Investigation of experimental issues concerning successful operation of quantum-logic-based $$^{27}\hbox {Al}^+$$ ion optical clock

Abstract

Successful operations of quantum-logic-based $$^{27}\hbox {Al}^+$$ 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 $$^{25}\hbox {Mg}^+$$ as the logic ion to ensure smaller time-dilation shift is discussed. We also compare several statistical methods to identify the $$^{27}\hbox {Al}^+$$ ion clock state. Finally, we report the observation of the $$^{27}\hbox {Al}^+$$ ion $${^1S_0} \rightarrow ^{3}{P_0}$$ clock transition based on $$^{25}\hbox {Mg}^+$$ – $$^{27}\hbox {Al}^+$$ ion pair. As a precondition for quantum logic spectroscopy (QLS), both the stretch (STR) mode and the center of mass (COM) mode of the $$^{27}\hbox {Al}^+$$ and $$^{25}\hbox {Mg}^+$$ 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.