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.
Prof. Boris Blinov, Department of Physics, University of Washington, USA. Prof. Blinov’s research focus on quantum computing, quantum simulation and ion trapping.