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Phys Rev Lett. 2010 Feb 19;104(7):070802. Epub 2010 Feb 17.

Frequency comparison of two high-accuracy Al+ optical clocks.

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Time and Frequency Division, National Institute of Standards and Technology, Boulder, Colorado 80305, USA.


We have constructed an optical clock with a fractional frequency inaccuracy of 8.6x10{-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 {1}S{0}<-->{3}P{0} clock transition is compared to that of a previously constructed Al+ optical clock with a statistical measurement uncertainty of 7.0x10{-18}. The two clocks exhibit a relative stability of 2.8x10{-15}tau{-1/2}, and a fractional frequency difference of -1.8x10{-17}, consistent with the accuracy limit of the older clock.

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