Liu Peiliang

Precision spectroscopy with a single Ca-40(+) ion in a Paul trap

Precision measurement of the 4s(2)S(1/2)-3d(2)D(5/2) clock transition based on Ca-40(+) ion at 729 nm is reported. A single Ca-40(+) ion is trapped and laser-cooled in a ring Paul trap, and the storage time for the ion is more than one month. The linewidth of a 729 nm laser is reduced to about 1 Hz by locking to a super cavity for longer than one month uninterruptedly. The overall systematic uncertainty of the clock transition is evaluated to be better than 6.5 x 10(-16). The absolute frequency of the clock transition is measured at the 10(-15) level by using an optical frequency comb referenced to a hydrogen maser which is calibrated to the SI second through the global positioning system (GPS). The frequency value is 411 042 129 776 393.0(1.6) Hz with the correction of the systematic shifts. In order to carry out the comparison of two Ca-40(+) optical frequency standards, another similar Ca-40(+) optical frequency standard is constructed. Two optical frequency standards exhibit stabilities of 1 x 10(-14) tau(-1/2) with 3 days of averaging. Moreover, two additional precision measurements based on the single trapped Ca-40(+) ion are carried out. One is the 3d(2)D(5/2) state lifetime measurement, and our result of 1174(10) ms agrees well with the results reported in [Phys. Rev. A 62 032503 (2000)] and [Phys. Rev. A 71 032504 (2005)]. The other one is magic wavelengths for the 4s(2)S(1/2)-3d(2)D(5/2) clock transition; lambda(vertical bar mj vertical bar =1/2) = 395.7992(7) nm and lambda(vertical bar mj vertical bar =3/2) = 395.7990 (7) nm are reported, and it is the first time that two magic wavelengths for the Ca-40(+) clock-transition have been reported.

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.

Correlation between the magic wavelengths and the polarization direction of the linearly polarized laser in the Ca+ optical clock*

The magic wavelengths for different Zeeman components are measured based on the Ca-40(+) optical clock. The dynamic dipole polarizability of a non-zero angular moment level has correlation with the polarization direction of the linearly polarized laser beam, and we show that the four hyperfine structure levels of 4s(1/2,m=+/-1/2) and 3d(5/2,m=+/-1/2) for Ca-40(+) have the same dynamic dipole polarizability at the magic wavelength and a certain polarization direction. In addition, the existence of a specific direction of polarization may provide a new idea for improving the precision of magic wavelength measurement in experiment.

The comparison of the 40 Ca + ion clocks with the improvement of the clock laser stability

Optical frequency standards have recently been significantly developed thanks to the invention of optical frequency comb technology and ultra-narrow-linewidth lasers. Optical frequency standards are expected to be used to test fundamental physical theory and the variance of the physical constant.

Determining the structural phase transition point from the temperature of Ca-40(+) Coulomb crystal

We observed the linear-to-zigzag structural phase transition of a Ca-40(+) crystal in a homemade linear Paul trap. The values of the total temperature of the ion crystals during the phase transition are derived using the molecular-dynamics (MD) simulation method. A series of simulations revealed that the ratio of the radial to axial secular frequencies has a dependence on the total temperature that obeys different functional forms for linear and zigzag structures, and the transition point occurs where these functions intersect; thus, the critical value of the ratio of secular frequencies that drives the structure phase transition can be derived.