Surface noise analysis using a single-ion sensor

We use a single-ion electric-field noise sensor in combination with in situ surface treatment and analysis tools, to investigate the relationship between electric-field noise from metal surfaces in vacuum and the composition of the surface. These experiments are performed in a setup that integrates ion trapping capabilities with surface analysis tools. We find that treatment of an aluminum-copper surface with energetic argon ions significantly reduces the level of room-temperature 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 noise levels after treatment are low enough to allow fault-tolerant trapped-ion quantum information processing on a microfabricated surface trap at room temperature.

Figure 1

(a) The vacuum system, integrating a surface trap (center) attached to a filter board (yellow, left), mounted on a 360${}^{\mathrm{o}}$ rotational holder (rotation about the $z$ axis), an Auger and low-energy electron-diffraction spectrometer (bottom), an Ar${}^{+}$ gun (behind the $xy$ plane), an observation channel (above), and a residual gas analyzer (RGA, upper right). The laser direction (blue arrow) is at a 7${}^{\mathrm{o}}$ angle with the trap axis [oriented along $(-1,0,1)$], defined by the radio-frequency trap electrodes (black lines on the trap). For clarity, the center of the coordinate system is offset with respect to the trapping position. (b) Photograph of the mounted trap.

Figure 2

The trap used in this work. (a) Schematic of the trap electrodes. The surface trap has two long, symmetric electrode rails along the trap axis for rf confinement (RF) and 21 electrodes used for the static confinement (E1–E21). The rf electrode can also be biased at a static potential. In the experiments described here, ions were trapped near the center of the trap (at the center of electrodes E5 and E15). The electrodes are wire bonded using 25-$\mu$m-diameter Pd wire at the pads near the periphery of the chip. (b) Optical microscope image of the trapping region. Electrodes E1 to E8 and E11 to E18 are shown. The trapping position, next to electrodes E5 and E15, is marked by an “x.”

Figure 3

(a) Copper and aluminum peaks after the vacuum bake (A) and after Ar${}^{+}$sputter treatment (B). At low energies, the peaks corresponding to oxidized Al($LMM$) and Cu($MVV$) dominate ($V$ represents valence-band electronic states). At higher energies, the Cu($LMM$) and Al($KLL$) peaks are three orders of magnitude weaker, due to poor sensitivity of the analyzer at high energies (note the expanded vertical axis). (b) Auger spectra showing the C($KLL$) and O($KLL$) peaks at different stages of surface treatments: after the bake (red, A), after the first treatment (blue, B), after 40 days in vacuum (black, C), and after the second treatment (green, D). In addition to carbon (270 eV) and oxygen (515 eV), traces of thorium and iridium (energies of 225, 245, and 255 eV) appear in B, a result of outgassing of the Auger filament. Lutetium (320 and 330 eV), possibly from the LEED fluorescent screen, is also visible in B. Traces of argon (200 and 215 eV) are visible in B, C, and D. A palladium peak, coming from the Pd wirebonds used, appears at 335 eV in C. The appearance of the palladium peak is a result of the large width of the Auger electron beam. Vertical axis units for all figures in (a) and (b) are the same. (c) Evolution of the signal strengths of oxygen (blue, circle) and carbon (black, diamond) in (b) normalized to the pretreatment carbon value.

Figure 4

Frequency scaling of motional heating in our trap. Color coding for measured data is the same as in Fig. 3; i.e., red filled circles is pretreatment (A), black open squares is 40 days after Ar${}^{+}$ treatment (C), and green filled triangles is after the second treatment (D). Dashed lines show fits of the form $\dot{\overline{n}}\sim f^{-(1+\alpha )}$ (which corresponds to $S_E\sim f^{-\alpha }$). The extent of each line shows the range of frequencies over which the fit was performed. The solid line shows the expected Johnson noise heating for our setup.

Figure 5

Representative electric-field noise measurements for various ion traps with planar electrode geometries. All measurements were done at room temperature. Each symbol in the label corresponds to a different trap. For measurements where noise was reported over a range of frequencies, we show multiple data points, corresponding to the extreme frequency values.