Mercury is the heaviest stable atom that could be laser cooled, and have a large nuclear charge number. So it has a distinct
advantage in quantum precision measurement such as fine-structure constant α and permanent electric dipole moment. Due
to its insensitivity of black body radiation, atomic mercury is a good candidate of optical clock. Here we report our recent
development of laser cooling of neutral mercury atom. By cooling the mercury source to about -70°C, an ultra-high
vacuum system was realized to produce ultracold mercury atoms. The commercial frequency quadrupled semiconductor
laser is locked on the cooling transition (<sup>1</sup>S<sub>0</sub>-<sup>3</sup>P<sub>1</sub> transition, wavelength of 253.7 nm) by sub-Doppler frequency modulation
spectroscopy. By the modification with feed-forward method, the UV laser becomes faster tunable and more stable. A
folded beam configuration was used to realize the magneto-optical trap (MOT) because of the shortage of cooling laser
power, and the ultracold mercury atoms were observed by fluorescence detection. All of six rich abundant isotopes have
been observed, and the atom number is about 1.5×10<sup>6</sup> with density of 3.5×10<sup>9</sup> /cm<sup>3</sup> for <sup>202</sup>Hg. With optical shutter and the
programmable system to control the time sequence, the temperature of ultracold atoms can be measured by time of flight
method. To enhance the laser power, a 1014.8 nm fiber laser amplifier was developed, which can work at room temperature.
After two stages of frequency doubling, about 75 mW of 253.7 nm UV laser were generated, and the saturated absorption
spectroscopy of mercury atom was also observed. More power of UV laser could help to trap more atoms in the future.
These works laid a good foundation to realize the mercury lattice clock.