Subkilohertz comparison of the single-ion optical-clock2S1/2−2D5/2transition in two88Sr+traps

A simultaneous observation of the optical-clock ${}^2{S}_{1/2}{-}^2{D}_{5/2}$ transition at 674 nm, in two separately trapped single ${}^{88}{\mathrm{Sr}}^{+}$ 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 locked 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 ${}^{88}{\mathrm{Sr}}^{+}$-ion 674-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 ${10}^{13}.$