88Sr+ 445-THz single-ion reference at the 10(-17) level via control and cancellation of systematic uncertainties and its measurement against the SI second.

We describe experiments and measurements on a trapped and laser-cooled single ion of (88)Sr(+) which, when probed on its reference 5s (2)S(1/2)→4d (2)D(5/2) transition at 445 THz, provides an optical frequency standard of evaluated accuracy outperforming the current realization of the SI second. Studies are presented showing that micromotion-associated shifts of the standard can be reduced to the 10(-18) level and uncertainties in the blackbody-induced shifts for the current system are at the low 10(-17) level due to the relatively well-known polarizability of the strontium ion system and careful choice of the trap structure. The current evaluated systematic shifts for the ion transition are at a fractional uncertainty of 2×10(-17). An absolute frequency measurement performed over a two-month period relative to a maser referenced to the SI second via Global Positioning System time transfer has determined the center frequency for the transition at ν(SD)=444 779 044 095 485.5±0.9  Hz (1σ).