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arXiv:1609.06973 · Physical Review A

Rapid crystallization of externally produced ions in a Penning trap

T. Murböck, S. Schmidt, G. Birkl, W. Nörtershäuser, Richard Claude Thompson, Manuel Vogel

Published Oct 11, 2016• 23 citations
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Abstract

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.

DOI
10.1103/physreva.94.043410

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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 \ensuremathμeV\ensuremath{\mu}\mathrm{eV}\ensuremathμeV and below within seconds. With ion numbers of the order of 1\ifmmode×\else\texttimes\fi1031\ifmmode\times\else\texttimes\fi{}{10}^{3}1\ifmmode×\else\texttimes\fi103 to 1\ifmmode×\else\texttimes\fi1051\ifmmode\times\else\texttimes\fi{}{10}^{5}1\ifmmode×\else\texttimes\fi105, 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.
In this work, laser-cooled ion crystals containing 1000 to 100000 singly charged magnesium ions (Mg+) were prepared in a Penning trap. The properties of the ion crystals and their structure displaying long-range ordering were analyzed by various non-destructive techniques. After creation of the Mg + ions in the form of ion bunches in an external source, the ions were injected into the Penning trap where their temperature was reduced by eight orders of magnitude within seconds using a combination of buffer gas cooling and Doppler laser cooling. The achieved temperatures in the millikelvin-regime were close to the theoretical Doppler-cooling limit and sufficiently low to induce the transition to a crystal phase exhibiting long-range ordering. The structure of these mesoscopic ion crystals is in agreement with a model describing the crystal as a set of planar shells. This allows for a derivation of properties such as the charge density or the temperature of the observed crystals. For the process of combined buffer-gas and Doppler laser cooling an analytical model has been developed, which explains the time development of the temperature and the fluorescence signal in agreement with the experimental results.
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 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.
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.
We report on the results on various aspects of the formation of ion Coulomb crystals in linear Paul traps, for plasmas containing from a few hundred to several thousand laser-cooled 24Mg+{}^{24}{\mathrm{Mg}}^{+}24Mg+ ions. The presented studies include observations of changes in emitted fluorescence, changes in plasma aspect ratio, and the formation of shell structure, during the transition from the cloudlike to the crystalline state. The competition between rf heating and laser cooling is observed to result in a very dramatic transition from the cloudlike disordered state to a liquidlike state. After the liquid state is reached ordering into shell structure takes place as a continuous process at even higher plasma coupling parameters. This is in contrast to earlier observations of few-ion plasmas in hyperbolic rf traps.
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.