Sympathetic Ground State Cooling and Time-Dilation Shifts in an super(27) Al super(+) Optical Clock

We report on Raman sideband cooling of super(25) Mg super(+) to sympathetically cool the secular modes of motion in a super(25) Mg super(+)? super(27) Al super(+) two-ion pair to near the three-dimensional (3D) ground state. The evolution of the Fock-state distribution during the cooling process is studied using a rate-equation simulation, and various heating sources that limit the efficiency of 3D sideband cooling in our system are discussed. We characterize the residual energy and heating rates of all of the secular modes of motion and estimate a secular motion time-dilation shift of ?(1.9+ or -0.1)x10 super(?18) for an super(27) Al super(+) clock at a typical clock probe duration of 150 ms. This is a 50-fold reduction in the secular motion time-dilation shift uncertainty in comparison with previous super(27) Al super(+) clocks.