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Implementation of a Rotationally Symmetric Ring Ion Trap and Coherent Control of Rotational States

Erik Urban

Published Jan 1, 2019• 1 citation
OpenAlex

Abstract

In this thesis, I present the design, implementation, and characterization of a novel, rotationally symmetric ion trap. Our compact design creates a toroidal trapping potential for charged atoms that has a radius that is much smaller than the height at which the ions are positioned above the surface of the trap. We trap ions at a height of 385~$\\mu$m and a radius of 45~$\\mu$m as expected from simulation. We demonstrate that this ring is rotationally symmetric with imperfections in the potential of much less than 3~mK. With two ions contained in a point Paul trap, we demonstrate the creation of a 100~kHz rotating ring using a rotating quadrupole. By releasing the quadrupole potential, we demonstrate the ability to create superpositions of rotational states using the modulation of the laser light produced by the classical rotation of the crystal in a symmetric potential. Superpositions of rotational states differing by up to four quanta are observed. With these rotational superpositions, we describe an experiment capable of observing the identical nature of two ions in a Coulomb crystal. By creating a rotational interferometer, one can coherently exchange two particles and observe their interference upon recombination. We describe what this interference signature would be and how particle indistinguishability would affect it. We present experimental efforts towards realizing this exchange and reflect on our unexpectedly short rotational superposition coherence times.

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In this thesis, I present the design, implementation, and characterization of a novel, rotationally symmetric ion trap. Our compact design creates a toroidal trapping potential for charged atoms that has a radius that is much smaller than the height at which the ions are positioned above the surface of the trap. We trap ions at a height of 385~mu\\mumum and a radius of 45~mu\\mumum as expected from simulation. We demonstrate that this ring is rotationally symmetric with imperfections in the potential of much less than 3mK. With two ions contained in a point Paul trap, we demonstrate the creation of a 100kHz rotating ring using a rotating quadrupole. By releasing the quadrupole potential, we demonstrate the ability to create superpositions of rotational states using the modulation of the laser light produced by the classical rotation of the crystal in a symmetric potential. Superpositions of rotational states differing by up to four quanta are observed. With these rotational superpositions, we describe an experiment capable of observing the identical nature of two ions in a Coulomb crystal. By creating a rotational interferometer, one can coherently exchange two particles and observe their interference upon recombination. We describe what this interference signature would be and how particle indistinguishability would affect it. We present experimental efforts towards realizing this exchange and reflect on our unexpectedly short rotational superposition coherence times.
We demonstrate the preparation and coherent control of the angular momentum state of a two-ion crystal. The ions are prepared with an average angular momentum of 7850ℏ freely rotating at 100 kHz in a circularly symmetric potential, allowing us to address rotational sidebands. By coherently exciting these motional sidebands, we create superpositions of states separated by up to four angular momentum quanta. Ramsey experiments show the expected dephasing of the superposition which is dependent on the number of quanta separating the states. These results demonstrate coherent control of a collective motional state described as a quantum rotor in trapped ions. Moreover, our Letter offers an expansion of the utility of trapped ions for quantum simulation, interferometry, and sensing.
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.
Laser-cooled trapped ions platform is one of the best candidates for the development of future quantum computing. This has generated a major worldwide research effort aimed at scaling and integrating trapping devices. As part of this effort, we are developing miniature atomic ion traps in the laboratory: Paul linear surface traps manufactured in collaboration with Nanyang Technology University and the Microelectronics Institute of Singapore. An original feature of the manufacturing process is that all the electrical contacts of the trap electrodes are made through the Silicon substrate (TSV: through silicon vias). In this way, the wire-bondings usually soldered directly to the electrodes can be offset or completely eliminated [1]. This opens the way to complex architectures, in particular cylindrically symmetrical ring traps, which cannot be made with surface connections that would break the desired symmetry. The trapping and laser cooling of ions in cylindrically symmetrical linear Paul traps (called "ion storage rings" in their macroscopic version, diameters of around 100 mm) was demonstrated in pioneering work at Garching [2] and subsequently extended to microfabricated traps (multilayer technology) at Sandia National Labs (diameter 2.5 mm) [3] and Berkeley (diameter 95 μm) [4]. These devices are of interest because, in the absence of defects, they enable rotational symmetry for the trapped ions and thus periodic boundary conditions for the confined cold ion system (Coulomb crystal). They are also candidates for observing and manipulating the rotational quantum state of trapped ion ensembles [4]. We will present our latest results concerning the performance of TSV surface ring traps (diameters between 150 and 210 μm) manufactured in Singapore. In particular, we loaded these traps with Dopplercooled 88Sr+ ions (from a single ion to several hundreds). Laser cooling and image acquisition enable us to estimate the defects of the trapping potential with respect to perfect rotational symmetry. The application of DC voltages to a set of segmented electrodes then makes it possible to compensate for most of these defects, which nevertheless remain the major problem to be solved in order to achieve sub milikelvin free motion.
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)
We report on development of a miniature linear rf ion trap and preliminary efforts to trap and laser-cool {sup 24}Mg{sup +} ions. This trap has a radial dimension of 50 {mu}m, one tenth the size of our previous trap. The rms confinement of a trapped ion scales as R{sup -2}, where R is the trap radius, so tight confinement of ions is possible at a modest rf voltage. Although the three-dimensional trapping potential for our trap cannot be calculated analytically, it can be described in terms of dimensionless parameters, measured by studying macroscopic charged particles in a much larger model of the trap. We expect that with this trap it will be possible to achieve secular frequencies high enough for resolved-sideband cooling of light ions using allowed dipole transitions. This will make it much easier to perform experiments on the quantum motion of trapped ions. This trap uses a two-wire design rather than the more common four-wire one, so the trap electrodes occlude a much smaller solid angle than either a four-wire linear trap or the more common quadrupole trap. Thus, it will be possible to couple optical devices, such as high-finesse cavities, much more efficiently to trapped ions.
We have developed a trapped ion system for producing two-dimensional (2D) ion crystals for applications in scalable quantum computing, quantum simulations, and 2D crystal phase transition and defect studies. The trap is a modification of a Paul trap with its ring electrode flattened and split into eight identical sectors and its two endcap electrodes shaped as truncated hollow cones for laser and imaging optics access. All ten trap electrodes can be independently DC-biased to create various aspect ratio trap geometries. We trap and Doppler cool 2D crystals of up to 30 Ba+ ions and demonstrate the tunability of the trapping potential both in the plane of the crystal and in the transverse direction.