References

• 

  PTB

  Physikalisch Technische Bundesanstalt

  Braunschweig, Germany
• 

  Menlo Systems

  Menlo Systems GmbH

  Planegg, Germany
• 

  NPL

  National Physical Laboratory

  Teddington, UK
• 

  NIST

  National Institute of Standards and Technology

  Boulder CO, U.S.A.
• 

  INRiM

  Istituto Nazionale Di Ricerca Metrologica

  Torino, Italia
• 

  Femto-ST

  Franche-Compté Electronique Méchanique Thermique et Optique - Sciences et Technologies

  Besancon, France
• 

  SJTU

  Jiao Tong University

  Shanghai, China
• 

  LPI

  Lebedev Physical Institute of the Russian Academy of Sciences

  Moscow, Russia
• 

  NICT

  National Institute of Information and Communications Technology - Space-Time Standards Laboratory

  Tokyo, Japan
• 

  KRISS

  Korea Research Institute of Standards and Science

  Daejeon, Republic of Korea