Evidence for multiple mechanisms underlying surface electric-field noise in ion traps

J. A. Sedlacek, J. Stuart, D. H. Slichter, C. D. Bruzewicz, R. McConnell, J. M. Sage, and J. Chiaverini

Phys. Rev. A 98, 063430 – Published 27 December 2018

Abstract

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 ${}^{88}{Sr}^{+}$ 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.

Received 24 September 2018

DOI:https://doi.org/10.1103/PhysRevA.98.063430

Physics Subject Headings (PhySH)

Atomic & molecular processes in external fields Quantum gates Quantum information with trapped ions Quantum sensing

Trapped ions

Atom & ion cooling

Quantum Information, Science & TechnologyAtomic, Molecular & Optical

Authors & Affiliations

J. A. Sedlacek^1,*, J. Stuart^1,2, D. H. Slichter³, C. D. Bruzewicz¹, R. McConnell¹, J. M. Sage^1,†, and J. Chiaverini^1,‡

¹Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02421, USA
²Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
³National Institute of Standards and Technology, Boulder, Colorado 80305, USA

^*Present address: Honeywell, Golden Valley, Minnesota, USA.
^†jsage@ll.mit.edu
^‡john.chiaverini@ll.mit.edu

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Vol. 98, Iss. 6 — December 2018

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Images

Figure 1
Electroplated gold traps used in this work. The figure is a photograph of the 1-cm-square trap chip attached and wire-bonded to the transfer stage, which is then mounted in either the ion-milling or experimental chamber. The aluminum cover, below the level of the trap surface, is meant to reduce sputtering of the trap electrode leads and interposer boards underneath. The inset is a micrograph of the central region of the trap electrodes; the RF-electrode rails are labeled, and all others are DC control electrodes. The ion is trapped $50 (1) μ m$ above the center of the linear trap section shown here. The niobium traps used in this work were of the same design.
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Figure 2
X-ray photoelectron spectroscopy (XPS) of the traps used in this work. Each graph shows data for typical solvent-cleaned chips before milling, after milling in the XPS chamber [2 keV ${Ar}^{+}$ ions, with a flux density of $6.4 \times 10^{- 1} (C / m^{2}) / s$ , for 2 min], and after a 30 min reexposure to atmosphere. Upper (lower) row is Nb (Au). Left: Nb 3d (Au 4f) peaks; center: C 1s peak; right: O 1s peak. After milling of Nb, the primarily niobium-pentoxide surface is removed, revealing metallic niobium peaks, while the carbon and oxygen present on the surface, both in carbon-containing compounds and in the oxide, are also eliminated. After reexposure, a mixture of niobium and niobium pentoxide is present, and carbon-containing compounds reappear, but at a lower level (visible in both the center and right panels of the top row). The O peak is the combination of a narrower, lower-binding-energy metallic oxide peak, and a broader higher-binding-energy peak that we associate with hydrocarbons or carbonates. After milling of Au, the carbon and oxygen present on the surface are eliminated. We associate these with carbon-containing compounds in part due to the O peak shift (cf. the O peak in niobium, upper right panel). After reexposure, some contamination returns, but the Au peaks remain single-component in nature; i.e., in contrast to Nb, we see no evidence of oxidation. The peak at slightly higher binding energy than the O peak is the Au 4p peak. Binding energy is referenced to the adventitious carbon C 1s peak at 284.8 eV. While the parameters of the milling done in the XPS chamber, particularly the higher ${Ar}^{+}$ ion flux, lead to a higher material removal rate than the ESIM, we believe these spectra are representative of what would be observed after ESIM since the ion energy and dose are essentially equivalent. The higher background levels visible in the right two panels in the upper row are primarily due to photoelectrons from Nb atoms which have lost various amounts of energy due to inelastic scattering on their way out of the sample. There are more such electrons in the “After milling” and “After reexposure” cases since there is a higher density of Nb atoms near the surface, due to the smaller amount of oxide and carbon-containing compounds, in these cases. The high-kinetic-energy edge of these broad backgrounds, appearing just to the left of the Nb peaks, can be seen in the upper left panel at high binding energy.
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Figure 3
The heating-rate plateauing behavior after increasing amounts of ex situ ion milling for two nominally identical gold traps (labeled A, depicted by the open symbols, and B, depicted by the closed symbols) with electrodes at 295 K and 4 K; here the heating rate is measured on the axial mode at 1.3 MHz. For each time step, each trap was exposed to air and transferred to and from the milling chamber. The milling time represents the total integrated time that the trap was milled. With the exception of duration, every milling step used nominally the same parameters: $5 \times 10^{- 6} Torr Ar$ , an ion beam energy of $2 keV$ , and an ion flux density of $3 \times 10^{- 2} (C / m^{2}) / s$ . The lines connecting data points are intended as a guide to the eye. Similar data (not shown) were acquired for a niobium trap (trap C), with a plateau time in that case of approximately 80 min.
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Figure 4
Comparison of temperature dependence of heating rates measured on the 1.3 MHz axial mode before and after ex situ ion milling for both electroplated gold (top, milling time 60 min) and sputtered niobium (bottom, milling time 100 min). The solid (red) lines are fits of the round points to Eq. (2), and the dashed (blue) lines are fits of the triangular points to Eq. (3). Key fit parameters for the top [bottom] graph: a pre-ESIM scaling exponent $β$ of 1.53(6) [1.48(7)], a pre-ESIM power-law thermal-activation temperature scale $T_{P}$ of 9(2) K [10(2) K], and a post-ESIM Arrhenius activation temperature scale $T_{0}$ of 41(9) K [63(4) K]. The post-ESIM heating rate in the gold trap was also measured at a trap frequency of 660 kHz (not shown), yielding an Arrhenius fit with a temperature scale $T_{0}$ of 51(11) K, equal within error to that determined from the 1.3 MHz heating rate data. Initial and ESIM plateau data for traps A and B (C) are also displayed in the top (bottom) figure to show trap-to-trap variability for each material. The right axes are translated to electric-field noise spectral density via Eq. (1).
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Figure 5
Frequency (top panel) and distance (bottom panel) scaling of ion heating rates in Nb traps at room temperature before and after ex situ ion milling (ESIM). The round, solid (red) data points are from a previous measurement [32] and were taken without ESIM. Using two traps of the same design (labeled F and G) as in that work, data were taken after ESIM [open circles (black) and triangles (green)]. The ion-electrode distance was $64 μ m$ for the measurement as a function of frequency (top), and the trap frequency $f$ was $860 kHz$ for the measurement as a function of distance. The lines are power-law fits with exponents as depicted in the legend. The traps used for these measurements are of a different electrode design than those used for the temperature-dependent measurements in this work (see Ref. [32] for details), though they were made in the same process run on the same wafers.
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