Stabilization of a specific configuration of a two-species ion chain by selective vibrational excitation

Two-species ion chains supply an access to the ions, which cannot be laser-cooled alone. The ions are cooled by coulomb coupling with laser-cooled ions, and manifest themselves as dark sites in the coulomb crystal. Localization within an optical wavelength can also be achieved. The applications include an optical clock based on atomic ions with a relative uncertainty in the order of 10^-18 and high-resolution vibrational spectroscopy of a molecular ion with an accuracy of 2ppb. We report on an experimental study of stabilizing a selected configuration of an ion chain consisting of ¹¹⁵In⁺ and ⁴⁰Ca⁺. Unwanted configurations are destabilized by applying amplitude modulation onto the Ca⁺ laser-cooling beam at frequencies resonant to the vibrational mode. As a result only the target configuration survives.