Imaging of Trapped Ions with a Microfabricated Optic for Quantum Information Processing

Trapped ions are a leading system for realizing quantum information processing (QIP). Most of the technologies required for implementing large-scale trapped-ion QIP have been demonstrated, with one key exception: a massively parallel ion-photon interconnect. Arrays of microfabricated phase Fresnel lenses (PFL) are a promising interconnect solution that is readily integrated with ion trap arrays for large-scale QIP. Here we show the first imaging of trapped ions with a microfabricated in-vacuum PFL, demonstrating performance suitable for scalable QIP. A single ion fluorescence collection efficiency of $4.2\pm 1.5\%$ was observed. The depth of focus for the imaging system was $19.4\pm 2.4\mu \mathrm{m}$ and the field of view was $140\pm 20\mu \mathrm{m}$. Our approach also provides an integrated solution for high-efficiency optical coupling in neutral atom and solid-state QIP architectures.

Figure 1

PFL based optical interconnects for trapped-ion QIP. (a) Proposed highly parallel readout of trapped-ion qubits with PFLs [12]. Fluorescence from ions trapped at many sites on a microfabricated trap [8, 10] is efficiently coupled into single optical modes by an array of microfabricated PFLs on a single substrate. (b) Electron microscope image near the center of the PFL. The 390 nm groove depth induces $\pi$ phase shifts at $\lambda =369.5\mathrm{nm}$. (c) Diagram of the experimental apparatus. A single ${}^{174}\mathrm{Yb}^{+}$ ion is trapped in the rf quadrupole field (dashed lines) produced between two tungsten needles and illuminated with resonant light at $\lambda =369.5\mathrm{nm}$ (arrow). Light scattered from the ion is collimated with an in-vacuum PFL and imaged onto a cooled CCD camera (not shown).

Figure 2

Imaging performance of the PFL in our apparatus. (a) Image of a single trapped ${}^{174}\mathrm{Yb}^{+}$ ion using the in-vacuum binary PFL with 12% solid angle coverage ($\mathrm{NA}=0.64$). Ion size is limited by residual ion motion. (b) Depth of focus measurement for ion image. Ion image size (FWHM) is plotted as a function of focal position. Fitting the size to $y_0\sqrt{1+(z/w_0{)}^2}$ gives a depth of focus $2w_0=19.4\pm 2.4\mu \mathrm{m}$ and a minimum spot size $y_0=3.7\pm 0.3\mu \mathrm{m}$. Uncertainty is dominated by pixel quantization.