A two-electron atom of ytterbium (Yb) is remarkable in the study of ultracold dilute gases in that it offers many interesting possibilities. The two valence electrons result in singlet and triplet states connected by extremely narrow intercombination transitions. The existence of rich varieties of stable isotopes of five bosons and two fermions will allow us to study various interesting quantum degenerate gases. In this presentation, we will report our recent experiments on quantum degenerate Yb atoms loaded into an optical lattice. In particular, we have studied the superfluid-Mott insulator transition by high-resolution laser spectroscopy using the ultranarrow intercombination transition. We have observed a drastic change of the spectra when we increase the lattice depth. The novel interatomic interaction between the ground and metastable triplet states is also revealed from the spectroscopic results. The resonant variation of the scattering length with a magnetic field is observed, but is not explained by a simple Feshbach resonance. Prospects are also discussed.
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