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arXiv:2312.05106 · Physical Review A

Sympathetic cooling of trapped Th3+ alpha-recoil ions for laser spectroscopy

G. Zitzer, Johannes Tiedau, Maksim V. Okhapkin, K. Zhang, C. Mokry, J. Runke, Ch. E. Düllmann, E. Peik

Published Mar 22, 2024• 12 citations
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Abstract

The authors experimentally demonstrate the sympathetic cooling of Th${}^{3+}$ ions, captured from uranium alpha-recoil sources, in a linear Paul trap using laser-cooled ${}^{88}$Sr${}^{+}$ ions. They show the efficacy of their method via high-resolution laser spectroscopy. This demonstration is an important step in enabling the use of these ions as the reference of a highly accurate nuclear optical clock.

DOI
10.1103/physreva.109.033116

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The authors experimentally demonstrate the sympathetic cooling of Th3+{}^{3+}3+ ions, captured from uranium alpha-recoil sources, in a linear Paul trap using laser-cooled 88{}^{88}88Sr+{}^{+}+ ions. They show the efficacy of their method via high-resolution laser spectroscopy. This demonstration is an important step in enabling the use of these ions as the reference of a highly accurate nuclear optical clock.
We demonstrate a modified integration of a linear ion trap with a time-of-flight mass spectrometer optimized to enhance the loading and detection of Th3+ ions. A phase-locked radiofrequency/high-voltage switch incorporating zero-crossing triggering and a programmable time delay is a key upgrade to minimize RF phase-dependent jitter and enable the unambiguous identification of Th3+ ions. To enhance purity, yield, and lifetime of trapped Th3+ ions, we optimize the ion loading parameters, including ion trap settings (RF amplitude, endcap voltages, and loading time), laser-ablation pulse energy, helium partial pressure, and ion storage time. These improvements extend trapping lifetimes of Th3+ ions to several hundred seconds, representing an order-of-magnitude increase compared to our previous work. Finally, single-pulse ablation of mixed Sr–Th nitrate targets enables the direct co-loading of Th3+ and 88Sr+ ions, and subsequent laser cooling forms multi-component Coulomb crystals for laser spectroscopy of Th3+ ions. Given the long trapping lifetime of Th3+ ions, this system will serve as a platform for developing a prototype trap-based thorium nuclear clock.
This paper reports on trapping and laser-cooling of singly-ionized strontium ions in a linear Paul trap. We demonstrate loading of large ion clouds containing as much as 106 ions and laser cooling down to the Coulomb crystal transition. We observe the spatial segregation of the different Sr+ isotopes due to the mass-dependent Paul trap stiffness. Sympathetic cooling of the different isotopes is demonstrated, either by laser-cooling of 88Sr+ (83% abundance) or of 86Sr+ (10% abundance). These demonstrations open the way to the use of a large ion Coulomb crystal for quantum optics and quantum information experiments, where the strong confinement, the long lifetime, and the absence of perturbation by cooling lasers are crucial.
Optical clocks have garnered widespread attention due to their unparalleled precision in the fields of time-frequency standards, geodetic measurements, and fundamental physics research. Among recent advances, highly charged ion (HCI)-based optical clocks have attracted significant scientific interest owing to their remarkable insensitivity to electromagnetic perturbations and enhanced sensitivity to potential variations of the fine-structure constant $(\ensuremath{\alpha})$. Although the recent successful demonstration of an Ar13+{\mathrm{Ar}}^{13+}Ar13+ optical clock has validated the feasibility of HCI-based systems, Ni12+{\mathrm{Ni}}^{12+}Ni12+ (featuring an ultranarrow clock transition linewidth) stands out as a superior candidate for achieving HCI optical clocks with ${10}^{\ensuremath{-}19}$ level uncertainty and stability. In this work, we report the Coulomb crystallization of highly charged nickel ions (Ni-HCIs). Through a precise deceleration and sympathetic cooling protocol in a room-temperature linear Paul trap, high-energy Ni-HCI bunches were sympathetically cooled from megakelvin energies down to the hundred-millikelvin range using laser-cooled Be+{\mathrm{Be}}^{+}Be+ ions. This work represents a pivotal step toward the realization of an optical clock based on Ni12+{\mathrm{Ni}}^{12+}Ni12+ ion.
The 27 Al + ion optical clock is one of the most attractive optical clocks due to its own advantages such as low black-body radiation shift at room temperature and insensitivity to the magnetic drift. However, it cannot be laser-cooled directly in the absence of 167nm laser to date. This problem can be solved by sympathetic cooling. In this work, a linear Paul trap is used to trap both 40 Ca + and 27 Al + ions simultaneously, and a single Doppler-cooled 40 Ca + ion is employed to sympathetically cool a single 27 Al + ion. Thus a 'bright-dark' two-ion crystal has been successfully synthesized. The temperature of the crystal has been estimated to be about 7 mK by measuring the ratio of carrier and sideband spectral intensities. Finally, the dark ion is proved to be an 27 Al + ion by precise measuring of the ion crystal's secular motion frequency, which means that it is a great step for our 27 Al + quantum logic clock.
Coulomb crystals, containing up to a few hundred ions of which more than 50% were cooled sympathetically by the Coulomb interaction with laser cooled Mg+ ions, have been produced in a linear Paul trap. By controlling the balance of the radiation pressure from the two cooling lasers, the Coulomb crystals could be segregated according to ion species. Previous studies of ion crystals and molecular dynamics simulations suggest that the temperature may be around 10 mK or lower. The obtained results indicate that a wide range of atomic and molecular ions, which due to their internal structures are not amenable to direct laser cooling, can be effectively cooled and localized (crystallized) in linear Paul traps. For high resolution spectroscopy of such ions this may turn out to be very useful.
Optical clocks have garnered widespread attention due to their unparalleled precision in time-frequency standards, geodetic measurements, and fundamental physics research. Among emerging developments, highly charged ion (HCI)-based optical clocks have attracted significant scientific interest owing to their exceptional resilience against electromagnetic perturbations and enhanced sensitivity to variations in the fine-structure constant (ααα). While the recent successful demonstration of an Ar13+^{13+}13+ optical clock has validated the feasibility of HCI-based systems, Ni12+^{12+}12+ -- featuring an ultranarrow clock transition linewidth -- stands out as a superior candidate for achieving HCI optical clocks with 10−1910^{-19}10−19 level uncertainty and stability. In this work, we report the Coulomb crystallization of nickel highly charged ions (Ni-HCIs). Through a precision deceleration and sympathetic cooling protocol in a room-temperature Paul trap, high-energy Ni-HCI bunches were sympathetically cooled from megakelvin to the 100-millikelvin range using laser-cooled Be+^{+}+ ions. This work represents a pivotal step toward the realization of an optical clock based on the Ni12+^{12+}12+ ion.