One of the most precise measurements

One of the most precise measurements ever comes from a new generation of atomic clocks that are based on optical transitions in single trapped ions. Like their cesium-based predecessors, optical clocks keep time by locking onto atomic resonances. The challenge is to trap an individual ion, cool it to a virtual standstill, and exactly count the number of oscillation cycles in a light source that is made synchronous with the natural oscillations of the single ion. Till Rosenband and his colleagues at NIST in Boulder, Colorado, used two atomic clocks—one based on an aluminum ion, the other based on a mercury ion—to meet the challenge. Direct comparison of the clocks is essential because uncertainties in the clock frequencies are smaller than the best Cs standards. Indeed, the ratio of the frequencies v _{Al ⁺} /v _{Hg ⁺} is accurate to within a mere 5.2 X 10⁻¹⁷, an order of magnitude improvement in achievable measurement accuracy. To reach that precision, the team accounted for subtle effects—among them the tiny jiggling of the ion in the Doppler-cooled trap, blackbody radiation, and external RF fields—that shift the resonance frequency. Another group, publishing in the same issue of Science, reports significant accuracy gains in optical atomic clocks that use not single ions but large ensembles of strontium atoms held in optical lattices. In that work Andrew Ludlow and colleagues from NIST and the University of Colorado demonstrated an all-optical comparison of clock signals transmitted over kilometers. In addition to testing the invariance of fundamental constants such as the fine structure constant, applications for the new clocks might include long-distance entanglement networks and geodesy measure ments. (T. Rosenband et al., Science, in press; A. D. Ludlow et al., Science, in press.)