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arXiv:2106.03945 · Physical review. B./Physical review. B

Electric-field noise in a high-temperature superconducting surface ion trap

Philip C. Holz, Kirill Lakhmanskiy, Dirk Rathje, Philipp Schindler, Yves Colombe, R. Blatt

Published Aug 27, 2021• 5 citations
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Abstract

Scaling up trapped-ion quantum computers requires new trap materials to be explored. Here we present experiments with a surface ion trap made from the high-temperature superconductor YBCO, a promising material for future trap designs. We show that voltage noise from superconducting electrode leads is negligible within the sensitivity ${S}_{V}=9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{2}/\mathrm{Hz}$ of our setup, and for lead dimensions typical for advanced trap designs. Furthermore, we investigate the frequency and temperature dependence of electric-field noise above a YBCO surface. We find a $1/f$ spectral dependence of the noise and a nontrivial temperature dependence, with a plateau in the noise stretching over roughly $60\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The onset of the plateau coincides with the superconducting transition, indicating a connection between the dominant noise and the YBCO trap material. We exclude the YBCO bulk as origin of the noise and suggest further experiments to decide between the two remaining options explaining the observed temperature dependence: noise screening within the superconducting phase, or surface noise activated by the YBCO bulk through some unknown mechanism.

DOI
10.1103/physrevb.104.064513

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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 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 from ion-trap electrode surfaces can limit the fidelity of multiqubit entangling operations in trapped-ion quantum information processors and can give rise to systematic errors in trapped-ion optical clocks. The underlying mechanism for this noise is unknown, but it has been shown that the noise amplitude can be reduced by energetic ion bombardment, or ``ion milling,'' of the trap electrode surfaces. Using a single trapped 88Sr+^{88}\mathrm{Sr}^{+}88Sr+ ion as a sensor, we investigate the temperature dependence of this noise both before and after ex situ ion milling of the trap electrodes. Making measurements over a trap electrode temperature range of 4 K to 295 K in both sputtered niobium and electroplated gold traps, we see a marked change in the temperature scaling of the electric-field noise after ion milling: power-law behavior in untreated surfaces is transformed to Arrhenius behavior after treatment. The temperature scaling becomes material-dependent after treatment as well, strongly suggesting that different noise mechanisms are at work before and after ion milling. To constrain potential noise mechanisms, we measure the frequency dependence of the electric-field noise, as well as its dependence on ion-electrode distance, for niobium traps at room temperature both before and after ion milling. These scalings are unchanged by ion milling.
Electric-field noise in microfabricated surface ion traps contributes to gate errors in trapped-ion quantum computers, but this noise has been challenging to predict. Here, we present a method to predict the electric-field noise arising from fluctuations in the bulk of dielectric and metallic materials. The method is valid for arbitrary trap geometries, and the only relevant material property is energy loss. We apply it to an ion trap with an electrically floating electrode and find the contribution from dielectrics to be dominant. Our model predicts the observed noise for this trap within a factor of four. In addition, we apply our model to typical surface trap designs and analyze how the electrode structures shield the noise.
Electric-field noise from ion-trap electrode surfaces can limit the fidelity of multiqubit entangling operations in trapped-ion quantum information processors and can give rise to systematic errors in trapped-ion optical clocks. The underlying mechanism for this noise is unknown, but it has been shown that the noise amplitude can be reduced by energetic ion bombardment, or “ion milling,” of the trap electrode surfaces. Using a single trapped ⁸⁸Sr⁺ ion as a sensor, we investigate the temperature dependence of this noise both before and after ex situ ion milling of the trap electrodes. Making measurements over a trap electrode temperature range of 4 K to 295 K in both sputtered niobium and electroplated gold traps, we see a marked change in the temperature scaling of the electric-field noise after ion milling: power-law behavior in untreated surfaces is transformed to Arrhenius behavior after treatment. The temperature scaling becomes material-dependent after treatment as well, strongly suggesting that different noise mechanisms are at work before and after ion milling. To constrain potential noise mechanisms, we measure the frequency dependence of the electric-field noise, as well as its dependence on ion-electrode distance, for niobium traps at room temperature both before and after ion milling. These scalings are unchanged by ion milling.