Investigation of experimental issues concerning successful operation of quantum-logic-based 27Al+ ion optical clock

Successful operations of quantum-logic-based Al-27(+) ion optical clocks have been reported by several groups. But there are still several lingering issues. The first is the proper choice of logic ions, and the second is the proper quantum state identification methods. In this paper, the advantage of using Mg-25(+) as the logic ion to ensure smaller time-dilation shift is discussed. We also compare several statistical methods to identify the Al-27(+) ion clock state. Finally, we report the observation of the Al-27(+) ion S-1(0) -> P-3(0) clock transition based on Mg-25(+)-Al-27(+) ion pair. As a precondition for quantum logic spectroscopy (QLS), both the stretch (STR) mode and the center of mass (COM) mode of the Al-27(+) and Mg-25(+) ion pair are cooled to the vibrational ground state by Raman sideband cooling. The mean phonon number is measured to be 0.10(1) for the STR mode and 0.01(1) for the COM mode, respectively. The heating rate is evaluated to be 0.23(5) phonons/s for the STR mode and 3.0(7) phonons/s for the COM mode, respectively. The clock transition is observed with a full-width-half-maximum (FWHM) of 38(4) Hz at 22 ms interrogation time.