Abstract: To study potential avenues for scaling up trapped ion crystal size, we built a modified Paul trap with an oblate aspect ratio designed for trapping radial 2-d ion crystals. In this geometry, the micromotion can only be minimized in the axial direction, transverse to the crystal plane. The in-plane micromotion is unavoidable, its amplitude increasing linearly with distance from the trap center. Thus, different ions in a large crystal experience different micromotion amplitudes, and different Doppler shifts, making traditional laser cooling techniques inefficient. We explored two avenues towards improving the laser cooling efficiency: the two-tone laser cooling and the micromotion-synchronized pulsed Doppler cooling, allowing us to stabilize larger 2-d crystals than with the traditional Doppler cooling.
These experiments were enabled in part by the novel imaging system based on a CMOS camera that allows 1.5 ns temporal resolution of single photon detection. Direct observation of the trapped ion micromotion is attainable with this camera, leading to a simplified way of micromotion detection and compensation. We also demonstrated how this camera can be used for robust, low crosstalk detection of a trapped Ba+ ion qubit register, with average single-qubit detection error of 4.2(1.5) ppm and a four-qubit state detection error of 17(2) ppm, limited by the decay lifetime of the qubit.