100-Fold Reduction of Electric-Field Noise in an Ion Trap Cleaned with In Situ Argon-Ion-Beam Bombardment

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.

Figure 1

Micrograph of ion-trap electrodes. The radio-frequency (rf) and static-potential electrodes are microfabricated using a $10\mathrm{\text{−}}\mu \mathrm{m}$-thick, electroplated Au film with $5\mathrm{\text{−}}\mu \mathrm{m}$ gaps between the electrodes (darker areas). The red dot represents the location of the ion.

Figure 2

Auger electron spectra of Au electrode surfaces. The vertical axis (same scale for both traces) displays the differential Auger electron intensity, $dI/dE$. Before ${\mathrm{Ar}}^{+}$ bombardment, carbon is observed as the only significant contaminant, probably resulting from oxygen-free hydrocarbon contamination. Oxygen, if present, would be indicated by a feature near 515 eV. The spectrum after ${\mathrm{Ar}}^{+}$ bombardment, offset downward by 1.7 units for clarity, shows only features that indicate a Au surface free of these contaminants. Typical ${\mathrm{Ar}}^{+}$ cleaning conditions in the surface analysis system are 0.5–2 kV beam voltage and $100–300\mathrm{C}/{\mathrm{m}}^2$ over $\sim 45$ minutes at $6\times {10}^{-3}\mathrm{Pa}$ Ar pressure. Inset: enhanced, false-color image of the ${\mathrm{Ar}}^{+}$ beam ($30°$ incidence from normal) in side view of the initial ion-trap apparatus, operated briefly at $3\times {10}^{-2}\mathrm{Pa}$ Ar pressure for imaging.

Figure 3

Heating rate measurements (a) before and (b) after treatment. Heating rates are obtained by measuring the average number of motional quanta, $\overline{n}$, with a variable delay time after initial laser cooling [5].

Figure 4

Heating rate vs trap frequency, fitted with $\dot{\overline{n}}\sim 1/{\omega }^{\alpha }$. The power-law exponents $\alpha =2.53\pm 0.07$ and $\alpha =2.57\pm 0.04$, (a) before and (b) after treatment in the initial experiments, respectively, are consistent with various adsorbate-induced noise models [4, 10, 11, 12].

Figure 5

Normalized electric-field noise spectral density, $\omega S_E(\omega )$, plotted versus ion-electrode distance, $d$. Data for initial experiments in this work (red crosses) indicate a reduction of anomalous heating by 2 orders of magnitude after ${\mathrm{Ar}}^{+}$-beam treatment. Data from other experiments employing room-temperature trap electrodes are depicted with filled symbols, whereas data from electrodes at cryogenic temperatures are represented with open symbols. The dash-dotted lines indicate the $d^{-4}$ trendlines, predicted by small-patch noise models [5, 10, 11] (vertical position not relevant).