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Formation of two-ion crystals by injection from a Paul-trap source into a high-magnetic-field Penning trap

J. Berrocal, E. Altozano, Francisco Domínguez, M. J. Gutiérrez, Fernández Baldomero, Francisco J., Daniel Rodríguez

Published May 16, 2022• 7 citations
Publisher

Abstract

Two-ion crystals constitute a platform for investigations of quantum nature that can be extended to any ion \nspecies or charged particle provided one of the ions in the crystal can be directly laser cooled and manipulated \nwith laser radiation. This paper presents the formation of two-ion crystals for quantum metrology in a 7-tesla \nopen-ring Penning trap. 40Ca+ ions are produced either internally by photoionization or externally in a (Paul-trap) \nsource, transported through the strong magnetic field gradient of the superconducting solenoid, and captured \nin-flight with a mean kinetic energy of a few electronvolts with respect to the minimum of the Penning-trap \npotential well. Laser cooling of the two-ion crystal in a strong magnetic field towards reaching the quantum \nregime is also presented, with particular emphasis on the cooling of the radial modes.

DOI
10.1103/physreva.105.052603

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Two-ion crystals constitute a platform for investigations of quantum nature that can be extended to any ion \nspecies or charged particle provided one of the ions in the crystal can be directly laser cooled and manipulated \nwith laser radiation. This paper presents the formation of two-ion crystals for quantum metrology in a 7-tesla \nopen-ring Penning trap. 40Ca+ ions are produced either internally by photoionization or externally in a (Paul-trap) \nsource, transported through the strong magnetic field gradient of the superconducting solenoid, and captured \nin-flight with a mean kinetic energy of a few electronvolts with respect to the minimum of the Penning-trap \npotential well. Laser cooling of the two-ion crystal in a strong magnetic field towards reaching the quantum \nregime is also presented, with particular emphasis on the cooling of the radial modes.
Two-ion crystals constitute a platform for investigations of quantum nature that can be extended to any ion species or charged particle provided one of the ions in the crystal can be directly laser cooled and manipulated with laser radiation. This paper presents the formation of two-ion crystals for quantum metrology in a 7-tesla open-ring Penning trap. 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ ions are produced either internally by photoionization or externally in a (Paul-trap) source, transported through the strong magnetic field gradient of the superconducting solenoid, and captured in-flight with a mean kinetic energy of a few electronvolts with respect to the minimum of the Penning-trap potential well. Laser cooling of the two-ion crystal in a strong magnetic field towards reaching the quantum regime is also presented, with particular emphasis on the cooling of the radial modes.
We have studied the formation and properties of two-species ion Coulomb crystals in the Penning trap of the SpecTrap experiment. These crystals have been formed by injection of admixture ions from an external source into a previously confined and laser-cooled cloud of magnesium ions. This kind of study, performed over a range of the admixture ions’ charge-to-mass ratios, indicates the conditions for their sympathetic cooling and the formation of two-species ion crystals. This mechanism allows efficient cooling of the admixed species such as highly charged ions which do not feature suitable laser-cooling transitions, and thus make them accessible to high-resolution laser spectroscopy.
We propose to measure the six motional frequencies of an unbalanced two-ion crystal in a Penning trap using a scheme that has been demonstrated in a quantum-metrology experiment with a single ion in a linear Paul trap. To study the feasibility, we derive the quantum Hamiltonian for the unbalanced crystal formed by a target and a sensor ion ( 40 Ca , and deduce characteristic coordinates , and , associated to the amplitudes of the displacement operator , and , respectively, when a dipolar time-varying field is applied. The final state after excitation is read out using rapid adiabatic passage. We also present the status of the 7 tesla open-ring Penning trap where first experiments on the 232 Th Ca crystal, cooled to the ground state, are foreseen in the short-term future. Finally, we underline possible experiments with this unique platform.
Results demonstrating laser cooling and observation of individual calcium ions in a Penning trap are presented. We show that we are able to trap, cool, image and manipulate the shape of very small ensembles of ions sufficiently well to produce two-ion 'Coulomb crystals' aligned along the magnetic field of a Penning trap. Images are presented which show the individual ions to be resolved in a two-ion crystal. A distinct change in the configuration of such a crystal is observed as the experimental parameters are changed. These structures could eventually be used as building blocks in a Penning trap based quantum computer.
We have studied the cooling dynamics, formation process, and geometric structure of mesoscopic crystals of externally produced magnesium ions in a Penning trap. We present a cooling model and measurements for a combination of buffer gas cooling and laser cooling which has been found to reduce the ion kinetic energy by eight orders of magnitude from several hundreds of eV to $\ensuremath{\mu}\mathrm{eV}$ and below within seconds. With ion numbers of the order of $1\ifmmode\times\else\texttimes\fi{}{10}^{3}$ to $1\ifmmode\times\else\texttimes\fi{}{10}^{5}$, such cooling leads to the formation of ion Coulomb crystals which display a characteristic shell structure in agreement with the theory of non-neutral plasmas. We show the production and characterization of two-species ion crystals as a means of sympathetic cooling of ions lacking a suitable laser-cooling transition.
We have performed laser cooling of Mg+{}^{+}+ ions confined in a Penning trap. The externally produced ions were captured in flight, stored, and laser cooled. Laser-induced fluorescence was observed perpendicular to the cooling laser axis. Optical detection down to the single-ion level together with the electronic detection of the ion oscillations inside the Penning trap have been used to acquire information on the ion storage time, ion number, and ion temperature. Evidence for the formation of ion crystals has been observed. These investigations are an important prerequisite for the sympathetic cooling of simultaneously stored highly charged ions and the precision laser spectroscopy of forbidden transitions in these.