Improvements of an 171 Yb Optical Lattice Clock at KRISS Dai-Hyuk Yu, Sangkyung Lee, Won-Kyu Lee, Chang Yong Park, Sang Eon Park, Myoung-Sun Heo, Jongchul Mun, Sang-Bum Lee, and Taeg Yong Kwon Center for Time and Frequency, Korea Research Institute of Standards and Science, Daejeon 305-340, Rep. of Korea dhyu@kriss.re.kr Abstract — The development of an Yb optical lattice clock at the Korea Research Institute of Standards and Science (KRISS) is reported with current systematic uncertainty of 2.9×10 -16 . A highly stable clock laser at 578 nm was developed with a short- term linewidth of 3.5 Hz. The laser was locked to the clock transition and the fractional frequency stability was 2.0×10 -15 at 1 s. Collisional frequency shift for Rabi spectroscopy was theoretically and experimentally investigated and the uncertainty was greatly reduced to 2.9×10 -17 . We identified conditions to further reduce the collisional shift uncertainty to 10 -18 level. Index Terms — Optical clock, optical lattice, Ytterbium, collisional shift, sub-Hz laser. I. INTRODUCTION Optical clocks using the narrow optical transitions of neutral atoms in an optical lattice and a single ion in a Paul trap have been actively developed and recently some of them reached the uncertainty levels below 10 -17 [1][2]. Optical lattice clocks, expected to show much better stability compared to the ion clock due to simultaneous interrogation of many atoms, have shown better stability surpassing the ion clock [3]. Not only for future new definition of the second, optical clocks, due to their ultimate accuracy, also serve to probe both temporal variations of fundamental constants like the fine structure constant and the coupling of fundamental constants to forces like, e.g., gravity. Due to their outstanding stability, optical clocks are also considered to be possible key instruments that could measure relativistic geodesy. Multiple atom species (Yb, Sr, Hg, Mg) are being actively investigated worldwide. In this paper, we report the current status of Yb lattice clock development at the Korea Research Institute of Standards and Science (KRISS). We improved the systematic uncertainty of the clock to 2.9×10 -16 . Previous dominant uncertainty factor, collisional frequency shift for Rabi spectroscopy was theoretically and experimentally investigated and the uncertainty was reduced to 2.9×10 -17 . We identified conditions to further reduce the collisional shift uncertainty to 10 -18 level. II. EXPERIMENT Most of the experimental setup is the same to the previous paper and details can be found in the reference [4]. The strong transition at 399 nm was used for Zeeman slowing and for the first stage cooling of the magneto-optical trap (MOT). The narrow-linewidth (182 kHz) transition at 556 nm was used for second-stage of the MOT. A linearly-polarized, vertically- aligned lattice laser at 759 nm was used to form a cavity enhanced optical lattice. After the MOT laser had been turned off, roughly 10 4 ~10 5 atoms were trapped in a 1-D optical lattice. The Yb atoms were spin-polarized by using a 556 nm lasers and its purity was over 95%. The lattice-trapped atom is interrogated by a Rabi pulse. The bias magnetic field was typically 0.12 mT inducing 460 Hz splitting between the two transitions. Because the excited state lifetime (~20 s) is much longer than the measurement cycle time (~1 s), excited atoms were transferred to the ground state by use of the 1389 nm repumping laser to get the normalized excitation probability. Fig. 1. Clock transition spectrum with 80 ms probe time. Fig. 1 shows a typical spectrum of the clock transition with one of the spin state obtained by using a single scan of the clock laser with 80 ms probe time. The spectrum shows almost Fourier-transform limited linewidth of 10.3 Hz. The laser was locked to the clock transition and the fractional frequency stability was 2.0×10 -15 at 1 s. We needed 4 s (4 cycles) to measure the clock transition frequency. III. ACCURACY EVALUATION The uncertainty of the blackbody radiation (BBR) shift was estimated to be 2.0×10 -16 by using the recent measured differential polarizability value [5] for 2 K uncertainty of the temperature of the vacuum chamber. Since we phase locked the lattice laser to the optical frequency comb at the magic wavelength [6] and used the lattice potential depth of 200 Er