Thorium-229 is a unique case in nuclear physics: it presents a metastable first excited state Th-229m, just a few electronvolts above the nuclear ground state. This so-called isomer is accessible by VUV lasers, which allows transferring the amazing precision of atomic laser spectroscopy to nuclear physics. Being able to manipulate the Th-229 nuclear states at will opens up a multitude of prospects, from studies of the fundamental interactions in physics to applications as a compact and robust nuclear clock. However, direct optical excitation of the isomer or its radiative decay back to the ground state has not yet been observed, and a series of key nuclear structure parameters such as the exact energies and half-lives of the low-lying nuclear levels of Th-229 are yet unknown. Here we present the first active optical pumping into Th-229m. Our scheme employs narrow-band 29 keV synchrotron radiation to resonantly excite the second excited state, which then predominantly decays into the isomer. We determine the resonance energy with 0.07 eV accuracy, measure a half-life of 82.2 ps, an excitation linewidth of 1.70 neV, and extract the branching ratio of the second excited state into the ground and isomeric state respectively. These measurements allow us to re-evaluate gamma spectroscopy data that have been collected over 40~years.
Exploring $α$- and $β$-decay-induced quenching of the $^{229}$Th nuclear-clock isomer in solid-state hosts
KU Leuven（荷语鲁汶大学）；Instituut voor Kern- en Stralingsfysica（核与辐射物理研究所）；Johannes Gutenberg University Mainz（美因茨约翰内斯·古腾堡大学）；CERN（欧洲核子研究组织）；Université de Caen Normandie（诺曼底卡昂大学）；ENSICAEN（卡昂高等师范学院）；CNRS/IN2P3（法国国家科学研究中心/粒子物理研究所）；Grand Accélérateur National d’Ions Lourds (GANIL)（重离子大加速器）；CEA（法国原子能和替代能源委员会）；Vienna Center for Quantum Science and Technology（维也纳量子科学与技术中心）；Atominstitut（原子研究所）；TU Wien（维也纳工业大学）
$^{229}$Th Nuclear Spectroscopy in an Opaque Material: Laser-Based Conversion Electron Mössbauer Spectroscopy of $^{229}$ThO$_2$
University of California, Los Angeles（加州大学洛杉矶分校）；University of Manchester（曼彻斯特大学）；University of Nevada, Reno（内华达大学雷诺分校）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；Eckert & Ziegler Analytics, Inc.（埃克特&齐格勒分析公司）；Johannes Gutenberg-Universität Mainz（美因茨约翰内斯古腾堡大学）；Ludwig-Maximilians-Universität München（慕尼黑路德维希-马克西米利安大学）
Toward nanophotonic platforms for solid-state $^{229}$Th nuclear clocks
KU Leuven（荷语鲁汶大学）；Institute for Nuclear and Radiation Physics（核与辐射物理研究所）；Institute of Science and Technology Austria（奥地利科学技术研究所）；Max Planck Institute for the Science of Light（马克斯·普朗克光科学研究所）；Friedrich-Alexander-Universität Erlangen-Nürnberg（埃尔兰根-纽伦堡弗里德里希·亚历山大大学）；Keio University（庆应义塾大学）；Ghent University（根特大学）
Probing the Linewidth of the 12.4-keV $^{45}$Sc Isomeric Resonance in Solids by Nuclear Forward Scattering
Argonne National Laboratory（阿贡国家实验室）；Max Planck Institute for Nuclear Physics (MPIK)（马克斯·普朗克核物理研究所）；Helmholtz-Institut Jena（耶拿亥姆霍兹研究所）；GSI Helmholtzzentrum für Schwerionenforschung（GSI重离子研究中心）；Friedrich-Schiller-Universität Jena（耶拿弗里德里希·席勒大学）；European X-Ray Free-Electron Laser Facility（欧洲X射线自由电子激光设施）；Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器）
Probing Ultralight Dark Matter at the Mega-Planck Scale with the Thorium Nuclear Clock
University of Delaware（特拉华大学）；High Energy Accelerator Research Organization (KEK)（高能加速器研究机构）；JILA, NIST and University of Colorado（JILA、美国国家标准与技术研究院和科罗拉多大学）；University of Colorado（科罗拉多大学）；Deutsches Elektronen-Synchrotron DESY（德国电子同步辐射研究所）；Leibniz Universität Hannover（汉诺威莱布尼茨大学）；Weizmann Institute of Science（魏茨曼科学研究所）；Physikalisch-Technische Bundesanstalt（德国物理技术联邦研究院）
Research Institute for Interdisciplinary Science, Okayama University（冈山大学跨学科科学研究所）；Faculty of Physics, TU Wien（维也纳工业大学物理学院）；Institute of Physics, Laboratory for Ultrafast Microscopy and Electron Scattering LUMESÉcole Polytechnique Fédérale de Lausanne（洛桑联邦理工学院超快显微镜与电子散射实验室LUMES物理研究所）
