Sympathetic Cooling of ^40Ca^+–^27Al^+ Ion Pair Crystal in a Linear Paul Trap

The ^27Al^+ ion optical clock is one of the most attractive optical clocks due to its own advantages such as low black-body radiation shift at room temperature and insensitivity to the magnetic drift. However, it cannot be laser-cooled directly in the absence of 167 nm laser to date. This problem can be solved by sympathetic cooling. In this work, a linear Paul trap is used to trap both ^40Ca^+ and ^27Al^+ ions simultaneously, and a single Doppler-cooled ^40Ca^+ ion is employed to sympathetically cool a single ^27Al^+ ion. Thus a 'bright-dark' two-ion crystal has been successfully synthesized. The temperature of the crystal has been estimated to be about 7 mK by measuring the ratio of carrier and sideband spectral intensities. Finally, the dark ion is proved to be an ^27Al^+ ion by precise measuring of the ion crystal's secular motion frequency, which means that it is a great step for our ^27Al^+ quantum logic clock.