Optical Clock Transition in Neutral Ytterbium

0031-9007= We report the direct excitation of the highly forbidden 6s2 S0 $ 6s6p P0 optical transition in two odd isotopes of neutral ytterbium. As the excitation laser frequency is scanned, absorption is detected by monitoring the depletion from an atomic cloud at 70 K in a magneto-optical trap. The measured frequency in 171Yb (F 1=2) is 518 295 836 591:6 4:4 kHz. The measured frequency in 173Yb (F 5=2) is 518 294 576 847:6 4:4 kHz. Measurements are made with a femtosecond-laser frequency comb calibrated by the National Institute of Standards and Technology cesium fountain clock and represent nearly a 106-fold reduction in uncertainty. The natural linewidth of these J 0 to J 0 transitions is calculated to be 10 mHz, making them well suited to support a new generation of optical atomic clocks based on confinement in an optical lattice.