Absolute frequency measurement of the 2S1/2?2F7/2 electric octupole transition in a single ion of 171Yb+ with 10?15 fractional uncertainty

An absolute frequency measurement has been made of the S-2(1/2)-F-2(7/2) electric octupole transition in a single ion of Yb-171(+). The implementation of a diode-based probe laser stabilized to this highly forbidden transition has resulted in an improvement of more than one order of magnitude upon the lowest published uncertainty. After correcting for systematic shifts, the frequency was determined to be 642 121 496 772 646.22 ( 67)Hz. This corresponds to a fractional uncertainty of 1.0 x 10(-15).