Preparation of Ultracold Li⁺ Ions by Sympathetic Cooling in a Linear Paul Trap

The ⁷Li⁺ ion is one of the most important candidates for verifying QED theory and obtaining the precise value of the fine-structure constant α. However, direct laser cooling of trapped Li⁺ ions will lead to strong background fluorescence which will influence the spectrum detection. The sympathetic cooling technique is a good choice to solve the problem. In this work, we report sympathetic cooling of ⁷Li⁺ ions to few mK using ⁴⁰Ca⁺ ions in a linear Paul trap. A mixed ion crystal of ⁴⁰Ca⁺ ions and ⁷Li⁺ ions are obtained. We also analyze the motion frequency spectra of pure ⁴⁰Ca⁺ ions and mixed ions.