  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.
