Absolute frequency measurement of the 1S0 - 3P0 transition of 171Yb
  We report the absolute frequency measurement of the unperturbed transition 1S0 - 3P0 at 578 nm in 171Yb realized in an optical lattice frequency standard. The absolute frequency is measured 518 295 836 590 863.55(28) Hz relative to a cryogenic caesium fountain with a fractional uncertainty of 5.4x10-16 . This value is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the International System of Units. 
Marco Pizzocaro, Pierre Thoumany, Benjamin Rauf, Filippo Bregolin,   Gianmaria Milani, Cecilia Clivati, Giovanni A. Costanzo...
physics.atom-ph