Progress Report of Yb Optical Clocks at KRISS

Accuracy of the Yb optical lattice clock being developed at KRISS has been improved due to better measurements of light shifts and collisional shift. Its accuracy uncertainty is now 5.6×10$^{\mathbf {-17}}$, three times better than our previous result. The second Yb optical lattice clock is now being built for lower blackbody radiation shift using a thermal shield made of copper. Its preliminary estimation of temperature uncertainty is below 50 mK, which corresponds to frequency uncertainty of low 10$^{\mathbf {-18}}$level.