Trapping and sympathetic cooling of single thorium ions for spectroscopy

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 ${}^{232}\mathrm{Th}^{+}$ ions in a linear Paul trap, embedded into and sympathetically cooled by small crystals of trapped ${}^{40}\mathrm{Ca}^{+}$ ions. Trapped thorium ions are identified in a nondestructive manner from the voids in the laser-induced calcium 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 thorium ions. We reach a time-of-flight detection efficiency of $\gtrsim 95\%$, 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 thorium, e.g., for future studies of ${}^{229\mathrm{m}}\mathrm{Th}$.

Figure 1

(a) Sketch of the experimental setup. (b) Observation of the laser-induced calcium ion fluorescence for crystals with and without embedded thorium and different numbers of calcium ions. Upper row: Calibration of the distance between two ions at a measured axial trap frequency of 310 kHz, corresponding to 7.2 pixel (px). The exposure time is 40 ms. Calcium ion crystals arrange in linear configuration. Lower row: Observation of mixed-species crystals. Thorium ion trapped at the location of dark void. Calculated ion positions for calcium (green) and thorium (red) are indicated with circles, assuming trap frequencies of ${\omega }_{x,y,z}=2\pi \times$ (1640, 1758, 310) kHz.

Figure 2

(a) Measured TOF distribution of laser-ablated and ionized thorium ions from the ion gun. (b) Sketch of the time-dependent electric potential for ion loading. (1) Incoming ions are decelerated by a repulsive potential $U^{\left(1\right)}=+350\mathrm{V}$. When they are inside the first endcap (2), the potential is switched to attractive $U^{\left(1\right)}=-256\mathrm{V}$ and therefore the ions are decelerated a second time since they do not see a potential change within the endcap. (3) Simultaneously the second endcap is switched to $U^{\left(2\right)}=+3\mathrm{kV}$, and thus the ions are repelled back into the trap volume. (c) Timing sequence for deceleration of incoming thorium ions for trap loading.

Figure 3

(a) TOF identification for ions extracted from the linear ion trap. Note that data points for isotopes of Xe and Pr/Ce in gray and black are partially overlapping. (b) Residuals of the TOF identification, including systematic errors due to the path into the detector. See text and Fig. 4. Total errors are 10 ns for thorium ions and 5 ns for all other ion species.

Figure 4

Determination of systematic TOF error due to the use of a tilted off-axis detector. (a) Thorium ions are steered onto different spots of the detector by changing the deflection voltage $U_x$, giving rise to a variation of the TOF signal. Row (A) shows the histogram for thorium ions which are steered into the detector edge in $x$ direction, (B) into the center of the detector, and (C) onto the position corresponding to the center of the detector as determined using calcium ions. The extracted number of thorium ions scales with the color bar, and the black lines indicate the mean value of the time-of-flight signals. (b) Measured TOF variation as calcium ions are directed over the entire detector surface in x and y direction.