The optical calcium frequency standards of PTB and NIST
  We describe the current status of the Ca optical frequency standards with laser-cooled neutral atoms realized in two different laboratories for the purpose of developing a possible future optical atomic clock.   Frequency measurements performed at the Physikalisch-Technische Bundesanstalt (PTB) and the National Institute of Standards and Technology (NIST) make the frequency of the clock transition of 40Ca one of the best known optical frequencies (relative uncertainty 1.2e-14) and the measurements of this frequency in both laboratories agree to well within their respective uncertainties.   Prospects for improvement by orders of magnitude in the relative uncertainty of the standard look feasible. 
U. Sterr, C. Degenhardt, H. Stoehr, Ch. Lisdat, H. Schnatz, J.   Helmcke, F. Riehle, G. Wilpers, Ch. Oates, L. Hollberg