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...