February 1, 2016
      
                  
        
  Author(s)
  Avinash  Kumar,   Neil  Anderson,   William D. Phillips,   Stephen P. Eckel,   Gretchen K. Campbell,   Sandro  Stringari
 
       
            
    
    
        The Doppler effect, the shift in the frequency of sound due to motion, is present in both classical gases and quantum superfluids. Here, we perform an in-situ, minimally destructive measurement, of the persistent current in a ring-shaped, superfluid Bose