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

The Li-7(+) ion is one of the most important candidates for verifying QED theory and obtaining the precise value of the fine-structure constant alpha. 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 Li-7(+) ions to few mK using Ca-40(+) ions in a linear Paul trap. A mixed ion crystal of Ca-40(+) ions and Li-7(+) ions are obtained. We also analyze the motion frequency spectra of pure Ca-40(+) ions and mixed ions.