Frequency Comparison of Two High-Accuracy Al+ Optical Clocks

We have constructed an optical clock with a fractional frequency inaccuracy of 8.6 x 10(-18), based on quantum logic spectroscopy of an Al+ ion. A simultaneously trapped Mg+ ion serves to sympathetically laser cool the Al+ ion and detect its quantum state. The frequency of the S-1(0) <-> P-3(0) clock transition is compared to that of a previously constructed Al+ optical clock with a statistical measurement uncertainty of 7.0 x 10(-18). The two clocks exhibit a relative stability of 2.8 x 10(-15)tau(-1/2), and a fractional frequency difference of -1.8 x 10(-17), consistent with the accuracy limit of the older clock.