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Nano Letters

Motional Heating in a Graphene-Coated Ion Trap

Amira M. Eltony, Hyesung Park, Shannon X. Wang, Jing Kong, Isaac L. Chuang

Published Aug 27, 2014• 12 citations
Publisher

Abstract

Electric field noise originating from metal surfaces is a hindrance for a variety of microengineered systems, including for ions in microtraps, but is not well understood at the microscopic level. For trapped ions, it is manifested as motional-state decoherence inexplicable by thermal noise of electrodes alone, but likely surface-dependent. Here, we investigate the role of surface properties in motional heating by creating an ion trap with a unique exterior. Using single trapped-ion probes, we characterize copper electrodes covered in monolayer graphene, a material free of surface charge and dangling bonds. Surprisingly, we measure an average heating rate of 1020 ± 30 quanta/s, which is ∼100 times higher than typical for an uncoated trap operated under similar conditions. This may be related to hydrocarbon deposits on the surface, which could be monitored on graphene to potentially elucidate the mechanisms of motional heating on the atomic scale.

DOI
10.1021/nl502468g

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Electric field noise originating from metal surfaces is a hindrance for a variety of microengineered systems, including for ions in microtraps, but is not well understood at the microscopic level. For trapped ions, it is manifested as motional-state decoherence inexplicable by thermal noise of electrodes alone, but likely surface-dependent. Here, we investigate the role of surface properties in motional heating by creating an ion trap with a unique exterior. Using single trapped-ion probes, we characterize copper electrodes covered in monolayer graphene, a material free of surface charge and dangling bonds. Surprisingly, we measure an average heating rate of 1020 ± 30 quanta/s, which is ∼100 times higher than typical for an uncoated trap operated under similar conditions. This may be related to hydrocarbon deposits on the surface, which could be monitored on graphene to potentially elucidate the mechanisms of motional heating on the atomic scale.
Motional heating of ions in microfabricated traps is one of the open challenges hindering experimental realizations of large-scale quantum processing devices. Recently, a series of measurements of the heating rates in surface-electrode ion traps characterized their frequency, distance, and temperature dependencies, but our understanding of the microscopic origin of this noise remains incomplete. In this work we develop a theoretical model for the electric field noise which is associated with a random distribution of adsorbed atoms on the trap electrode surface. By using first-principles calculations of the fluctuating dipole moments of the adsorbed atoms we evaluate the distance, frequency, and temperature dependence of the resulting electric field fluctuation spectrum. Our theory reproduces correctly the ${d}^{\ensuremath{-}4}$ dependence with distance of the ion from the electrode surface and calculates the noise spectrum beyond the standard scenario of two-level fluctuators by incorporating all the relevant vibrational states. Our model predicts a regime of 1/f1/f1/f noise which commences at roughly the frequency of the fundamental phonon transition rate and a thermally activated noise spectrum which for higher temperatures exhibits a crossover as a function of frequency.
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.
Trapped ions have proven to be effective quantum bits; but increasing electric field noise as traps are miniaturized limits gate fidelity and progress towards a large-scale quantum computer. Removing contamination from surfaces is important for noise suppression; but cleaning techniques like argon ion bombardment are difficult to integrate with current systems and are too harsh for traps incorporating optical devices. We investigate an alternative solution: a protective coating against surface contamination. We fabricate copper traps with a graphene passivation layer and characterize them with single ions. Surprisingly, we find worse noise performance than for an uncoated metal trap.
Abstract In order to realize the full potential of ion trap quantum computers, an improved understanding is required of the motional heating that trapped ions experience. Experimental studies of the temperature-, frequency-, and ion–electrode distance-dependence of the electric field noise responsible for motional heating, as well as the noise before and after ion bombardment cleaning of trap electrodes, suggest that fluctuations of adsorbate dipoles are a likely source of so-called ‘anomalous heating,’ or motional heating of the trapped ions at a rate much higher than the Johnson noise limit. Previous computational studies have investigated how the fluctuation of model adsorbate dipoles affects anomalous heating. However, the way in which specific adsorbates affect the electric field noise has not yet been examined, and an electric dipole model employed in previous studies is only accurate for a small subset of possible adsorbates. Here, we analyze the behavior of both in-plane and out-of-plane vibrational modes of twenty-one adsorbate–electrode combinations within the independent fluctuating dipole model, utilizing accurate first principles computational methods to determine the surface-induced dipole moments. We find the chemical specificity of the adsorbate can change the electric field noise by seven orders of magnitude and specifically that soft in-plane modes of weakly-adsorbed hydrocarbons produce the greatest noise and ion heating. We discuss the dynamics captured by the fluctuating dipole model, namely the adsorbate-dependent turn-on temperature and electric field noise magnitude, and also discuss the model’s failure to reproduce the measured 1/ω noise frequency scaling with a single adsorbate species. We suggest future research directions for improved, quantitatively predictive models based on extensions of the present framework to multiple interacting adsorbates.
We measure and characterize anomalous motional heating of an atomic ion confined in the lowest quantum levels of a novel rf ion trap that features moveable electrodes. The scaling of heating with electrode proximity is measured, and when the electrodes are cooled from 300 to 150 K, the heating rate is suppressed by an order of magnitude. This provides direct evidence that anomalous motional heating of trapped ions stems from microscopic noisy potentials on the electrodes that are thermally driven. These observations are relevant to decoherence in quantum information processing schemes based on trapped ions and perhaps other charge-based quantum systems.
Motional heating of trapped atomic ions is a major obstacle to their use as quantum bits in a scalable quantum computer. The detailed physical origin of this heating is not well understood, but experimental evidence suggests that it is caused by electric-field noise emanating from the surface of the trap electrodes. In this study, we have investigated the role of adsorbates on the electrodes by identifying contaminant overlayers, implementing an in situ argon-ion-beam cleaning treatment, and measuring ion heating rates before and after treating the trap electrodes' surfaces. We find a 100-fold reduction in heating rate after treatment. The experiments described here are sensitive to low levels of electric-field noise in the MHz frequency range. Therefore, this approach could become a useful tool in surface science that complements established techniques.