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arXiv:1610.00206 · Journal of Physics B Atomic Molecular and Optical Physics

Surface-electrode trap with an integrated permanent magnet for generating a magnetic-field gradient at trapped ions

Yuji Kawai, Kenji Shimizu, Atsushi Noguchi, Shinji Urabe, Utako Tanaka

Published Dec 22, 2016• 5 citations
arXivHTMLPDF

Abstract

Abstract We report on a surface-electrode trap with SmCo magnets arranged in a quadrupole configuration underneath the trap electrode. Because the distance between the magnets and the trapped ions can be as little as several hundred micrometers, a large magnetic field is produced without any heat management. The magnetic-field gradient was measured using the Zeeman splitting of a single trapped 40 Ca + ion at several positions, and a field gradient of 36 T m −1 was obtained. Such a field gradient is useful for the generation of a state-dependent force, which is important for quantum simulation and/or quantum gate operation using radio-frequency or microwave radiation.

DOI
10.1088/1361-6455/50/2/025501

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•Nov 1, 2016•0 citations
Abstract We report on a surface-electrode trap with SmCo magnets arranged in a quadrupole configuration underneath the trap electrode. Because the distance between the magnets and the trapped ions can be as little as several hundred micrometers, a large magnetic field is produced without any heat management. The magnetic-field gradient was measured using the Zeeman splitting of a single trapped 40 Ca + ion at several positions, and a field gradient of 36 T m −1 was obtained. Such a field gradient is useful for the generation of a state-dependent force, which is important for quantum simulation and/or quantum gate operation using radio-frequency or microwave radiation.
Abstract A major challenge for quantum computers is the scalable simultaneous execution of quantum gates. One approach to address this in trapped ion quantum computers is the implementation of quantum gates based on static magnetic field gradients and global microwave fields. In this paper, we present the fabrication of surface ion traps with integrated copper current carrying wires embedded inside the substrate below the ion trap electrodes, capable of generating high magnetic field gradients. The copper layer’s measured sheet resistance of 1.12 mΩ/sq at room temperature is sufficiently low to incorporate complex designs, without excessive power dissipation at high currents causing a thermal runaway. At a temperature of 40 K the sheet resistance drops to 20.9 μΩ/sq giving a lower limit for the residual resistance ratio of 100. Continuous currents of 13 A can be applied, resulting in a simulated magnetic field gradient of 144 T m−1 at the ion position, which is 125 μm from the trap surface for the particular anti-parallel wire pair in our design.
For quantum computers to become a reality, high-fidelity logic gates are required to be implemented on a scalable architecture. The IQT group follows an approach based on using a magnetic field gradient and microwave radiation to implement the required operations. To demonstrate their scalability to many quantum bit systems, these operations need to be implemented on micro-fabricated surface ion traps. In this thesis I describe the technologies I developed for surface ion traps with magnetic field gradients. This entailed the design, assembly and operation of two surface ion trap systems with permanent magnets for generating large magnetic field gradients of 140T=m, novel in-vacuum radio-frequency and microwave emitters for implementing high-speed quantum state manipulation and development of robust and reliable laser control systems required for stable long-term experimental operation.
In modern ion quantum technology the fields of quantum information and microfabrication are being merged to build devices in which ions are confined near trap electrodes at the micrometer scale -with novel functionality where ion qubits enabling quantum devices. A long term goal in this field is to build a trapped-ion based quantum computer. Today’s quest, however, requires a progressive ion trap development in which materials selection, trap’s design and fabrication methods play a fundamental role. Here, we report on two fabrication methods for planar traps. We present the characterization of a planar trap with integrated electromagnets which is designed to coherently manipulate 40Ca+ ions’ internal states via magnetic field gradients and long-wavelength radiation. By implementing radiofrequency spectroscopy, in the presence of static magnetic fields, and by moving a single 40Ca+ ion about 600 µm along the trap axis we measured the total static magnetic field which yields a gradient of 1.11 +/- 0.18 T/m*A in this direction. We anticipate in this trap magnetic gradients of 10 T/m*A perpendicular to the trap axis. A second generation of this trap, a novel hybrid planar trap, is built by combining the capabilities of thick film technology and laser-induced glass micromachining to structure independent current-carrying wires beneath an ion trap. Into the vacuum chamber, we also incorporate surface cleaning tools such as ion-beam bombardment to diminish possible adsorbed contaminants which may reflect in excessive ion heating rates. Based on numerical simulations, we predict that the wires underneath the ion trap, although far from trap center, will be capable of producing high magnetic field gradients of about 3 T/m*A. Already such robust fields will allow to investigate strong v_ss=30 kHz spin-spin interactions in 40Ca+ ion crystals in planar traps.
A major challenge for quantum computers is the scalable simultaneous execution of quantum gates. One approach to address this in trapped ion quantum computers is the implementation of quantum gates based on static magnetic field gradients and global microwave fields. In this paper, we present the fabrication of surface ion traps with integrated copper current carrying wires embedded inside the substrate below the ion trap electrodes, capable of generating high magnetic field gradients. The copper layer's measured sheet resistance of 1.12 mΩΩΩ/sq at room temperature is sufficiently low to incorporate complex designs, without excessive power dissipation at high currents causing a thermal runaway. At a temperature of 40 K the sheet resistance drops to 20.9 μΩμΩμΩ/sq giving a lower limit for the residual resistance ratio of 100. Continuous currents of 13 A can be applied, resulting in a simulated magnetic field gradient of 144 T/m at the ion position, which is 125 μμμm from the trap surface for the particular anti-parallel wire pair in our design.
A dense array of ions in microfabricated traps represents one possible way to scale up ion trap quantum computing. The ability to address individual ions is an important component of such a scheme. We demonstrate individual addressing of trapped ions in a microfabricated surface-electrode trap using a magnetic field gradient generated on-chip. A frequency splitting of 310(2) kHz for two ions separated by 5 μm is achieved. Selective single qubit operations are performed on one of two trapped ions with an average of 2.2(±1.0%) crosstalk. Coherence time is reduced by the magnetic field gradient, but the spin-echo technique effectively restores the coherence time.
This thesis describes experimental work towards the development of a trapped-ion quantum computer based on microchip ion traps and long-wavelength radiation, using magnetic field gradients. The relationship between experimental parameters and two-qubit gate fidelity is investigated for microchips with two different static magnetic field gradient generation methods. For current-carrying wires and under-chip permanent magnets, optimum ion heights of 110 μm and 200 μm are found respectively. Construction of an experiment capable of demonstrating high-fidelity gates is reported, including innovations for the use of microchip ion traps with permanent magnets. The development of a vacuum system for versatile microchip experiments is described, including new methods for impedance-matched RF delivery, in-vacuum filtering and liquid nitrogen microchip cooling. Protection of both the microchip surface from atomic flux and of ions from the charged imaging viewport are both investigated in detail. A new preparation framework for microchip ion traps before their use in experiments is developed. In order to remove unwanted deposited layers on the microchips, a process of multiple chemical treatments is used. In addition, these characterisation efforts lead to refinement of the microfabrication process for future microchips. The application of large currents to microchips is of fundamental importance to scalable trapped-ion quantum computing using static magnetic field gradients. As part of the characterisation process, currents of ≈ 10A are successfully applied to microfabricated current-carrying wires, demonstrating the viability of these structures for generation of local magnetic fields and gradients in a quantum computing device. The operation of a microchip ion trap experiment with under-chip permanent magnets for a high magnetic field gradient (≈ 140Tm−1) is described. The successful trapping of ytterbium-174 and -171 ions is reported, as well as their use to measure and optimise the ion trap parameters. The thesis concludes with consideration of the expected future results from the ongoing operation of the experiment.