A measurement with 17 significant digits: The ratio of Al+ and Hg+ optical clock frequencies

The single-ion Mercury optical clock at NIST, Boulder has been the world’s most accurate atomic clock for several years.Recently, a new type of optical clock was built, which relies on quantum logic techniques to interrogate a single Aluminum ion. Both standards have systematic uncertainties below 3 x 10-17, which allows us to measure their frequency ratio with a total uncertainty of 5 x 10-17, making this ratio the best known constant of nature. By looking for changes of the ratio, we can search for changes of the fine-structure constant.