Agreement between two 88Sr+ optical clocks to 4 parts in 1017

The frequencies of two nominally identical Sr-88(+) trapped single ion optical clocks, based on the 674 nm 5s(2)S(1/2)-4d(2)D(5/2) electric quadrupole clock transition, have been compared over a period of nine months. The frequencies of the two clocks were found to agree within a total uncertainty of 4 x 10 (17), demonstrating that the individual Sr-88(+) optical clocks are reproducible at the 3 x 10(-17) level. The absolute frequency of the clock transition was measured to be f = 444 779 044 095 486.71(24) Hz using an optical frequency comb referenced to a cesium fountain primary frequency standard. The standard uncertainty of 0.24 Hz (5.3 x 10(-16) of the optical frequency) is dominated by measurement statistics and cesium fountain systematics and is around four times lower than previously published.