The electromagnetic manipulation of isolated ions has led to many advances in atomic physics, from laser cooling to precision metrology and quantum control. As technical capability in this area has grown, so has interest in building miniature electromagnetic traps for the development of large-scale quantum information processors. This thesis will primarily focus on using microfabrication techniques to build arrays of miniature ion traps, similar to techniques used in fabricating high component density microprocessors. A specific focus will be on research using a gallium arsenide/aluminum gallium arsenide heterostructure as a trap architecture, as well as the recent testing of different ion traps fabricated at outside foundries. The construction and characterization of a conventional ceramic trap capable of shuttling an ion through a junction will also be detailed, and reveal the need for moving towards lithographically fabricated traps. Combined, these serve as a set of proof-of-principle experiments pointing to methods for designing and building large scale arrays of ion traps capable of constituting a quantum information processor. As traps become smaller, electrical potentials on the electrodes have greater influence on the ion. This not only pertains to intentionally applied voltages, but also to deleterious noise sources, such as thermal Johnson noise and the more significant ``patch potential" noise, which both cause motional heating of the ion. These problematic noise sources dovetail with my thesis research into trap miniaturization since their effects become more pronounced and impossible to ignore for small trap sizes. Therefore characterizing them and investigating ways to suppress them have become an important component of my research. I will describe an experiment using a pair of movable needle electrodes to measure the ion heating rate corresponding to the harmonic frequency of the trap, the ion-electrode distance, and the electrode temperature. This information is used for characterizing the fluctuating potentials and exploring the possibility of suppressing motional heating by cooling the trap electrodes. This source of noise is also observed in other systems, and its characterization could potentially improve other precision experiments, such as those measuring deviations in the gravitational inverse square law with proximate masses.
The electromagnetic manipulation of isolated ions has led to many advances in atomic physics, from laser cooling to precision metrology and quantum control. As technical capability in this area has grown, so has interest in building miniature electromagnetic traps for the development of large-scale quantum information processors. This thesis will primarily focus on using microfabrication techniques to build arrays of miniature ion traps, similar to techniques used in fabricating high component density microprocessors. A specific focus will be on research using a gallium arsenide/aluminum gallium arsenide heterostructure as a trap architecture, as well as the recent testing of different ion traps fabricated at outside foundries. The construction and characterization of a conventional ceramic trap capable of shuttling an ion through a junction will also be detailed, and reveal the need for moving towards lithographically fabricated traps. Combined, these serve as a set of proof-of-principle experiments pointing to methods for designing and building large scale arrays of ion traps capable of constituting a quantum information processor. As traps become smaller, electrical potentials on the electrodes have greater influence on the ion. This not only pertains to intentionally applied voltages, but also to deleterious noise sources, such as thermal Johnson noise and the more significant ``patch potential" noise, which both cause motional heating of the ion. These problematic noise sources dovetail with my thesis research into trap miniaturization since their effects become more pronounced and impossible to ignore for small trap sizes. Therefore characterizing them and investigating ways to suppress them have become an important component of my research. I will describe an experiment using a pair of movable needle electrodes to measure the ion heating rate corresponding to the harmonic frequency of the trap, the ion-electrode distance, and the electrode temperature. This information is used for characterizing the fluctuating potentials and exploring the possibility of suppressing motional heating by cooling the trap electrodes. This source of noise is also observed in other systems, and its characterization could potentially improve other precision experiments, such as those measuring deviations in the gravitational inverse square law with proximate masses.
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.
Trapped ions within Paul traps have shown to be a promising architecture in the realisation of a quantum information processor together with the ability of providing quantum simulations. Linear Paul traps have demonstrated long coherence times with ions being well isolated from the environment, single and multi-qubit gates and the high fidelity detection of states. The scalability to large number of qubits, incorporating all the previous achievements requires an array of linear ion traps. Microfabrication techniques allow for fabrication and micron level accuracy of the trap electrode dimensions through photolithography techniques. The first part of this thesis presents the experiential setup and trapping of Yb+ ions needed to test large ion trap arrays. This include vacuum systems that can host advanced symmetric and asymmetric ion traps with up to 90 static voltage control electrodes. Demonstration of a single trapped Yb+ ion within a two-layer macroscopic ion trap is presented. with an ion-electrode distance of 310(10) µm. The anomalous heating rate and spectral noise density of the trap was measured, a main form of decoherence within ion traps. The second half of this thesis presents the design and fabrication of multi-layer asymmetric ion traps. This allows for isolated electrodes that cannot be accessed via surface pathways, allowing for higher density of electrodes as well as creating novel trap designs that allow for the potential of quantum simulations to be demonstrated. These include two-dimensional lattices and ring trap designs in which the isolated electrodes provide more control in the ion position. For the microfabrication of these traps I present a novel high-aspect ratio electroplated electrode design that provides shielding of the dielectric layer. This provides a means to mitigate stray electric field due to charge build up on the dielectric surfaces. Electrical testing of the trap structures was performed to test bulk breakdown and surface flashover of the ion trap architectures. Results showed sufficient isolation between electrodes for both radio frequency and static breakdown. Surface flashover voltage measurements over the dielectric layer showed an improvement of more than double over previous results using a new fabrication technique. This will allow for more powerful ion trap chips needed for the next generation of microfabricated ion trap arrays for scalable quantum technologies.
The electromagnetic confinement of atomic ions has provided a useful testbed for many different applications in atomic physics, including laser cooling {1}, mass spectrometry {2}, and precision control of nearly-pure quantum states {3}. Recently, ion traps have been effectively applied to the growing field of quantum computation {4}, where the ability to isolate a single ion from its environment has made it an attractive architecture for a large-scale quantum information processor {5, 6, 7}. While many of the fundamental quantum computing building blocks have been demonstrated with trapped ions {8}, the technology for scaling to large numbers of ion quantum bits (qubits) is just beginning to develop. In this paper, we describe an important milestone on this path with the successful operation of an ion trap fabricated on an integrated gallium arsenide (GaAs) heterostructure, which could in principal scale to host a large array of ions.
Over the past 15 years ion traps have demonstrated all the building blocks required of a quantum computer. Despite this success, trapping ions remains a challenging task, with the requirement for extensive laser systems and vacuum systems to perform operations on only a handful of qubits. To scale these proof of principle experiments into something that can outperform a classical computer requires an advancement in the trap technologies that will allow multiple trapping zones, junctions and utilize scalable fabrication technologies. I will discuss the construction of an ion trapping experiment, focussing on my work towards the laser stabilization and ion trap design but also covering the experimental setup as a whole. The vacuum system that I designed allows the mounting and testing of a variety of ion trap chips, with versatile optical access and a fast turn around time. I will also present the design and fabrication of a microfabricated Y junction and a 2- dimensional ion trap lattice. I achieve a suppression of barrier height and small variation of secular frequency through the Y junction, aiding to the junctions applicability to adiabatic shuttling operations. I also report the design and fabrication of a 2-D ion trap lattice. Such structures have been proposed as a means to implement quantum simulators and to my knowledge is the first microfabricated lattice trap. Electrical testing of the trap structures was undertaken to investigate the breakdown voltage of microfabricated structures with both static and radio frequency voltages. The results from these tests negate the concern over reduced rf voltage breakdown and in fact demonstrates breakdown voltages significantly above that typically required for ion trapping. This may allow ion traps to be designed to operate with higher voltages and greater ion-electrode separations, reducing anomalous heating. Lastly I present my work towards the implementation of magnetic fields gradients and microwaves on chip. This may allow coupling of the ions internal state to its motion using microwaves, thus reducing the requirements for the use of laser systems.
With all the key elements of quantum computing in ion traps demonstrated by the research community, the focus is now placed on building more sophisticated traps with larger numbers of ions to allow practical scale information processing. One promising avenue is to store ions in and shuttle them between many independent traps which serve as potential interaction sites. The core of the work described in this thesis is the experimental evaluation of a microfabricated segmented ion trap, built by Sandia National Laboratories ("Sandia trap"). These experiments required construction of a wholly new optical setup including laser and detection systems, a vacuum system and control electronics. Among our experimental achievements were: successful loading of single and pairs of ions in the microscale trap, measurement of ion storage lifetime, measurement of the motional heating rate with a time-resolved Doppler-cooling method — which showed above than average heating, and implemented a single-ion shuttling method — which reliably transferred the ion through a distance of 360 μm (two DC electrode widths away) and back. These results have been used to improve the next version of the Sandia trap design. We also used computer modelling to study several aspects of ion traps: a mesoscopic ion trap designed for fast ion separation, simulated ion loading to quantify requirements for successful trapping in small and shallow traps, and analyzed a precise shuttling method — where the time dependence of the trapping potential is engineered such that there is minimal motional heating. The results show that ion trap arrays at the 100 μm distance scale are feasible and suggests that such multiple trap designs merit further study.
While quantum information processing offers a tantalizing possibility of a significant speedup in execution of certain algorithms, as well as enabling previously unmanageable simulations of large quantum systems, it remains extremely difficult to realize experimentally. Recently, fundamental building blocks of a quantum computer, including one and two qubit gates, teleportation and error correction, were demonstrated using trapped atomic ions. Scaling to a larger number of qubits requires miniaturization of the ion traps, currently limited by the sharply increasing motional state decoherence at sub-100 μm ion-electrode distances. This thesis explores the source and suppression of this decoherence at cryogenic temperatures, and demonstrates fundamental logic gates in a surface electrode ion trap.
Construction of the apparatus requires the development of a number of experimental techniques. Design, numerical simulation and implementation of a surface electrode ion trap is presented. Cryogenic cooling of the trap to near 4 K is accomplished by contact with a bath cryostat. Ions are loaded by ablation or photoionization, both of which are characterized in terms of generated stray fields and heat load.
The bulk of new experimental results deals with measurements of electric field noise at the ion's position. Upon cooling to 6 K, the measured rates are suppressed by up to 7 orders of magnitude, more than two orders of magnitude below previously published data for similarly sized traps operated at room temperature. The observed noise depends strongly on fabrication process, which suggests further improvements are possible. The measured dependence of the electric field noise on temperature is inconsistent with published models, and can be explained using a continuous spectrum of activated fluctuators.
The fabricated surface electrode traps are used to demonstrate coherent operations and the classical control required for trapped ion quantum computation. The necessary spectral properties of coherent light sources are achieved with a novel design using optical feedback to a triangular, medium finesse, cavity, followed by electronic feedback to an ultra-high finesse reference cavity. Single and two qubit operations on a single ion are demonstrated with classical fidelity in excess of 95%. Magnetic field gradient coils built into the trap allow for individual addressing of ions, a prerequisite to scaling to multiple qubits. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)