January 31, 2017
      
                  
        
  Author(s)
  David R. Leibrandt,   Samuel M. Brewer,   Jwo-Sy  Chen,   Aaron M. Hankin,   David B. Hume,   David J. Wineland,   Chin-Wen  Chou
 
       
            
    
    
        Frequency and its inverse, time, are the most accurately measured quantities. Historically, improvements in the accuracy of clocks have enabled advances in navigation, communication, and science. Since 1967, the definition of the SI second has been based