Sympathetic cooling of $^{40}\textbf{Ca}^+$ - $^{27}\textbf{Al}^+$ ion pair crystal in a linear Paul trap

TL;DR

This paper demonstrates sympathetic cooling of a $^{27}$Al$^+$ ion using a $^{40}$Ca$^+$ ion in a linear Paul trap, enabling the development of an $^{27}$Al$^+$ quantum logic clock without direct laser cooling.

Contribution

It presents the first successful sympathetic cooling of $^{27}$Al$^+$ ions with $^{40}$Ca$^+$ ions in a linear Paul trap, facilitating advanced quantum clock development.

Findings

Achieved cooling to approximately 7 mK temperature.

Successfully identified the dark ion as $^{27}$Al$^+$.

Created a stable two-ion crystal with sympathetic cooling.

Abstract

The $^{27}$Al$^+$ ion optical clock is one of the most attractive optical clocks due to its own advantages, such as low blackbody radiation shift at room temperature and insensitive 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 $^{40}$Ca$^{+}$ and $^{27}$Al$^+$ ions simultaneously, and a single Doppler-cooled $^{40}$Ca$^+$ ion is employed to sympathetically cool a single $^{27}$Al$^+$ 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 $^{27}$Al$^+$ ion by precise measuring of the ion crystal`s secular motion frequency, which means that it is a great step for our $^{27}$Al$^+$ quantum logic clock.