This thesis describes the development of two segmented ion trap systems for quantum information experiments and investigations of novel quantum measurements. We report successful loading of 40Ca+ ions in one of the trap systems and the successful demonstration of the partial collapse and revival of the wavefunction of an ion qubit using the system. We describe the operation of ion trap systems with segmented electrodes and develop a practical strategy for choosing the voltages to apply to the electrode segments to obtain desired trapping potentials and correct for stray fields. The linearity of the Laplace equation results in a linear map from the configuration space of electrode voltages to the function space of electric potentials in the trap. We decompose the electrode configuration space into basis vectors whose corresponding potentials have intuitive meaning. We describe attempts at loading Ca+ ions in a micro-scale segmented ion trap built at Lucent Technologies. The Lucent Trap is a planar ion trap fabricated using lithographic techniques on a silicon substrate. We experienced several problems with this trap and were unable to successfully load ions in it. A second ion trap, built at the University of Liverpool, is described. The Liverpool trap has miniature segmented electrodes constructed using conventional machining. We describe loading of Ca+ ions, and the demonstration of stable trapping with long ion lifetimes and stable electric and magnetic fields. The Liverpool trap is used to demonstrate non-projective “partial” measurements of a qubit wavefunction, which result in non-unitary, non-projective evolution of the wavefunction dubbed partial collapse. This effect can be reversed by performing another partial measurement, but only if the second measurement results in a particular outcome. When this outcome is observed, the qubit wavefunction is restored to the original state. An exact restoration occurs when both measurements are ideal, and we quantify the success of our demonstration by quantum process tomography. We obtain process fidelity >0.65 for a wide variation in strength of the partial collapse from 0−0.94 (where 0 leaves the system unaffected and 1 is the limit of a complete projective measurement).
This thesis describes the development of two segmented ion trap systems for quantum information experiments and investigations of novel quantum measurements. We report successful loading of 40Ca+ ions in one of the trap systems and the successful demonstration of the partial collapse and revival of the wavefunction of an ion qubit using the system. We describe the operation of ion trap systems with segmented electrodes and develop a practical strategy for choosing the voltages to apply to the electrode segments to obtain desired trapping potentials and correct for stray fields. The linearity of the Laplace equation results in a linear map from the configuration space of electrode voltages to the function space of electric potentials in the trap. We decompose the electrode configuration space into basis vectors whose corresponding potentials have intuitive meaning. We describe attempts at loading Ca+ ions in a micro-scale segmented ion trap built at Lucent Technologies. The Lucent Trap is a planar ion trap fabricated using lithographic techniques on a silicon substrate. We experienced several problems with this trap and were unable to successfully load ions in it. A second ion trap, built at the University of Liverpool, is described. The Liverpool trap has miniature segmented electrodes constructed using conventional machining. We describe loading of Ca+ ions, and the demonstration of stable trapping with long ion lifetimes and stable electric and magnetic fields. The Liverpool trap is used to demonstrate non-projective “partial” measurements of a qubit wavefunction, which result in non-unitary, non-projective evolution of the wavefunction dubbed partial collapse. This effect can be reversed by performing another partial measurement, but only if the second measurement results in a particular outcome. When this outcome is observed, the qubit wavefunction is restored to the original state. An exact restoration occurs when both measurements are ideal, and we quantify the success of our demonstration by quantum process tomography. We obtain process fidelity >0.65 for a wide variation in strength of the partial collapse from 0−0.94 (where 0 leaves the system unaffected and 1 is the limit of a complete projective measurement).
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.
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.
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.
This thesis describes experimental work towards developing a trapped ion quantum information processor. An existing ion trap apparatus was capable of trapping and laser-cooling single ions or small ion strings of 40 Ca+, and had been used for studies of quantum jumps and natural lifetime measurements in Ca. This thesis describes improvements in this apparatus, which have allowed the stability and the flexibility of experimental control of the ions to be greatly increased. This enabled experiments to read out the spin state of a single trapped ion, and to load ions with isotope selectivity through photoionization. The optical systems were improved by installation of new lasers, optical reference cavities, and a system of acousto-optic modulators for laser intensity switching and frequency control. The photon counting for fluorescence detection was improved, and a new photon time-of-arrival correlation circuit developed. This has permitted rapid and more sensitive detection of micromotion, and hence cancellation of stray fields in the trap. A study of resonant circuits in the low RF, high voltage (10 MHz, 1 kV) regime was carried out with a view to developing a new RF supply for the Paul trap with reduced noise and increased power. A new supply based on a helical resonator was built and used to trap ions. This technique has reduced noise and will permit higher secular frequencies to be attained in the future. A magnetic field B in the ion trap is used to define a quantization axis, and in one series of experiments was required to be of order 100 G to provide a substantial Zeeman splitting. A set of magnetic field coils to control the size and direction of B is described. The design of these posed some problems owing to an unforseen issue with the vacuum chamber. In short, it is magnetizable and acts to first approximation like a magnetic shield. The field coils had to be sufficiently substantial to produce the desired field at the ion even in the presence of this shielding effect, and dark resonance (and other) spectra with Zeeman splitting were obtained to calibrate the field using the ion as a probe. Finally, the thesis describes the successful loading of the ion trap by laser photoionization from a weak atomic beam. This involved two new lasers at 423 nm and 389 nm. Saturated absorption spectroscopy of neutral calcium is first described, then transverse excitation of an atomic beam in our vacuum chamber is used to identify all the main isotopes of calcium and confirm their abundances in our source (a heated sample of natural calcium). Finally, photoionization is used to load the trap. This has three advantages over electron-impact ionization. By avoiding an electron gun, we avoid charging of insulating patches and subsequent electric field drift as they discharge; the flux in the atomic beam and hence calcium (and other) deposits on the electrodes can be greatly reduced; and most importantly, the photoionization is isotope selective. Evidence is presented which suggests that even with an non-enriched source, the rare isotope 43 Ca can be loaded with reasonable efficiency. This isotope is advantageous for quantum information experiments for several reasons, but chiefly because its ground state hyperfine structure can act as a stable qubit.
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.
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.)