Eur. Phys. J. D (2025) 79: 127
https://doi.org/10.1140/epjd/s10053-025-01071-1

Research - Atoms, Molecules, Ions, and Clusters

A cryogenic Paul trap for probing the nuclear isomeric excited state $_{}^{229\text{m}}$Th$_{}^{3+}$

¹ Fakultät für Physik, LMU München, 85748, Garching bei München, Germany
² MPIK Heidelberg, 69117, Heidelberg, Germany
³ II. Physikalisches Institut, Justus-Liebig-Universität Gießen, 35392, Gießen, Germany
⁴ GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291, Darmstadt, Germany
⁵ Department of Chemistry - TRIGA site, Johannes Gutenberg-Universität, 55099, Mainz, Germany
⁶ Helmholtz Institute Mainz, 55099, Mainz, Germany
⁷ Department of Physics and Astronomy, UCLA, 90095, Los Angeles, California, USA
⁸ Qatar Center for Quantum Computing, College of Science and Engineering, Hamad Bin Khalifa University, Doha, Qatar
⁹ Department of Physics, Tsinghua University, 100084, Beijing, China
¹⁰ Institute for Nuclear and Radiation Physics, KU Leuven, 3001, Leuven, Belgium
¹¹ TUM School of Natural Sciences, Department of Physics, Technische Universität München, 85748, Garching bei München, Germany
¹² Europäisches Patentamt, 80469, München, Germany
¹³ Institut für Physik, Johannes Gutenberg-Universität, 55128, Mainz, Germany

^a markus.wiesinger@lmu.de

Received: 4 August 2025
Accepted: 16 September 2025
Published online: 17 October 2025

Abstract

While laser excitation of the nuclear isomeric transition in $_{}^{229}$Th has been recently achieved for thorium atoms embedded in large-bandgap crystals, laser excitation and characterization of the nuclear transition in trapped $_{}^{229}$Th$_{}^{3+}$ ions has not yet been accomplished. To address these experiments, a cryogenic Paul trap setup has been designed, built, and commissioned at LMU Munich. Here, we present the specifications of the new experimental platform and demonstrate its successful operation, showing the extraction, subsequent ion guiding, mass purification, and trapping of $_{}^{229}$Th$_{}^{3+}$ and $_{}^{229\text{m}}$Th$_{}^{3+}$ ions from a newly designed buffer-gas stopping cell as well as of $_{}^{88}$Sr$_{}^{+}$ ions from laser ablation of a solid target. Further, we show sympathetic laser cooling of $_{}^{229\text{(m)}}$Th$_{}^{3+}$ by Doppler-cooled $_{}^{88}$Sr$_{}^{+}$ ions and the formation of mixed-species Coulomb crystals.