Endcap-Type Paul Trap for Precision Spectroscopy and Studies of Controlled Interactions

TL;DR

The paper presents an endcap-type Paul trap optimized for Ca$^{+}$ and Yb$^{+}$ that traps at the rf node with high optical access. The authors design, fabricate, and characterize the apparatus, achieving a quadrupole coefficient of $A_2\approx 0.30$ in close agreement with the design value and demonstrating an excess micromotion-induced relative frequency shift of $3.5\times 10^{-18}$ for $^{40}$Ca$^{+}$. A custom UV imaging system ($NA=0.14$, $\approx 22\times$) resolves 2- and 3-ion Coulomb clusters, with structural transitions in good agreement with molecular dynamics simulations, validating near-cylindrical trap symmetry. These capabilities enable high-precision spectroscopy, optical clock tests, and controlled ion–atom interactions, paving the way for automated EMM minimization and exploration of mesoscopic Coulomb physics.

Abstract

We present the design and fabrication of an endcap-type Paul trap. The trap is designed for studies with Ca$^{+}$ and Yb$^{+}$. The design, fabrication process, and characterization are presented in detail with a focus on trapping a single compensated ion at the rf node. A custom-built imaging system of $NA = 0.14$ and magnification $\approx 22 \times$ performs close to diffraction-limit and resolves multi-ion clusters. Controlled ion loading and characterization of the trap are performed using $^{40}$Ca$^{+}$. The experimentally determined quadrupole coefficient of the trap is $\approx 0.3$, which is very close to the design value. The relative frequency shift along the spectroscopy beam due to excess micromotion (EMM) is at the level of $3.5\times 10^{-18}$ for $^{40}$Ca$^{+}$. Applications of this trap encompass single-ion-based optical frequency standards, tests of fundamental physics, the study of mesoscopic Coulomb clusters, and the controlled interaction of a single ion with co-trapped atoms.

Figures (10)

• Figure 1: (a) A $2D$ schematic diagram of the trap electrodes showing the relevant design parameters. (b) Central plane view of the designed trap. (c) (Top) M2 tap holes on one face, (Bottom) Slit on the perpendicular face to align the inner electrodes.
• Figure 2: (a) (Top) AFM image of a selected region of the first inner electrode. (Bottom) Surface profile along the line shown in the top image. (b) (Top) AFM image of a selected region of the second inner electrode. (Bottom) Surface profile along the line shown in the top image. (c) SEM image showing dimensions of the inner electrode and the distance between them in the aligned assembly. (d) SEM image showing the alignment of the outer electrode with respect to the inner electrode in one direction of the trap assembly.
• Figure 3: (a) Complete vacuum setup. (b) A close view of the trap assembly with connecting wires mounted inside the chamber.
• Figure 4: (a) Schematic diagram of the helical resonator. Parameters of the resonator are: d $= 53$ mm, D $= 90$ mm, d$_{0} = 6$ mm, b $= 80$ mm, $\tau = 10$ mm, d$_{a} = 31$ mm, $\tau_{a} = 5$ mm, d$_{aa} = 2$ mm. The number of turns in the main coil and the antenna coils are 8 and 2, respectively. (b) Voltage measured across the probe at different drive frequencies.
• Figure 5: Schematic diagram of the fluorescence imaging system. The 4-lens objective is housed in a home-built brass tube with brass spacers. The objective guides the fluorescence into a light-tight box, where a 70:30 beam splitter transmits 30$\%$ of the fluorescence onto an EMCCD camera and reflects 70$\%$ of the fluorescence onto a PMT. A spatial filter before the PMT ensures a high signal-to-noise ratio (SNR) for photon counting.