Observation of Superradiant and Subradiant Spontaneous Emission of Two Trapped Ions

We report the first observation of a microscopically resolved interaction between two trapped atoms. This is also the first observation of two-ion superradiance and subradiance, the most elementary example of Dicke's theory. Its signature is the variation of the spontaneous emission decay rate of a laser-cooled two-ion crystal, $+1.5%$ and -1.2%, as the ion-ion distance is changed from 1380 to 1540 nm, in agreement with a no-free-parameter theory. The experiment uses an ion microtrap (80 {μ}m radius) with planar electrodes and a new few-atom technique for measuring lifetimes.