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arXiv:2601.07328 · AIP Advances

Endcap-type Paul trap for precision spectroscopy and studies of ion-crystal dynamics

Anand Prakash, Akhil Ayyadevara, E. Krishnakumar, M. Ibrahim, K. M. Yatheendran, Subhadeep De, Sayan Patra, S. A. Rangwala

Published Jul 1, 2026• 0 citations
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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 of ≈22× performs close to the diffraction limit and resolves multi-ion clusters. Controlled ion loading and characterization of the trap are performed using 40Ca+. The experimentally determined quadrupole coefficient of the trap is ≈0.3, which is very close to the design value. The relative frequency shift along the spectroscopy beam due to excess micromotion is at the level of 3.5 × 10−18 for 40Ca+. 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.

DOI
10.1063/5.0334531

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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 of ≈22× performs close to the diffraction limit and resolves multi-ion clusters. Controlled ion loading and characterization of the trap are performed using 40Ca+. The experimentally determined quadrupole coefficient of the trap is ≈0.3, which is very close to the design value. The relative frequency shift along the spectroscopy beam due to excess micromotion is at the level of 3.5 × 10−18 for 40Ca+. 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.
We report on the design of a segmented linear Paul trap for optical clock applications using trapped ion Coulomb crystals. For an optical clock with an improved short-term stability and a fractional frequency uncertainty of 10−18, we propose 115In+ ions sympathetically cooled by 172Yb+. We discuss the systematic frequency shifts of such a frequency standard. In particular, we elaborate on high-precision calculations of the electric radiofrequency field of the ion trap using the finite element method. These calculations are used to find a scalable design with minimized excess micromotion of the ions at a level at which the corresponding second-order Doppler shift contributes less than 10−18 to the relative uncertainty of the frequency standard.
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We have fabricated a miniature three-segment linear Paul trap with r 0=0.6 mm and 2Z 0=4 mm. In order to perform quantum-state manipulation of trapped 40 Ca+ ions, we discuss the required harmonic oscillation frequencies which allow us to perform effective sideband cooling and have measured those that depend on the dimensions of the trap. A laser beam that has a component in the radial direction of the linear trap as well as that propagating along the trap axis has been used to detect the fluorescence spectra of the 42 S 1/2–42 P 1/2 transition. We have observed that the profile of the fluorescence spectrum obtained with the side laser beam is markedly different from that obtained with the axial laser beam due to micromotion. By applying additional DC voltages, the micromotion has been compensated, its amplitude has been estimated, and crystallization has been observed for two 40 Ca+ and four 40 Ca+ ions.
This thesis reports on the setup of a new ion trap apparatus designed for experiments with single ⁴⁰Ca⁺ ions to perform molecular spectroscopy. The calcium ion is laser cooled, allowing for sympathetic cooling of the nonfluorescing molecular ion. The aim of these experiments is to explore loading and identifying molecular ions in RF-Paul traps, as well as developing new spectroscopic tools to measure transitions of molecular ions via the fluorescence of co-trapped ⁴⁰Ca⁺ ions. Ground state cooling of a mixed ion pair is implemented as a first step towards increasing the sensitivity of our technique to the level necessary to measure transitions with low scattering rates (like those present in molecular ions). Doppler cooling on the S(1/2)->P(1/2) transition of the calcium ion results in the formation of a Coulomb crystal, the behavior of which may be used to infer properties of the molecular ion. Following cooling, sideband spectroscopy on the narrow S(1/2)->D(5/2) quadrupole transition of calcium may be used to identify the mass of single molecular ions. This method is verified via a non-destructive measurement on ⁴⁰CaH⁺ and ⁴⁰Ca¹⁶O⁺. The normal modes of the Coulomb crystal can also be used to extract information from the target ion to the control ion. By driving the blue side of a transition, laser induced heating can be put into the two ion system, which leads to changes in fluorescence of the ⁴⁰Ca⁺ ion, first demonstrated with two Ca⁺ isotopes. Increasing the sensitivity of this technique requires ground state cooling of both the ⁴⁰Ca⁺ ion and the ion of interest, enabling the transfer of the ion's motional state into the ground state with high probability. This thesis demonstrates ground state cooling of the atomic ion and sympathetic cooling of a second ion (⁴⁴Ca⁺). Once in the ground state, heating of the Coulomb crystal by scattering photons off of the spectroscopy ion can be measured by monitoring the resolved motional sidebands of the S(1/2)->D(5/2) transition of ⁴⁰Ca⁺, allowing for spectral lines to be inferred. Future experiments will investigate this technique with molecular ions.
We describe the design, fabrication, and operation of a novel surface-electrode Paul trap that produces a radio-frequency-null along the axis perpendicular to the trap surface. This arrangement enables control of the vertical trapping potential and consequentially the ion-electrode distance via dc-electrodes only. We demonstrate the confinement of single 40Ca+ ions at heights between 50 μm and 300 μm above planar copper-coated aluminum electrodes. Laser-cooling and coherent operations are performed on both the planar and vertical motional modes. This architecture provides a platform for precision electric-field noise detection and trapping of vertical ion strings without excess micromotion and may have applications for scalable quantum computers with surface ion traps.
We describe a configuration for a linear Paul rf ion trap. This trap can store a long string of ions with a small second-order Doppler shift, comparable to that achieved with a single ion in a quadrupole Paul trap. Crystallized strings of trapped ions, as well as more complicated structures, have been observed in the trap. We report an observation of the 40.5-GHz ground-state hyperfine interval of 199Hg+^{199}\mathrm{Hg}^{+}199Hg+ by microwave-optical double-resonance spectroscopy and discuss prospects for a microwave frequency standard based on a trapped string of ions.