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Progress of Theoretical and Experimental Physics

Simulation study of ultrahigh-precision single-ion extraction from a linear Paul trap

Kento Muroo, H. Okamoto, Nobumasa Miyawaki, Yosuke Yuri

Published Jun 1, 2023• 6 citations
Publisher

Abstract

Abstract A detailed simulation study is conducted to explore the possibility of transporting a single ion precisely along a particular design orbit. As a practically useful example, we consider nitrogen ions sympathetically cooled with ultracold calcium ions produced in a linear Paul trap (LPT) by the Doppler laser-cooling technique. A single nitrogen almost completely frozen in the laboratory frame is extracted from the LPT by manipulating the axial potential barriers. A similar idea was recently implemented by Groot-Berning et al. [New J. Phys. 23, 063067 (2021)] to create nitrogen-vacancy centers in diamond as a platform for some important quantum technologies. We here perform systematic 3D simulations taking a standard LPT structure into account and figure out the necessary conditions to achieve the ultrahigh-precision irradiation of a single nitrogen onto any target. In addition to the string Coulomb crystal used in the previous work above, we show that even the shell crystalline structure can be employed for our purpose. When several specific conditions are satisfied, a normalized root-mean-squared emittance of ejected nitrogen ions can be made of the order of 10−16 m·rad in transverse dimensions perpendicular to the design orbit.

DOI
10.1093/ptep/ptad071

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Abstract A detailed simulation study is conducted to explore the possibility of transporting a single ion precisely along a particular design orbit. As a practically useful example, we consider nitrogen ions sympathetically cooled with ultracold calcium ions produced in a linear Paul trap (LPT) by the Doppler laser-cooling technique. A single nitrogen almost completely frozen in the laboratory frame is extracted from the LPT by manipulating the axial potential barriers. A similar idea was recently implemented by Groot-Berning et al. [New J. Phys. 23, 063067 (2021)] to create nitrogen-vacancy centers in diamond as a platform for some important quantum technologies. We here perform systematic 3D simulations taking a standard LPT structure into account and figure out the necessary conditions to achieve the ultrahigh-precision irradiation of a single nitrogen onto any target. In addition to the string Coulomb crystal used in the previous work above, we show that even the shell crystalline structure can be employed for our purpose. When several specific conditions are satisfied, a normalized root-mean-squared emittance of ejected nitrogen ions can be made of the order of 10−16 m·rad in transverse dimensions perpendicular to the design orbit.
Deterministic ion implantation and color-center creation methods at the nanometer scale provide a route towards scalable quantum information processing. For this purpose, we have developed an inherently deterministic single-ion implantation method based on a linear Paul trap (LPT) as a single-ion source since the laser-cooling technique can be applied to the trap system to obtain ultralow-emittance ions and manipulate single ions one by one . Doubly charged nitrogen ions (N2+) produced in a sputter ion source is introduced into the LPT and sympathetically cooled with laser-cooled calcium ions (Ca+). Both the sputter ion source and the LPT chamber are elevated to +50kV for the 100keV-implantation of doubly charged single ions into a grounded diamond crystal.
Ion implantation is an accelerator-based technology used to create defects and introducing impurities into solid materials. A research and development study is currently underway toward ultrahigh-precision single-ion implantation using laser-cooling techniques at Takasaki Institute, National Institutes for Quantum Science and Technology. To achieve this, we incorporate a linear Paul trap as an ultracold single-ion source, where trapped ions can be made “Coulomb-crystallized” through Doppler laser cooling. In our scheme, nitrogen or silicon ions, useful for ion implantation to create color centers in materials, are sympathetically cooled through Coulomb collisions by co-trapping them with laser-cooled calcium ions to form a two-component crystal. These cold ions are then selectively extracted from the trap to be accelerated and focused through a 50-kV electrostatic bipotential-lens system. Our goal is to achieve beam focusing for ultrahigh-precision implantation on the order of 10 nm. The implanter system has already been assembled, and the commissioning is currently ongoing to enable single-ion extraction using a Coulomb crystal. In these proceedings, we describe the scheme for selective ion extraction and nanobeam focusing based on multiparticle simulations and briefly report recent progress in system implementation.
Nitrogen-vacancy (NV) centers in diamond are gaining attention as room-temperature quantum bits, with potential applications in quantum sensing, quantum information processing, and quantum communication. Precise positioning of NV centers is crucial for creating efficient single-photon sources using optical resonators and quantum memories for quantum interface, etc. Generally, broad ion beam implantation through nanohole fabricated by electron beam lithography has been widely used for precise creation of NV centers. There are several reports of deterministic NV creation by using pulsed fs- or ps-laser irradiations. In addition, focusing ion beam implantation provides a route towards scalable quantum information processing [3-6]. In this study, we are developing an inherently deterministic single-ion implantation method based on a linear Paul trap (LPT) as a single-ion source, since the laser-cooling technique can be applied to the trap system to obtain ultralow-emittance ions and manipulate single ions one by one. The QST system uses doubly charged ions cooled with calcium ions, allowing 100 keV ion implantation in diamond.
A basic performance test of a novel ion source is carried out to attain an extremely narrow beam for pinpoint irradiation of single ions onto an intended target. In principle, the normalized root-mean-squared emittance of the beam can reach a sub-femtometer level, which makes a transverse spot size of nanometer order available. The system is based on a compact linear Paul trap equipped with a Doppler-cooling device. The trap is the most popular, economical type consisting simply of a few metal plates and rods. Among several possible choices of ion species, we picked Ca 40 + for the present investigation. A small number of Ca 40 + ions are first laser cooled to near absolute zero where they form a spatially ordered configuration called a . The axial-potential well provided by dc bias voltages on two planar electrodes is then deformed gradually to eject the ions from the trap along the design orbit. We demonstrate that it is possible to launch the ultracold ions one by one at arbitrary timing with no serious radio-frequency heating. These ions are accelerated to 1 keV immediately after the extraction. The effectiveness of a few different ion-ejection schemes is experimentally verified and supported by three-dimensional molecular dynamics simulations.
Ion beams with micrometer-order spot size, so-called “microbeams,” have been utilized in a variety of fields. These include the biological response of radiation in cells and the single event effects of a semiconductor, etc. On the other hand, certain applications, such as the creation of a color center in a diamond, require an ion beam with not only the higher precision of a nanometer-order diameter but also the ability to implant ions one by one. The ion machine gun (IMG) is the possible choice for the realization of this distinctive ion beam. It is mainly composed of a linear Paul trap (LPT) and Doppler-cooling lasers. Ions laser-cooled to nearly absolute zero in the LPT reach a “Coulomb crystalline state.” If we extract ions in this state without heating, a unique ion beam can be attained. Most ion species, even if they cannot be directly laser-cooled, can be cooled sympathetically and crystallized through Coulomb collisions with laser-cooled ions simultaneously confined in the LPT. At Hiroshima University, we conduct proof-of-principle experiments of the IMG using laser-cooled calcium ions and sympathetically cooled nitrogen ions. Here we discuss the fundamental principle and experimental results of this study.
Ion implantation is an accelerator technology essential for creating defects or introducing impurities into materials. A research and development study is currently underway at QST Takasaki Institute toward ultrahigh-precision single-ion implantation based on laser-cooling techniques. To achieve this, we incorporate a linear Paul trap as an ultracold single-ion source, where trapped ions can be cooled to the order of mK or even “Coulomb-crystallized” by Doppler laser cooling. In our scheme, N or Si ions, useful for ion implantation to create color centers, are sympathetically cooled down to the mK range through Coulomb collisions by co-trapping them with laser-cooled Ca ions. Then, the ions are extracted selectively from the trap to be accelerated and focused through a 50-kV electrostatic bipotential lens system. We aim to focus the ions on the order of 10 nm for ultrahigh-precision implantation. The implantation system has already been assembled, and the commissioning is currently underway to enable ion extraction using a Coulomb crystal and focusing extracted cold ions. We present the status of system development and outline the scheme for selective ion extraction and nanobeam focusing based on multiparticle simulations.