Progress toward an Ytterbium Optical Clock

We report progress toward an optical frequency standard based on the narrow $^{1}$S$_{0} \quad \leftrightarrow \quad ^{3}$P$_{0}$ transition of odd ytterbium isotopes at 578 nm. Recoil-free spectroscopy of neutral atoms tightly confined to an optical lattice could support fractional frequency instabilities of 10$^{-18 }$in one second. Loading the lattice from cold atomic clouds will provide large numbers of atoms and good signal-to-noise. ~We present initial results of two stage cooling and trapping experiments. ~The first stage consists of collecting and cooling atoms from an atomic beam using the broad 399 nm line and InGaN diode lasers. Second stage cooling to $\sim $50 mK is performed on the 556 nm intercombination line using frequency-doubled light from a narrow linewidth infrared fiber laser. A single pass of 1 W fundamental power through a periodically-poled lithium niobate crystal produces $\sim $30 mW of 556 nm light. With a fast linewidth of $\sim $60 kHz, locking the laser frequency to the atomic beam fluorescence is sufficient for cooling on this $\sim $180 kHz transition. This all-solid-state laser architecture enables simple and robust production of large numbers of atoms at ultracold temperatures for precision spectroscopy in a lattice.