Trapping and sympathetic cooling of single thorium ions for spectroscopy

Abstract

Precision optical spectroscopy of exotic ions reveals accurate information about nuclear properties such as charge radii and magnetic and quadrupole moments. Thorium ions exhibit unique nuclear properties with high relevance for testing symmetries of nature. We report loading and trapping of single 232 Th + ions in a linear Paul trap, embedded into and sympathetically cooled by small crystals of trapped 40 Ca + ions. Trapped Th ions are identified in a non-destructive manner from the voids in the laser-induced Ca fluorescence pattern emitted by the crystal, and alternatively, by means of a time-of-flight signal when extracting ions from the Paul trap and steering them into an external detector. We have loaded and handled a total of 231 individual Th ions. We reach a time-of-flight detection efficiency of 95 %, consistent with the quantum efficiency of the detector. The sympathetic cooling technique is expected to be applicable for other isotopes and various charge states of Th e.g., for future studies of 229m Th. PACS numbers:The low-lying isomeric excitation in 229 Th is a unique case where a nuclear transition may be accessible by direct laser excitation. Recently, 229m Th deexcitation to the ground state was directly observed via detection of conversion electrons emitted in the decay of neutral 229m Th [1], and the half-time in neutral 229m Th was determined to be 7(1) µs [2]. Numerous applications of this low-lying isomer have been proposed, including a nuclear laser [3] and nuclear quantum optics [4], as well as its use as a reference for an optical clock of unprecedented accuracy [5][6][7][8]. Such a clock is of interest as a tool to search for dark matter [9] and gravitational waves [10], as well as in relativistic geodesy [11]. It also promises ultra-high sensitivity to time variations of fundamental constants [12,13] making the 229/229m Th system to be of great interest for both fundamental science and applications.The first characterization of the 229m Th nucleus via hyperfine spectroscopy [13] revealed discrepancies between the experimentally determined value of the magnetic moment of the isomeric state and theoretical estimates based on the Nilsson deformed-shell model, which were attributed to neglecting effects like collective quadrupoleoctupole coupling. More precise spectroscopic measurements are needed to resolve the uncertainties about the sensitivity of the 229 Th system to the variation of fundamental constants [13,14].In the present work, we aim to develop and exploit single-ion trapping and spectroscopy of 232 Th + . Trapped single ions or linear crystals with only a few ions allow for cooling to temperatures in the µK regime, which, in combination with near-to-perfect micro motion compensation [15], leads to much reduced systematic frequency shifts. Eventually ions and ion crystals are cooled to the motional ground state [16] and offer options for quantum logic spectroscopy [17], a technique which has been used for frequency measurements exceeding relative accuracy of 10 −17 [18]. However, ...

Catching, trapping and in-situ-identification of thorium ions inside Coulomb crystals of 40Ca+ ions

“…Here, a voltage ramp at the endcaps steers the ion crystal to the SEM detector and determines the time of flight. A detailed model description of the determination of the masses from the flight times is described in [5]. The results show a clear agreement.…”

“…1. To achieve this, we apply optimized deceleration voltage ramps to the entrance endcap of the trap [5]. Ions are then confined in the dynamic and static potential of the segmented linear Paul trap, see thus forming a X-blade design.…”

“…(iii) When applying a voltage ramp to the exit endcap of the trap, ions may be extracted, and travel about 30 cm distance before impacting into a a secondary electron multiplier (SEM) detector which registers the impact. Via time-of-flight measurement the ion mass-to-charge ratio is determined [5]. Even though this method provides an accurate identification at the single ion level, it is destructive, and may only be used as a cross-check for verifying the previous non-destructive observation and identification of single thorium ions.…”

“…IV, the distance for a two-ion crystal with this axial frequency is 11.98 µm. Thus, we determine an accounting factor of 1.66(2) µm/px and a magnification of 14.5 (5). Figures 4 and 5 each show the comparison of a three and four ion crystal with and without impurity.…”

Development of a recoil ion source providing slow Th ions including 229(m)Th in a broad charge state distribution

“…The absence of gas encounters provides the opportunity to study 229(m) Th in various charge states like in the case of 222 Ra from 226 Thdecay [13]. In addition to the recoil ion source, a laser-ablation ion source [11] will also be installed to provide ions of Th isotopes that are available as macroscopic samples. The combination of the ablation and recoil ion sources offers the possibility to study a variety of Th isotopes (see Fig.…”