A nanoparticle stored with an atomic ion in a linear Paul trap

Abstract

Radiofrequency (RF) traps enable highly controlled interactions between charged particles, including reactions between cold molecular ions, sympathetic cooling of one ion species with another, and quantum logic spectroscopy. However, the charge-to-mass ($Q/m$) selectivity of RF traps limits the range of objects that can be confined simultaneously in the same trap. Here, we confine two particles - a nanoparticle and an atomic ion - in the same radiofrequency trap although their charge-to-mass ratios differ by six orders of magnitude. The confinement is enabled by a dual-frequency voltage applied to the trap electrodes. We introduce a robust loading procedure under ultra-high vacuum and characterize the stability of both particles. It is observed that slow-field micromotion, an effect specific to the dual-field setting, plays a crucial role for ion localization. Our results lay the groundwork for controlled interactions between diverse charged particles, regardless of the difference in their charge or mass, with applications from antimatter synthesis to the generation of macroscopic quantum states of motion.

Figures (4)

• Figure 1: Experimental setup. (a) Dual-frequency drive of the linear Paul trap. (b) Schematic of the procedure for trapping a nanoparticle (NP) and an ion. Along the $z$ axis, DC endcap electrodes are indicated in gray. Not shown: two pairs of compensation electrodes for displacement in the $xy$ plane. (c) Camera image of a $^{40}$Ca$^+$ ion and a nanoparticle confined in the same Paul trap. (d) A second image in which two ions are confined with a nanoparticle.
• Figure 2: Influence of the slow field on the ion. (a) Blue dots: instability threshold measured with the ion, plotted in terms of the stability parameters $q_\text{(i)}$ and $a_\text{eff}$. Green area: stability diagram, calculated from experimental parameters. Insets are camera images of the ion for different stability regimes; the scale bar is 10µm. (b) Left: Composite image of the ion at four positions along the $z$ axis for $q_\text{(i)} = 0.4$, $a_\text{eff} = 0.06$. The positions along the $z$ axis are set by the endcap voltage. Inset: schematic of the ion positions along $z$ axis in the absence of micromotion.
• Figure 3: Ion-particle interaction. Circles: fixed nanoparticle positions in the dual-frequency trap. Diamonds: each ion position is calculated from the corresponding nanoparticle position. Squares: ion positions extracted from Fig. \ref{['fig:fig_4']}. Triangles: nanoparticle position extracted from Fig. \ref{['fig:fig_4']}. The labels next to the data points correspond to the labels in Fig. \ref{['fig:fig_4']}c,f. The error bars correspond to the size of the nanoparticle and the ion images in Fig. \ref{['fig:fig_4']}. Ion-nanoparticle pairs are indicated with the same color. Calculation parameters: the nanoparticle's charge is 800, and the ion's secular frequencies along the $x$ and $z$ axes are $\omega_{\text{(i)},x} = 4MHz$, $\omega_{\text{(i)},z} = 800kHz$.
• Figure 4: Ion-nanoparticle configurations. The $z$ axis points in the same direction in all images. (a) EM-CCD images of the $xy$ and (b) $z$ configurations. (c) Starting from an $xy$ pair, we decrease the compensation-electrode voltages that have been used to displace the nanoparticle from the trap center. As a result, the nanoparticle shifts towards the center of the trap, and the ion is repelled along the axis of weakest confinement, which is the $z$ axis here. (d) The voltage on one endcap is increased to compensate for this shift; both particles shift away from that endcap. (e) The compensation-electrode voltages are decreased further until the nanoparticle lies on the $z$ axis. (f) The endcap voltage is increased further until the ion is at the original position. The red cross in (c)-(f) indicates the potential minimum of the trap for the ion in the absence of the nanoparticle. The labels I-III correspond to the data-point labels in Fig. \ref{['fig:fig_3']}.