Achievement Of Sympathetic Cooling Of Lithium Ions With Calcium Ions

The fine structure constant-a physical quantity called the magic number, has attracted the attention of scientists. As the simplest system, Li+has significant application in determining the fine-structure constant and verification of quantum electrodynamics (QED) theory because that the spectrum of Li+ion can be calculated accurately in theory. But the direct laser cooled lithium ion has all sorts of drawbacks, so the sympathetic cooling method is adopt to cool the lithium ion. However, direct laser cooling of trapped Li+ions has some disadvantages in fluorescence detection, so sympathetic cooling technique is an alternative option. Therefore, the temperature of lithium ions can be effectively reduced in order to get the spectrum data with sufficient high precision to accurately determine the fine structure constant. The main content of this thesis concerning with the sympathetically cooled Li+ion is divided into the following several aspects:1. A set of ion trap system suitable for trapping both 40Ca+and 7Li+ ion was built, the ion trap was placed in the vacuum chamber with the vacuum degree up to 5×10-8 Pa. And a set of radio frequency high voltage source was developed, the frequency and voltage are 2π×7.20 MHz and 600 V respectively.2. Simulated the geometrical factor of the linear ion trap using the finite element analysis method, which is used to determine the work point of the trap for trapping Calcium and Lithium ions simultaneously. The pseudopotential well depths of different trapping field are also given.3. A set of laser system for the production, laser cooing of Ca+ ions and the production of Li+ ion was set up. All the lasers passed through the trap center, and the 397 nm and 866 nm laser for cooling Ca+ are stabilized by transfer cavity with long-term stability better than 2MHz/hour.4. Trapping and cooling of Ca+ ions were realized, and the fluorescence signal was observed by PMT and EMCCD respectively. The micromotion amplitude of ions was reduced using position compensation method, and the Coulomb crystal Ca+ ions was finally cooled to the order of mK. The second order structural phase transformation of Ca+ion was studied. The ion’s temperature was obtained by means of molecular dynamics simulation. Besides, by analysis of the temperature of ion crystal under different structures, we obtained the transition point of second order structural phase transition.5. In theory, the sympathetic cooling limit temperature is obtained by analysis of the single Ca+ion and single Li+ion in the equilibrium state. In experiment, we obtained ultracold Li+ions in a linear Paul trap by sympathetic cooling with Ca+ ions. Mixed ionic crystal is obtained, and the motion frequency spectra of the ions were measured. The temperature of lithium ion crystal was estimated to be mK by means of molecular dynamics simulation.