Figure 2: Optical trapping probability in dependence on the total trap depth U_tot.

For each U_tot, controlled by the power of the dipole laser beam, we prepare the ions in the rf trap (see Fig. 1a), compensate stray electric fields and run a performance test at maximal optical trap depth, U_max≈k_B × 62 mK (black open circle). Subsequently, we reduce U_tot to one of four values (blue filled circles). Under optimized conditions (see text), we observe improved probabilities for optical trapping (red diamond). We choose a trapping duration sufficiently short (Δt=100 μs) to avoid shelving into the |D› manifolds currently limiting the lifetime at U_max to milliseconds. To demonstrate optical trapping, we compensate the stray electric field to a level of ~80 mV m⁻¹ only. The corresponding electric force results in a threshold for the critical optical potential U_crit depicted as the vertical dashed line. Applying our novel compensation method to its current limitation of 8.7 mV m⁻¹ (see text) will allow us to further reduce the laser power of the dipole beam and to increase the lifetime of the ion substantially. The vertical error bars denote the statistical error (1σ) of the respective measurement.