Measurements of electric field noise and light-inducedcharging in cryogenic surface electrode ion traps; Measurements of electric field noise and light-inducedcharging in Al and Cu surface electrode ion traps at cryogenictemperatures
Ion traps provide an excellent tool for controlling
and observing the state of a single trapped ion. For this reason,
ion traps have been proposed as a possible system for large-scale
quantum computation. However, many obstacles must be overcome
before quantum computing can become a reality. In particular,
perturbations in the electric field due to noise and electrode
charging must be reduced to increase coherence of the motional
quantum state. Gold has been a popular choice in the past due to
its inert properties; however, it is undesirable due its
incompatibility with CMOS technology. This has led to increased
research into alternative CMOS-compatible materials, such as
aluminum and copper. This thesis presents measurements of electric
field noise and light-induced charging in aluminum, copper, and
gold surface electrode traps. In addition, the effect of oxide
growth on field noise and electrode charging is explored by
controlling the thickness of aluminum oxide on several aluminum
traps. The measurements show that electric field noise can be
suppressed in aluminum traps to approximately 10-18 V2 cm-2 Hz-1,
matching the noise exhibited in gold traps, and that copper traps
exhibit noise within an order of magnitude of that in aluminum and
gold. However, the natural oxide of aluminum poses many problems
towards high-performance aluminum ion traps. The electric field
noise is shown to be strongly dependent on the oxide thickness,
increasing the noise by a factor of about 10 until saturation at a
thickness of 13 nm. Charging of surface electrodes is shown to be
highly dependent upon the material, but the model presented does
not match the experimental data and is found to be incomplete.
These results indicate that ion traps made out of CMOS-compatible
materials can perform as well as more traditional traps fabricated
from gold with respect to heating and charging as long as methods
are developed for controlling oxide growth.
Ion traps provide an excellent tool for controlling
and observing the state of a single trapped ion. For this reason,
ion traps have been proposed as a possible system for large-scale
quantum computation. However, many obstacles must be overcome
before quantum computing can become a reality. In particular,
perturbations in the electric field due to noise and electrode
charging must be reduced to increase coherence of the motional
quantum state. Gold has been a popular choice in the past due to
its inert properties; however, it is undesirable due its
incompatibility with CMOS technology. This has led to increased
research into alternative CMOS-compatible materials, such as
aluminum and copper. This thesis presents measurements of electric
field noise and light-induced charging in aluminum, copper, and
gold surface electrode traps. In addition, the effect of oxide
growth on field noise and electrode charging is explored by
controlling the thickness of aluminum oxide on several aluminum
traps. The measurements show that electric field noise can be
suppressed in aluminum traps to approximately 10-18 V2 cm-2 Hz-1,
matching the noise exhibited in gold traps, and that copper traps
exhibit noise within an order of magnitude of that in aluminum and
gold. However, the natural oxide of aluminum poses many problems
towards high-performance aluminum ion traps. The electric field
noise is shown to be strongly dependent on the oxide thickness,
increasing the noise by a factor of about 10 until saturation at a
thickness of 13 nm. Charging of surface electrodes is shown to be
highly dependent upon the material, but the model presented does
not match the experimental data and is found to be incomplete.
These results indicate that ion traps made out of CMOS-compatible
materials can perform as well as more traditional traps fabricated
from gold with respect to heating and charging as long as methods
are developed for controlling oxide growth.
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.)
All surfaces generate electric-field noise, yet the physical origins of this noise are not well understood. This has been an active area of research in the ion trapping community for the past two decades, as ions are highly sensitive to electric-field fluctuations. With our work, we aim to illuminate the microscopic processes that drive charge dynamics on metal surfaces, so as to enable the engineering of low noise quantum devices. We use single trapped calcium ions as detectors to study the 1/f noise generated by the surfaces of ion traps. We the study this noise by observing how it responds to changes in the properties of the trap surface. The surface properties are altered using treatments including prolonged heating, argon ion milling, and electron bombardment. In situ characterization tools are used to monitor the effects of these treatments. Our measurements are consistent with noise produced by an ensemble of thermally activated fluctuators, so our data is discussed in this context. In this dissertation, we present results from a lengthy series of surface treatment experiments, the majority of which took place on a single aluminum-copper substrate. The results of these experiments indicate that argon ion milling can lower noise both by removing contaminants and by altering the morphology of the trap surface. The effects of morphology are isolated from the effects of contaminant removal via heat treatments, which alter the structure of the surface without changing its chemical composition. Through electron bombardment experiments, we begin an exploration of the relationship between hydrocarbon adsorbate structure and electric-field noise. In addition, we compare the noise characteristics of a set of similarly fabricated traps, and determine that atmosphere exposure has a major impact on noise produced by aluminum-copper films.These experiments establish links between the electric-field noise characteristics and the microscopic properties of a contaminated metal trap surface. The insights we draw in this work can inform the next generation of ion trap engineering, storage, and treatment.
Electric-field noise is a major limiting factor in the performance of ion traps and other quantum devices. Despite intensive research over the past decade, the nature and cause of electric field noise near surfaces is not well understood. This dissertation reports the high- temperature dependence of electric-field noise above an Al-Cu surface using a trapped 40Ca+ ion as a probe.We employ a novel setup with a surface ion trap mounted on a heater for studies of the temperature dependence of electric-field noise. To characterize the Al-Cu material, we explore the effects of heat treatment through ex situ annealing followed by inspection in a scanning electron microscope. To calibrate the temperature of the trap, we demonstrate the use of thermal imaging for monitoring the temperature of an ion trap in vacuum.The temperature and frequency dependence of electric-field noise above the surface is measured using the heating rate of a single ion in a surface-electrode Paul trap. We find that the heating rate saturates at temperatures greater than 450 K. We find that the frequency dependence shows a 1/f behavior, and has a lower frequency scaling exponent at high temperatures than at room temperature. We show that these results are a reflection of the surface-related noise by eliminating other possible sources of noise.Building on historical data for resistance fluctuations in thin films, we develop the thermally-activated fluctuator model to describe the results. We find that a broad distribution of fluctuators with energy barriers peaked around 0.5 eV accurately models both the temperature and frequency dependence of the electric-field noise measured. We present the interpretation of this model as a way to infer that the cause of electric-field noise in the trap is likely defect motion in the metal surface, connecting the problems faced in ion trapping to a large body of work in solid state physics.
Electric field noise produced by the surface of ion trap electrodes reduces the fidelity of quantum computing operations. Despite decades of investigation its microscopic origins remain unclear.This thesis investigates the origins of surface electric field noise by examining its dependence on trap temperature. A set of Al-Cu surface Paul traps are fabricated together and sent to MIT Lincoln Labs (MITLL) and the University of California, Berkeley (UCB). MITLL measures noise from 7 K to room temperature above a first trap and UCB measures noise from room temperature to 590 K above a second trap. The noise measurements from the two experiments are combined to study the noise dependence over the full temperature range from 7-590 K. This is the largest temperature range ever studied using a single trap design and connects the temperature ranges used in all previous studies.We find qualitatively different behaviors at low and high temperatures: from 7-430 K we find that the noise follows the same power-law scaling reported in previous cryogenic studies. At temperatures from 430-590 K, we find that the noise increases much more rapidly than predicted by the power-law scaling. We note a particularly sharp rise in noise at 560 K, which is similar to the sharp rise in noise reported in a previous high-temperature study of a different Al-Cu trap. To determine which physical processes are associated with the change in noise at elevated trap temperatures, we investigate how the material state of the trap electrodes changes when they are heated. We find that the primary physical process associated with the change in noise at high temperature is thermally-activated atom diffusion within the films.This thesis also investigates if the surface structure of room-temperature Au ion traps affects noise levels. We use a heat treatment to restructure the surface of the traps and measure the resulting change in noise. We find that the heat treatment leads to significant changes to the surface structure but causes only a modest change to noise levels. This indicates that the heat-induced changes to the surface do not strongly affect noise. We also note that the noise magnitude observed above these Au traps is state-of-the-art for room-temperature, untreated surface ion traps. This noise level can be explained by modeling the contamination that exists on the trap surface as a dielectric with a loss tangent of tan θ ≈ 0.01.Finally, this thesis investigates the origin of elevated noise levels in one region of a Au surface ion trap. We find that micron-sized particles are located where noise levels are highest, indicating that these particles are likely responsible for the noise. We can explain the magnitude, frequency, and distance dependence of the noise if we treat the particles as a lossy dielectric with a loss tangent tan θ ≈ 0.3.
Electric field noise from fluctuating patch potentials is a significant problem for a broad range of precision experiments, including trapped ion quantum computation and single spin detection. Recent results demonstrated strong suppression of this noise by cryogenic cooling, suggesting an underlying thermal process. We present measurements characterizing the temperature and frequency dependence of the noise from 7 to 100 K, using a single Sr+ ion trapped 75 mum above the surface of a gold plated surface electrode ion trap. The noise amplitude is observed to have an approximate 1/f spectrum around 1 MHz, and grows rapidly with temperature as T;{beta} for beta from 2 to 4. The data are consistent with microfabricated cantilever measurements of noncontact friction but do not extrapolate to the dc measurements with neutral atoms or contact potential probes.
An ion trap is a device to confine charged particles by utilizing electromagnetic fields.Since the trapped ions have a feasibility of the individual state manipulation, a long coherence time, and an ideal isolation from the surroundings, the ion-trap technology becomes one of the leading candidates for the physical implementation of quantum information processing.In order to build a large-scale integrated iontrap system for realizing complex quantum algorithms, microfabrication technologies have been applied to construct ion traps.The use of micro-electromechanical system (MEMS) ion-trap chip allows a scalable architecture of ion-trap arrays and integration of functional components to the trap chip, but a few side effects also arise.One of the most particular problems is stray fields generated by the charges accumulated on the sidewalls of thick dielectric pillars or the native oxide grown on metal surfaces, since the stray fields lead to ion micromotions which can cause heating and escape of the trapped ions.This dissertation presents a silicon surface ion-trap chip with dielectric sidewalls shielded by metal films.The oxide pillars supporting the top electrodes are fabricated to have overhang structures, and the upper and lower part of the pillars are coated by separate aluminum layers which are electrically isolated from each other.In order to evaluate the effects of the native ii metal oxide on the electrode surface to the charging phenomenon, a trap chip with an additional gold layer on the aluminum electrodes is also fabricated.An ultra-high vacuum (UHV) chamber, electrical connections, and an optical setup are prepared to trap 174 Yb + ions, and the ions are successfully trapped by using the experimental setup including the fabricated ion-trap chip.To evaluate the effectiveness of the proposed electrode structures, charging is intentionally induced on the chip surface by injecting a 355-nm pulse laser with 40-μW power perpendicularly to the chip surface.The intensity of the stray field is estimated by measuring the displacement of ion position after charging is induced.When the Al-based chips with exposed and Al-coated dielectric sidewalls are used, the standard deviation of the intensities of stray fields are 3.53, and 5.63 V/m, respectively.However, the standard deviation of the intensity of stray fields in the case of using Au-coated chip is 1.56 V/m, and any stray field is measured when the laser was injected on the surface or sidewalls of the gold-coated electrodes.These experimental results indicate that the gold coating on the surface of the aluminum electrodes and the sidewalls of dielectric pillars is effective for eliminating the generation of stray fields induced by static charges.