride

   https://doi.org/10.1364/OE.441741  

   Shaw, J_C; Hannig, S.; McCarron, D_J (October 2021, Optics Express)

   We present a high-power tunable deep-ultraviolet (DUV) laser that uses two consecutive cavity enhanced doubling stages with LBO and CLBO crystals to produce the fourth harmonic of an amplified homebuilt external cavity diode laser. The system generates up to 2.75 W of 261.5 nm laser light with a ∼2 W stable steady-state output power and performs second harmonic generation in a largely unexplored high intensity regime in CLBO for continuous wave DUV light. We use this laser to perform fluorescence spectroscopy on theA¹Π ← X¹Σ⁺transition in a cold, slow beam of AlCl molecules and probe the A¹Π|v′ = 0,J′ = 1〉 state hyperfine structure for future laser cooling and trapping experiments. This work demonstrates that the production of tunable, watt-level DUV lasers is becoming routine for a variety of wavelength-specific applications in atomic, molecular and optical physics. 

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5. Prospects of a thousand-ion Sn2+ Coulomb-crystal clock with sub-10−19 inaccuracy

   https://doi.org/10.1038/s41467-024-49241-w  

   Leibrandt, David_R; Porsev, Sergey_G; Cheung, Charles; Safronova, Marianna_S (July 2024, Nature Communications)

   Abstract Optical atomic clocks are the most accurate and precise measurement devices of any kind, enabling advances in international timekeeping, Earth science, fundamental physics, and more. However, there is a fundamental tradeoff between accuracy and precision, where higher precision is achieved by using more atoms, but this comes at the cost of larger interactions between the atoms that limit the accuracy. Here, we propose a many-ion optical atomic clock based on three-dimensional Coulomb crystals of order one thousand Sn²⁺ions confined in a linear RF Paul trap with the potential to overcome this limitation. Sn²⁺has a unique combination of features that is not available in previously considered ions: a¹S₀ ↔ ³P₀clock transition between two states with zero electronic and nuclear angular momentum (I = J = F = 0) making it immune to nonscalar perturbations, a negative differential polarizability making it possible to operate the trap in a manner such that the two dominant shifts for three-dimensional ion crystals cancel each other, and a laser-accessible transition suitable for direct laser cooling and state readout. We present calculations of the differential polarizability, other relevant atomic properties, and the motion of ions in large Coulomb crystals, in order to estimate the achievable accuracy and precision of Sn²⁺Coulomb-crystal clocks. 

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