88Sr+ single-ion optical clock with a stability approaching the quantum projection noise limit

A number of optical frequency standards have been evaluated with fractional uncertainties between 10(-17) and 10(-18). Reduction of the statistical uncertainty of a clock comparison to this level is challenging, requiring the best possible stability to avoid excessively long averaging times. We report recent improvements of our Sr-88(+) single-ion standard that have reduced its 1-s Allan deviation from 1 x 10(-14) to 3 x 10(-15), yielding an order of magnitude decrease in averaging time for a given statistical uncertainty level. Among the improvements made are the implementation of a clear-out laser that transfers the ion from the metastable state to the ground state at each cycle, followed by a state preparation step that transfers the ion to the ground-state magnetic sublevel of the probed transition. With these changes, the ion clock transition interacts with the probe laser at every interrogation cycle. The stability of our optical standard is essentially limited by the quantum projection noise for pulse lengths up to approximate to 100 ms.