Integrated Optical Addressing of a Trapped Ytterbium Ion

We report on the characterization of heating rates and photoinduced electric charging on a microfabricated surface ion trap with integrated waveguides. Microfabricated surface ion traps have received considerable attention as a quantum information platform due to their scalability and manufacturability. Here, we characterize the delivery of 435-nm light through waveguides and diffractive couplers to a single ytterbium ion in a compact trap. We measure an axial heating rate at room temperature of $0.78\pm 0.05$ q/ms and see no increase due to the presence of the waveguide. Furthermore, the electric field due to charging of the exposed dielectric outcoupler settles under normal operation after an initial shift. The frequency instability after settling is measured to be 0.9 kHz.

Popular Summary

State-of-the-art trapped-ion atomic clocks are among the best timekeeping devices in the world, losing just 1 s every $300\times {10}^9$ years. The operation of such clocks relies upon tightly confining the individual ions, manipulating their quantum state via lasers, and reading the quantum states with sensitive detectors. Microfabricated “surface traps” provide the necessary electrical fields for trapping ions in a compact and scalable chip and have opened the doors to further integration of light-delivering waveguides and detectors; however, there are several challenges in this integration effort. To overcome those challenges, we here fabricate and characterize a surface trap with integrated waveguides.

Previous efforts at integrating surface traps with waveguides have run into three problems. First, many ions emit ultraviolet light whose transmission through standard waveguides is inefficient. Second, ions confined by microfabricated traps experience anomalous heating, leading to detrimental effects that are expected to increase in dielectric materials used for waveguides and detectors. Finally, photoinduced charging from ultraviolet light on the dielectric materials is expected to impact the performance and operation of the clock.

In our setup, we deliver 435-nm light to a trapped ytterbium ion and characterize the proximal electric field and heating rate. While we do observe trap frequency shifts due to dielectric charging, remarkably, we see no increase in the heating rate over the dielectric output coupler, despite having the lowest published ion-surface distance to date.

Our study offers encouraging evidence that photonics integrated chips will be a promising technology for deployable quantum timekeeping.

Figure 1

(a) False-colored scanning electron micrograph (SEM) image of the loading hole region of the trap and dc control electrodes (yellow) in the topmost metal layer. The dark gray dotted line indicates cutout beneath the overhanging metal layer to allow backside loading. (b) False-colored SEM image of the region above the leftmost output diffraction grating, shown in blue beneath the overhanging metal layer. (c) False-colored SEM image of the trapping region of the surface trap showing dc control electrodes (yellow), rf electrodes (orange), and dc bias electrodes (brown). Triangles above and below dc bias electrodes are for beam alignment. The trap region shows the loading hole on the left and four output diffraction gratings. (d) Dark field optical micrograph image of the input diffraction gratings for coupling light to the waveguides. (e) False-colored SEM imaged sectional view of the trap showing the overhanging top metal electrodes and Al reflector below an output grating. (f) SEM image of a diffractive output coupler etched in ${\mathrm{SiN}}_x$ for coupling light from the waveguides and focusing light $20\mu \mathrm{m}$ above the chip surface.

Figure 2

$⟨n⟩$ as a function of time using sideband amplitudes over the loading hole. Insets show the corresponding sideband amplitude measurements for $⟨n⟩=0.1$ (top) and $⟨n⟩=3$ (bottom). These measurements were taken using a free-space clock beam. The red and blue data correspond to the red and blue sidebands.

Figure 3

Heating rate as a function of position overlayed on the trap region, where the output coupler for the waveguide is at $0\mu \mathrm{m}$, the loading hole is at $80\mu \mathrm{m}$, and the maximum overlap between the ion and the waveguide output light is at $70\mu \mathrm{m}$. No measurable change in heating rate is observed as the ion is shuttled from the loading hole to the output grating where it is closest to the exposed dielectric material.

Figure 4

Heating rate as a function of the ion-surface distance. The heating rate has been normalized for ion mass and secular motional trap frequency. We plot our data as the green star compared to previously published data from Refs. [14] (black circles), [24] (blue squares), and [15] (red diamonds). The scaling of each fit is indicated in the legend. Data are reprinted with permission from the authors of Ref. [24].

Figure 5

Heating rate as a function of axial secular motional trap frequency following a $1/f^{2.2\pm 0.3}$ trend consistent with $1/f$ scaling of electric field spectral density [15, 24].

Figure 6

Measured (black) and simulated (blue) Rabi frequency of the $\mathrm{\Delta }{m}_F=+1$ transition using the waveguide-delivered beam as a function of the ion’s distance from the loading hole. The two maxima, indicating highest beam intensities, occur when the ion is $10\mu \mathrm{m}$ and $12\mu \mathrm{m}$ from the output grating. The dip is attributed to variations in the distance between the grating and reflective layer and can be reproduced in simulations that vary this distance by 60 nm (blue). The simulated Rabi frequency is calculated from the simulated waveguide output intensity and is scaled to match the experimental Rabi frequency. Inset: simulated 2D waveguide-output-intensity cross section at the ion height.

Figure 7

Secular frequency vs time as waveguide-coupled light is turned on and off. Secular frequency is measured with a free-space beam tuned to the $\mathrm{\Delta }{m}_F=+2$ transition. Between 500 s and 2500 s, the waveguide-coupled light is turned on (green) and servoed to a constant power. The two opposing charge effects are attributed to the metal and dielectric layers as described in Ref. [25]. Fitted lines for the charging (blue dot dashed) and discharging (red dashed) are described by Eqs. (1) and (2).

Figure 8

Secular frequency over time during typical clock operation, as measured by the waveguide-coupled light tuned to the $\mathrm{\Delta }{m}_F=1$ transition. Inset: histogram of the settled frequency (red points) difference from the fitted curve (blue line). The histogram is fitted to a Gaussian with $\sigma =0.9\mathrm{kHz}$.