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arXiv:1805.00793 · Physical Review A

Observation of superconductivity and surface noise using a single trapped ion as a field probe

Kirill Lakhmanskiy, Philip C. Holz, Dominic Schärtl, Ben Ames, Réouven Assouly, Thomas Monz, Yves Colombe, R. Blatt

Published Feb 4, 2019• 18 citations
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Abstract

Measuring and understanding electric-field noise from bulk material and surfaces is important for many areas of physics. In this work, we demonstrate the probing of electric-field noise from different sources with an ion, $225\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ above the trap surface. We detect noise levels as small as ${S}_{E}=5.2(11)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{4pt}{0ex}}{\mathrm{V}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1}$ at ${\ensuremath{\omega}}_{\text{z}}=2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}1.51\phantom{\rule{4pt}{0ex}}\mathrm{MHz}$ and $T=12\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Our setup incorporates a controllable noise source utilizing a high-temperature superconductor. This element allows us, first, to benchmark and validate the sensitivity of our probe. Second, it allows us to probe noninvasively the bulk properties of the superconductor, observing a superconducting transition with an ion. For temperatures below the transition, we use our setup to assess different surface noise processes. The measured surface noise shows a deviation from a power law in the frequency domain. However, the temperature scaling of the data is not in a good agreement with existing surface noise models. Our results open perspectives for models in surface science and pave the way to test them experimentally.

DOI
10.1103/physreva.99.023405

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Measuring and understanding electric-field noise from bulk material and surfaces is important for many areas of physics. In this work, we demonstrate the probing of electric-field noise from different sources with an ion, 225\ensuremathμm225\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}225\ensuremathμm above the trap surface. We detect noise levels as small as SE=5.2(11)\ifmmode×\else\texttimes\fi10\ensuremath−16V2m\ensuremath−2Hz\ensuremath−1{S}_{E}=5.2(11)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{4pt}{0ex}}{\mathrm{V}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1}SE​=5.2(11)\ifmmode×\else\texttimes\fi10\ensuremath−16V2m\ensuremath−2Hz\ensuremath−1 at \ensuremathωz=2\ensuremathπ\ifmmode×\else\texttimes\fi1.51MHz{\ensuremath{\omega}}_{\text{z}}=2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}1.51\phantom{\rule{4pt}{0ex}}\mathrm{MHz}\ensuremathωz​=2\ensuremathπ\ifmmode×\else\texttimes\fi1.51MHz and T=12KT=12\phantom{\rule{4pt}{0ex}}\mathrm{K}T=12K. Our setup incorporates a controllable noise source utilizing a high-temperature superconductor. This element allows us, first, to benchmark and validate the sensitivity of our probe. Second, it allows us to probe noninvasively the bulk properties of the superconductor, observing a superconducting transition with an ion. For temperatures below the transition, we use our setup to assess different surface noise processes. The measured surface noise shows a deviation from a power law in the frequency domain. However, the temperature scaling of the data is not in a good agreement with existing surface noise models. Our results open perspectives for models in surface science and pave the way to test them experimentally.
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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.
We probe electric-field noise near the metal surface of an ion-trap chip in the temperature range from 295 to 530 K. We observe a nontrivial temperature dependence with the noise amplitude at 1 MHz frequency saturating around 500 K. Measurements of the noise spectrum reveal a $1/{f}^{\ensuremath{\alpha}\ensuremath{\approx}1}$ dependence and a small decrease in $\ensuremath{\alpha}$ between low and high temperatures. This behavior can be explained by considering noise from a distribution of thermally activated two-level fluctuators with activation energies between 0.35 and 0.65 eV. Processes in this energy range may be relevant to understanding electric-field noise in ion traps; for example, defect motion in the solid state and surface adsorbate binding energies. The study of these processes may aid in identification of the origin of excess electric-field noise in ion traps---a major source of ion motional decoherence limiting the performance of surface traps as quantum devices.
We report room-temperature electric field noise measurements combined with in-situ surface characterization and cleaning of a microfabricated ion trap. We used a single-ion electric field noise sensor in combination with surface cleaning and analysis tools, to investigate the relationship between electric field noise from metal surfaces in vacuum and the composition of the surface. These experiments were performed in a novel setup that integrates ion trapping capabilities with surface analysis tools. We find that surface cleaning of an aluminum-copper surface significantly reduces the level of electric field noise, but the surface does not need to be atomically clean to show noise levels comparable to those of the best cryogenic traps. The post-cleaning noise levels are low enough to allow fault-tolerant trapped-ion quantum information processing on a microfabricated surface trap.
How can the electric noise in the vicinity of a metallic body be measured and understood? Trapped ions, known as unique tools for metrology and quantum information processing, also constitute very sensitive probes of this electric noise for distances from micrometers to millimeters. This paper presents various models for the origin of the electric noise, provides a critical review of the experimental findings, and summarizes the important questions that are still open in this active research area.