July 14, 2025
      
                  
        
  Author(s)
  Mason  Marshall,   Daniel  Rodriguez Castillo,   Willa  Dworschack,   Alexander  Aeppli,   Kyungtae  Kim,   Dahyeon  Lee,   William  Warfield,   Nicholas  Nardelli,   Tara  Fortier,   Jun  Ye,   David Ray  Leibrandt,   David  Hume
 
       
            
    
    
        We report a single-ion optical atomic clock with fractional frequency uncertainty of 5.5 x 10^-19 and frequency stability of 3.5 x 10^-16/sqrttau/s}, based on quantum logic spectroscopy of a single 27Al+ ion. A co-trapped 25Mg+ ion provides sympathetic