Sympathetic ground state cooling and time-dilation shifts in an $^{27}\text{Al}^+$ optical clock

TL;DR

This paper demonstrates near-ground-state sympathetic cooling of a $^{27} ext{Al}^+$ optical clock using Raman sideband cooling of ${^{25} ext{Mg}^+}$ ions, significantly reducing the motion-induced time-dilation shift and its uncertainty.

Contribution

It introduces a method for effective 3D ground-state cooling of $^{27} ext{Al}^+$ clocks via ${^{25} ext{Mg}^+}$ ions, reducing time-dilation shift uncertainties by 50 times.

Findings

Achieved near-ground-state cooling of $^{27} ext{Al}^+$ ions.

Measured a secular motion time-dilation shift of ${-(1.9 imes 10^{-18})}$.

Reduced the uncertainty in time-dilation shift by a factor of 50.

Abstract

We report on Raman sideband cooling of ${^{25}\text{Mg}^+}$ to sympathetically cool the secular modes of motion in a $^{25}\text{Mg}^+$-$^{27}\!\text{Al}^+$ 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 ${-(1.9 \pm 0.1)\times 10^{-18}}$ for an ${^{27}\text{Al}^+}$ 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 ${^{27}\text{Al}^+}$ clocks.