Subkilohertz comparison of the single-ion optical-clock 2S1/2-2D5/2 transition in two 88Sr+ traps

A simultaneous observation of the optical-clock S-2(1/2)-D-2(5/2) transition at 674 nm, in two separately trapped single Sr-88(+) ions, is reported. Two nominally identical miniature rf Paul traps were used together with a 674-nm sideband-injection-locked extended-cavity diode laser. This "slave" laser was optically phase locked about 650 MHz away from a "master" diode laser, which was itself lacked to a high-finesse, ultra-low-expansion (ULE) cavity. The ULE cavity was temperature-stabilized and suspended in an evacuated enclosure, and provided an "optical flywheel" reference standard with a relative drift rate of better than 1 part in 10(11) per hour. The difference between center frequencies of the single Sr-88(+)-ion 673-nm (445-THz) transition multiplet in two traps was 120(90) Hz (one standard uncertainty). Thus the two trap center frequencies agree to 3 parts in 10(13). [S1050-2947(99)50405-8].