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arXiv:1605.02143 · Physical Review Letters

Realization of Translational Symmetry in Trapped Cold Ion Rings

Haokun Li, Erik Urban, Crystal Noel, Alexander Chuang, Yang Xia, Anthony Ransford, Boerge Hemmerling, Yuan Wang, Tongcang Li, Hartmut Häffner, Xiang Zhang

Published Jan 31, 2017• 46 citations
arXivHTMLPDF

Abstract

We crystallize up to 15 ^{40}Ca^{+} ions in a ring with a microfabricated silicon surface Paul trap. Delocalization of the Doppler laser-cooled ions shows that the translational symmetry of the ion ring is preserved at millikelvin temperatures. By characterizing the collective motion of the ion crystals, we identify homogeneous electric fields as the dominant symmetry-breaking mechanism at this energy scale. With increasing ion numbers, such detrimental effects are reduced. We predict that, with only a ten-ion ring, uncompensated homogeneous fields will not break the translational symmetry of the rotational ground state. This experiment opens a door towards studying quantum many-body physics with translational symmetry at the single-particle level.

DOI
10.1103/physrevlett.118.053001

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We crystallize up to 15 ^{40}Ca^{+} ions in a ring with a microfabricated silicon surface Paul trap. Delocalization of the Doppler laser-cooled ions shows that the translational symmetry of the ion ring is preserved at millikelvin temperatures. By characterizing the collective motion of the ion crystals, we identify homogeneous electric fields as the dominant symmetry-breaking mechanism at this energy scale. With increasing ion numbers, such detrimental effects are reduced. We predict that, with only a ten-ion ring, uncompensated homogeneous fields will not break the translational symmetry of the rotational ground state. This experiment opens a door towards studying quantum many-body physics with translational symmetry at the single-particle level.
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Laser-cooled trapped ions platform is one of the best candidates for the development of future quantum computing. This has generated a worldwide research effort for scaling and integrating trapping devices. As part of this effort, we are developping cylindrically symmetrical linear Paul traps manufactured in collaboration with Nanyang Technology University and the Microelectronics Institute of Singapore. The electrical contacts of these traps electrodes are made through the Silicon substrate (TSV: through silicon vias), allowing not to break the desired symmetry with surface connections. The trapping and laser cooling of ions in ring traps was demonstrated at Garching [2] and subsequently extended to microfabricated traps at Sandia National Labs [3] and Berkeley [4]. In the absence of defects, these devices enable rotational symmetry for the trapped ions. They are also candidates for manipulating the rotational quantum state of trapped ion ensembles [4].We will present our results concerning the performance of TSV surface ring traps manufactured in Singapore. In particular, we loaded these traps with Doppler-cooled 88Sr+ ions and we estimate the defects of the trapping potential with respect to perfect rotational symmetry. The application of DC voltages to a set of segmented electrodes makes it possible to compensate for most of these defects for observing sub milikelvin free motion.[1] P. Zhao et al., Appl. Phys. Lett. 118, 124003 (2021).[2] I. Waki et al.,Phys. Rev. Lett. 68, 2007 (1992).[3] B. Tabakov et al., Phys. Rev. Applied 4, 031001 (2015). [4] E. Urban et al., Phys. Rev. Lett. 123, 133202 (2019)
A laser-cooling experiment with Ca+{\mathrm{Ca}}^{+}Ca+ ions trapped in a linear rf octupole ion trap is presented. The phase transition of the laser-cooled Ca+{\mathrm{Ca}}^{+}Ca+ ions from the cloud to the crystal state is observed by an abrupt dip of the laser-induced fluorescence spectrum and indicates that mK temperatures are obtained. We have also performed molecular dynamics simulations under various conditions to confirm this property by deducing axially symmetric structures of Coulomb crystals and by evaluating the translational temperatures of the laser-cooled ions. The simulation results show that for small numbers of ions novel ring-shaped crystals are produced. As the number of ions is increased, cylindrical layers in the ring crystal are sequentially formed. For more than 100 ions, also hexagonal and spiral structures emerge in parts of the large-size ion crystal, which has a length on the order of millimeters for the present geometrical arrangement and voltages. An advantage of the linear rf octupole trap is its large almost-field-free region in the middle of the trap, where the micromotion amplitude is small for trapped ions. These results demonstrate that such a multipole trap has attractive features for quantum computing and ultracold ion-atom collision studies.
Abstract Rings of trapped ions are an attractive system to study quantum-many body dynamics with closed boundary conditions as well as symmetry breaking. One of the biggest challenges towards such experiments is to sufficiently suppress rotational imperfections of the trapping potential and to allow for instance freely rotating ring structures. We show how to overcome this challenge with a surface trap design and perform numerical calculations to analyze the consequences of various imperfections in detail. We conclude that trap electrode imperfections, external stray electric fields, and local charging of the trap electrodes can be controlled sufficiently well to allow ion rings to rotate freely even near their rotational ground state.
Working with MgH+molecular ions in a linear Paul trap, we routinely cool their translational degrees of freedom by sympathetic cooling with Doppler laser cooled Mg+ions. Giving the time for the molecules to equilibrate internally to room temperature blackbody radiation, the vibrational degree of freedom is frozen out, leaving only the rotational degree of freedom to be cooled. We have implemented a new technique for laser-induced rotational ground-state cooling of vibrationally and translationally cold MgH+ions, using a scheme based on excitation of a single rovibrational transition. This technique is generalizable to any diatomic polar molecular ion, given an appropriate mid-infrared laser source such as a quantum cascade laser.
We demonstrate the Doppler laser cooling of 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ ions confined in a segmented linear Paul trap in the presence of a strong quadrupolar magnetic field generated by two permanent ring magnets. Magnetic field gradients of 800 to 1600 G/mm give rise to a highly position-dependent Zeeman shift on the energy levels of the trapped ions. Efficient laser cooling is demonstrated using two 397 nm cooling laser beams with appropriate wavelengths and polarizations and one 866 nm repumper laser beam. Coulomb crystals of ions are found to exhibit similar secular temperatures to those trapped in absence of the magnetic field. In addition, the position dependency of the Zeeman effect is used to generate a map of the field strength. This work forms the basis for developing hybrid trapping experiments for cold ions and neutral molecules that consist of an ion and a magnetic trap to study cold interactions between these species, and opens up new possibilities for quantum-science experiments that employ trapped ions in inhomogeneous magnetic fields.