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Towards high-fidelity microwave driven multi-qubit gates on microfabricated surface ion traps

Tomas Navickas

Published Sep 21, 2018• 0 citations
OpenAlex

Abstract

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.

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•May 1, 2019•2 citations
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.
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.
Microfabricated ion traps are an important tool in the development of scalable quantum systems. Tremendous advancements towards an ion quantum computer were made in the past decade and most requirements for a quantum computer have been fulfilled in individual experiments. Incorporating all essential capabilities in a fully scalable system will require the further advancement of established quantum information technologies and development of new trap fabrication techniques. In my thesis I will discuss the theoretical background and experimental setup required for the operation of ion traps. Measurement of the important ion trap heating rate was performed in the setup and I will discuss the results in more detail. I will give a review of microfabrication processes used for the fabrication of traps, outlining advantages, disadvantages and issues inherent to the processes. Following the review I will present my work on a concept for a scalable ion trap quantum system based on microwave quantum gates and shuttling through X-junctions. Many of the required building blocks, including ion trap structures with current-carrying wires intended to create strong magnetic field gradients for microwave gates were investigated further. A novel fabrication process was developed to combine current-carrying wires with advanced multilayered ion trap structures. Several trap designs intended for proof of principle experiments of high fidelity microwave gates, advanced detection techniques and shuttling between electrically disconnected ion traps will be presented. Also the electrode geometry of an optimized X-junction design with strongly suppressed rf barrier height will be presented. Further, I developed several modifications for the experimental setup to extend the existing capabilities. A plasma source capable of performing in-situ cleans of the trap electrode surfaces, which has been demonstrated to dramatically reduce the heating rate in ion traps, was incorporated. I will also present a vacuum system modification designed to cool ion traps with current-carrying wires and transport the generated heat out of the vacuum system. In addition a novel low-noise, high-speed, multichannel voltage control system was developed by me. The device can be used in future experiments to precisely shuttle ions from one trapping zone to another and also to shuttle ions through ion trap junctions. Lastly I will outline the process optimization and microfabrication of my ion trap designs. A novel fabrication process which makes use of the extremely high thermal conductivity of diamond substrates and combines it with thick copper tracks embedded in the substrate was developed. Large currents will be passed through the wires creating a strong and controllable magnetic field gradient. Ion trap designs with isolated electrodes connected via buried wires can be placed on top of the current-carrying wires, allowing the most advanced electrode designs to be fabricated with current-carrying wires.
The major challenges in trapped-ion quantum computation are to scale up few-ion experiments to many qubits and to improve control techniques so that quantum logic gates can be carried out with higher fidelities. This thesis reports experimental progress in both of these areas. In the early part of the thesis we describe the fabrication of a surface-electrode ion trap, the development of the apparatus and techniques required to operate it and the successful trapping of 40Ca+ ions. Notably we developed methods to control the orientation of the principal axes and to minimise ion micromotion. We propose a repumping scheme that simplifies heating rate measurements for ions with low-lying D levels, and use it to characterise the electric field noise in the trap. Surface-electrode traps are important because they offer a route to dense integration of electronic and optical control elements using existing microfabrication technology. We explore this scaling route by testing a series of three traps that were microfabricated at Sandia National Laboratories. Investigations of micromotion and charging of the surface by laser beams were carried out and improvements to future traps are suggested. Using one of these traps we also investigated anomalous electrical noise from the electrode surfaces and discovered that it can be reduced by cleaning with a pulsed laser. A factor of two decrease was observed; this represents the first in situ removal of this noise source, an important step towards higher gate fidelities. In the second half of the thesis we describe the design and construction of an experiment for the purpose of replacing laser-driven multi-qubit quantum logic gates with microwave-driven ones. We investigate magnetic-field-independent hyperfine qubits in 40Ca+ as suitable qubits for this scheme. We make a design study of how best to integrate an ion trap with the microwave conductors required to implement the gate and propose a novel integrated resonant structure. The trap was fabricated and ions were successfully loaded. Single-qubit experiments show that the microwave fields above the trap are in excellent agreement with software simulations. There are good prospects for demonstrating a multi-qubit gate in the near future. We conclude by discussing the possibilities for larger-scale quantum computation by combining microfabricated traps and microwave control.
This thesis describes the experimental work towards the development of a scalable quantum computer based on microfabricated ion trap using long-wavelength radiation and magnetic field gradient. There are three key elements in implementing such a quantum computer: the junction trap to shuttle ions, the structure to generate high gradient magnetic field and the structure to induce strong microwave coupling to the ions. A new dynamic simulation tool was developed addressing the problems faced by static solvers. This tool was used to aid the design and optimisation of an X junction geometry allowing the ions to be shuttled with minimised motional heating gain. The design and fabrication techniques were reported on the structure to generate a high gradient magnetic field. A discussion was given on the design of producing microwave and maximise the coupling to the cold ions. A review was given on the far-field methods and near-field methods. A novel design was reported where the single-qubit gate is predicted to be 45 times faster than a conventional setup. Two essential topics on the microfabrication of a reliable scalable quantum computer unit were discussed: breakdown and RF loss. Investigation using numerical simulations showed that dielectric can breakdown due to high voltage and local heating which are results of impedance mismatch. Microfabrication processes were improved, high-quality films were reported to have twice as much breakdown voltages. The mechanisms of RF loss were reviewed. Novel structures and smooth electroplating technique were developed to minimise the loss. A low loss ion trap was produced and tested. An experimentally observed anomalous glow discharge phenomenon was reported and investigated.
Trapped ions, together with superconducting qubits, are one of the two leading hardware platforms for scalable quantum information processing. The development of quantum computers represents a major technological breakthrough comparable to the introduction of classical computing. The benefits of this technology are currently limited by the technical capability to perform high fidelity entangling operations on the qubits. When gate fidelities surpass the fault-tolerance threshold it becomes possible, through error correction, to increase the system size to an arbitrary number of qubits. In this cumulative thesis we address some of the issues in the scalability of the trapped-ion quantum computer based on microwave near-fields. In this approach, gate operations on one or multiple ions are driven by an oscillating magnetic field generated by a current flowing through a conductor. In the first part of this work we discuss the design of traps toward the implementation of large scale systems. We introduce the basic design of a surface-electrode ion trap with embedded microwave conductors. The oscillating magnetic field required to perform the operations is generated by a single optimized conductor. We discuss the simulation and characterization of the magnetic field pattern, which is fixed by the microwave conductor design. In addition, we demonstrate the capability to simulate, fabricate and characterize a multilayer surface ion trap. Multilayer traps are a key aspect for scalability since they are necessary to achieve large system sizes, with many `ion registers', where the registers are interconnected by physically transporting ions between them. In the second part of this thesis we demonstrate the implementation of a two-qubit entangling gate and we explore the possibilities offered by quantum control methods to improve its fidelity. We perform an entangling gate on two 9Be+ ions and measure a Bell state fidelity of 98.2(1.2)%. Error characterization shows that the gate result is limited by technical issues connected to the ions' motional states. To reduce these errors we apply amplitude modulation of the gate microwave drive. After stabilization of the ions' radial modes, we obtain an amplitude modulated gate with infidelity in the 10^-3 range. The result is confirmed by analyzing the data using three different methods. Using additional dynamic decoupling techniques, these results could bring microwave near-field gates past the fault-tolerance threshold.
This thesis describes the development of technology and experimental techniques for scalable trapped-ion quantum computing. An architecture for scalable quantum computing is presented in which 171Yb+ ions are trapped in linear R.F. Paul traps and their hyper-fine states are used to form qubits. Quantum logical operations can be carried out by applying R.F. and microwave fields. This thesis presents significant improvements in the experimental setup used for quantum computing, as well as new theoretical developments and experimental demonstrations of new high- fidelity quantum control methods. A new procedure for carrying out high- delity two qubit gates using static magnetic eld gradients and resonant microwave elds is presented. It is shown that this has the potential to enable signi cantly improved delities compared to current methods. Improved methods for measurement and statistical analysis in trapped-ion experiments are also shown. These should enable more accurate measurement of two-qubit gate delities, which becomes increasingly important as delities increase. A new system for generating arbitrary R.F. and microwave waveforms is presented, including a bespoke software control system allowing the user to de ne custom pulse sequences, which represents a signi cant improvement in the scalability of the quantum control system. Experimental developments towards a system for carrying out on-chip trapped-ion quantum logic in a static magnetic eld gradient are also presented. A new technique for generating multi-level control methods is introduced, based on reducing a multi-level system to an e ective two-level system. High- delity quantum control methods generated using this technique, which form a will form a key part of the scalable quantum computing architecture, are implemented experimentally.