Trapping, Shaping and Isolating of Ion Coulomb Crystals via State-selective Optical Potentials

For conventional ion traps, the trapping potential is close to independent of the electronic state, providing confinement for ions dependent primarily on their charge-to-mass ratio Q/m. In contrast, storing ions within an optical dipole trap results in state-dependent confinement. Here we experimentally study optical dipole potentials for ¹³⁸Ba^+ ions stored within two distinctive traps operating at 532 nm and 1064 nm. We prepare the ions in either the 6S_1/2 electronic ground or the 5D_3/2/ 5D_5/2 metastable excited state and probe the relative strength and polarity of the potential. On the one hand, we apply our findings to selectively remove ions from a Coulomb crystal, despite all ions sharing the same Q/m. On the other hand, we deterministically purify the trapping volume from parasitic ions in higher-energy orbits, resulting in reliable isolation of Coulomb crystals down to a single ion within a radio-frequency trap.