Sympathetic cooling of a large ${}^{113}\mathrm{Cd}^{+}$ ion crystal with ${}^{40}\mathrm{Ca}^{+}$ in a linear Paul trap

Abstract

We have sympathetically cooled and crystallized 113 Cd + ions with laser-cooled 40 Ca + ions, and directly observed the complete large bicrystal structure in a linear Paul trap. The large two-component crystal contains up to 3.5 × 10 3 40 Ca + ions and 6.8 × 10 3 113 Cd + ions. The temperature of the crystallized 113 Cd + ions was measured to be as low as 41 mK with a large mass ratio. Moreover, we have studied several properties and structures of the ultracold sample. The factors affecting the sympathetic cooling effect were studied, including the electrical parameters and the number ratio between laser-cooled ions and sympathetically-cooled ions. The results of this paper enrich the experimental research of large two-component ion crystals, and the ultracold sample of 113 Cd + ions makes it possible to further improve the accuracy of the cadmium-ion microwave frequency standard.

High-Performance Microwave Frequency Standard Based on Sympathetically Cooled Ions

“…The core of the whole system is a linear quadrupole trap (see Figure 2). Each electrode is divided into three segments: the middle segment with length of 40 mm that acts as radio frequency (RF) electrodes for radial confinement, and the two ends of length 20 mm that act as end-cap (EC) electrodes for axial confinement [27,28]. The radius of each electrode is r e = 7.1 mm and the distance to the main central axis of the trap is r 0 = 6.2 mm to obtain the most ideal quadrupole field and to reduce RF heating [29].…”

“…Thereafter, 113 Cd + ions are produced by ionizing evaporated atoms. By scanning the frequency of the cooling laser (397 nm) and reducing the amplitude of the RF voltage, 113 Cd + ions are sympathetically cooled to the crystal phase [28]. A typical image of the twocomponent ion crystal (Figure 3) displays an ellipsoidal 40 Ca + crystal located in the center of the trap; the 113 Cd + ions form a thin shell outside the 40 Ca + crystal.…”

“…However, because the cadmium ion shell is thin, deriving the volume from the EMCCD image directly results in a large error. The number of 113 Cd + is calculated by comparing the image with that obtained from a molecular dynamics simulation [28]. In our sympatheticallycooled ion frequency standard experiment, a typical value for the number of 40 Ca + is 7 500 whereas the number of 113 Cd + is 5 200.…”

“…Because the power of the 397 nm laser beam is much larger than that for the 214 nm beam, the fluorescence signal of 40 Ca + is much brighter than that of 113 responding to a temperature of 87(3) mK. We studied the effects of different RF and EC voltages, ion ratios [28], detuning of the 397 nm laser beam, and other factors on the temperature of cadmium ions. The lowest temperature was 36(5) mK, corresponding to Gaussian linewidth of 17.8(1.3) MHz.…”

“…However, prior to our work, the only sympathetically-cooled ion microwave frequency standard realized was that constructed by J. J. Bollinger and colleagues using a Penning trap with Be + sympathetically cooled by Mg + [25]. In past work, we have demonstrated the laser-cooled cadmium ion microwave frequency standard [11][12][13], and have made much progress in sympathetically-cooled large ion crystals [26][27][28].…”