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

TL;DR

This paper reports the development and successful operation of a cryogenic Paul trap system for trapping, cooling, and studying $^{229}$Th$^{3+}$ ions, enabling future nuclear transition experiments.

Contribution

The paper introduces a new cryogenic Paul trap setup specifically designed for $^{229}$Th$^{3+}$ ions, demonstrating ion trapping, purification, and sympathetic cooling capabilities.

Findings

Successful trapping of $^{229}$Th$^{3+}$ and $^{229m}$Th$^{3+}$ ions.

Demonstration of sympathetic laser cooling with $^{88}$Sr$^{+}$ ions.

Formation of mixed-species Coulomb crystals.

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.