UC Berkeley

This work presents the techniques used to create a freely rotating trapped-ion Coulomb crystal and to establish quantum coherent control over this rotational motion. This is done using a highly symmetric surface ion trap with circular trapping electrodes. We derive how a trapped-ion rotor couples to coherent laser light, including the motional transition sideband spectrum and the corresponding coupling strengths. We also show how rotational motion modifies the coupling of the usual trapped-ion vibrational motion to laser light. In our experiments, light which is coupled to the rotor's motion propagates nearly normal to the trap surface, which is thus also sensitive to vibrational motion in this direction. We thus also present the design of and benchmark the performance of a Faraday cage which has successfully protected this motion from harmful electric field noise.

A prerequisite to clean, coherent manipulation of the rotational quantum state of our rotor is preparing it in a rapidly rotating state with a small uncertainty in its angular momentum. This rotational state preparation is done with by accelerating a rotating quadrupole field via time-dependent voltages applied directly to the trap electrodes, resulting in rotation frequencies of 100's of kHz with uncertainties within 1 kHz. We show how this procedure allows for creation of superpositions of angular momentum states, and present considerations and measurements pertaining to optimizing this procedure. We find that the coherence of these superpositions is limited by angular momentum diffusion processes induced by coupling to noisy electric field gradients. Careful measurements of these rotor decoherence dynamics demonstrate close agreement with the corresponding theory for the first time.

Finally, we present a proposal to use angular momentum superpositions of our trapped ion rotor as an interferometer in which we may exchange the positions of two ions. This experiment would serve as a test of the symmetrization of their mutual quantum state, and would be sensitive to the phase of the exchange operation without requiring the two particles to coincide spatially. We interpret the physical meaning of this exchange phase measurement and present detailed considerations of potential errors.