Observation of the $^{1}$S$_{0}$ -- $^3$P$_0$ clock transition at 578 nm in atomic Yb

We report the first observation of the strongly forbidden $6s^2$ $^{1}$S$_{0}$ -- $6s6p$ $^3$P$_0$ transition near 578 nm in atomic Yb, which is weakly† allowed in the odd isotopes ($^{171, 173}$Yb) through internal hyperfine coupling† of the nuclear spin.† This transition has been proposed as one of the† primary† candidates for future optical frequency standards using alkaline earth atoms [1]. In our experiment, a 578 nm laser beam strikes cold Yb atoms† held in a magneto-optical trap (MOT). When this light is tuned to resonance† with the clock transition, a decrease in the MOT fluorescence is detected† as atoms are pumped into the metastable $^3$P$_0$ state and escape† from the trap. By chopping the MOT and clock beams out of phase, we avoid line shifts and broadening due to near-resonant trap light. Nearby 578 nm iodine lines, observed by Doppler-free† saturated absorption, are used as a frequency reference. †We find the $^ {171}$Yb transition to be about $2.5$ GHz below the first hyperfine component of the iodine line 1852. We will present our measurements of the clock transition frequency, including a determination of the relative shift between the $^{171}$Yb and $^{173}$Yb isotopes.[1]S. G.† Porsev, A. Derevianko, E. N. Fortson, Phys. Rev. A {\bf 69}, 021403(R)† (2004); H. Katori, in {\it Proc. 6th Symposium Frequency Standards and Metrology}, edited by P. Gill (World Scienti.c, Singapore, 2002), pp. 323-330