Ever since the mid-twentieth century,atomic clocks have become the most accurate time and frequency standards known.In 1967,the 13th CGPM(Gen-eral Conference of Weights and Mea-sures)decided to replace the defini-tion of the second by 'the duration of 9192631770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the Cs-133 atom'.Optical atomic clocks are the next generation of atomic clocks with clock frequencies ＞10000 times higher(100-1000 THz)and lie in the op-tical spectral range.Their accuracy could be 4 orders of magnitude better.Nowa-days,the best optical atomic clocks have surpassed the best Cs fountain clocks by two orders of magnitude in both accu-racy and stability(Fig.l)[1-4].Scien-tists believe the definition of the second will be referenced to the optical atomic clock transition frequencies in the near future.There are different kinds of opti-cal clocks,including the neutral atomic lattice clocks referenced to Sr,Yb or Hg atoms and single-ion clocks referenced to Sr+,Hg+,Yb+,Al+,Ca+,etc.For dif-ferent referenced atoms or ions,differ-ent technologies have been used,so each one of them has its own advantages and disadvantages[1].Which type of opti-cal clock will be the best? No one re-ally knows.However,the race to find the best clock of the future has already started.